| Oil and Gas Exploration and Production Industries Disclosures [Text Block] | 
20.  Supplemental Oil and Gas Disclosures—UNAUDITED        Geographic Area of Operation       All of our proved reserves are located within the United States in the Gulf of Mexico. Therefore, the following disclosures about our costs incurred, results of operations and proved reserves are on a total-company basis.         Net capitalized costs related to our oil, NGLs and natural gas producing activities are as follows (in millions):         |   |    |     |    |     |   |    |     |    |    |     |    |    |     |    |     |    |    |    |    |     |    |    |    |     |    |    |    |     |     |   Proved oil and natural gas properties and equipment   |    |  $ |  8,532.2  |    |    |  $ |  8,169.9  |    |    |  $ |  8,102.0  |    |     |   Accumulated depreciation, depletion and amortization related to oil, NGLs and natural gas activities   |    |    |  (7,793.3  |  ) |    |    |  (7,665.1  |  ) |    |    |  (7,525.0  |  ) |     |   Net capitalized costs related to producing activities   |    |  $ |  738.9  |    |    |  $ |  504.8  |    |    |  $ |  577.0  |    |            Costs Incurred In Oil and Gas Property Acquisition, Exploration and Development Activities       The following costs were incurred in oil and gas acquisition, exploration, and development activities (in millions):         |   |    |     |    |     |   |    |     |    |    |     |    |    |     |    |     |    |    |    |    |     |    |    |    |     |    |    |    |     |     |   Proved properties acquisitions   |    |  $ |  223.8  |    |    |  $ |  24.1  |    |    |  $ |  1.1  |    |     |   Exploration (2) (3)   |    |    |  30.6  |    |    |    |  49.9  |    |    |    |  62.0  |    |     |   Development   |    |    |  114.5  |    |    |    |  56.2  |    |    |    |  92.5  |    |     |   Total costs incurred in oil and gas property acquisition, exploration and development activities   |    |  $ |  368.9  |    |    |  $ |  130.2  |    |    |  $ |  155.6  |    |               |   |     |   Includes net additions from capitalized ARO of  $37.5  million,  $20.3  million and  $21.3  million during  2019,  2018  and  2017,  respectively.  These adjustments for ARO are associated with acquisitions, liabilities incurred, divestitures and revisions of estimates.    |          |   |     |   Includes seismic costs of  $7.8  million,  $1.5  million and  $0.5  million incurred during  2019,  2018  and  2017,  respectively.    |          |   |     |   Includes geological and geophysical costs charged to expense of  $5.7  million,  $5.4  million and  $4.2  million during  2019,  2018  and  2017,  respectively.    |          Depreciation, depletion, amortization and accretion expense       The following table presents our depreciation, depletion, amortization and accretion expense per barrel equivalent (“Boe”) of products sold:         |   |    |     |    |     |   |    |     |    |    |     |    |    |     |    |     |   Depreciation, depletion, amortization and accretion per Boe   |    |  $ |  10.01  |    |    |  $ |  11.24  |    |    |  $ |  10.68  |    |                 Oil and Natural Gas Reserve Information      There are numerous uncertainties in estimating quantities of proved reserves and in providing the future rates of production and timing of development expenditures. The following reserve information represents estimates only and are inherently imprecise and   may  be subject to substantial revisions as additional information such as reservoir performance, additional drilling, technological advancements and other factors become available.  Decreases in the prices of oil, NGLs and natural gas could have an adverse effect on the carrying value of our proved reserves, reserve volumes and our revenues, profitability and cash flow.  We are  not  the operator with respect to  10.7%  of our proved developed non-producing reserves as of   December 31, 2019  so we   may  
not  be in a position to control the timing of all development activities.  We are the operator for substantially all of our proved undeveloped reserves as of   December 31, 2019.   In prior years, we were  not  the operator of substantially all proved undeveloped reserves.      The following sets forth estimated quantities of our net proved, proved developed and proved undeveloped oil, NGLs and natural gas reserves.  All of the reserves are located in the United States with all located in state and federal waters in the Gulf of Mexico.  The reserve estimates exclude insignificant royalties and interests owned by the Company due to the unavailability of such information.  In addition to other criteria, estimated reserves are assessed for economic viability based on the unweighted average of  first -day-of-the-month commodity prices over the period   January  through   December  for the year in accordance with definitions and guidelines set forth by the SEC and the FASB.  The prices used do  not  purport, nor should it be interpreted, to present the current market prices related to our estimated oil and natural gas reserves.  Actual future prices and costs   may  differ materially from those used in determining our proved reserves for the periods presented.  The prices used are presented in the section below entitled “ Standardized Measure of Discounted Future Net Cash Flows”. 
         |   |    |    |     |     |    |     |     |    |     |     |   Total Energy Equivalent Reserves (1)    |    |     |   |    |     |    |     |    |     |    |     |    |   Natural Gas Equivalent (Bcfe)    |    |     |   Proved reserves as of Dec. 31, 2016   |    |    |  32.9  |    |    |  8.2  |    |    |  197.8  |    |    |  74.0  |    |    |  444.0  |    |     |   Revisions of previous estimates (2)   |    |    |  4.5  |    |    |  0.7  |    |    |  25.8  |    |    |  9.6  |    |    |  57.4  |    |     |   Extensions and discoveries (3)   |    |    |  4.1  |    |    |  0.3  |    |    |  5.4  |    |    |  5.2  |    |    |  31.3  |    |     |   Production   |    |    |  (7.1  |  ) |    |  (1.4  |  ) |    |  (36.8  |  ) |    |  (14.6  |  ) |    |  (87.4  |  ) |     |   Proved reserves as of Dec. 31, 2017   |    |    |  34.4  |    |    |  7.8  |    |    |  192.2  |    |    |  74.2  |    |    |  445.3  |    |     |   Revisions of previous estimates (4)   |    |    |  11.6  |    |    |  2.8  |    |    |  40.4  |    |    |  21.1  |    |    |  126.7  |    |     |   Extensions and discoveries (5)   |    |    |  0.5  |    |    |  0.3  |    |    |  7.7  |    |    |  2.1  |    |    |  12.6  |    |     |   Purchase of minerals in place (6)   |    |    |  1.5  |    |    |  0.4  |    |    |  9.4  |    |    |  3.4  |    |    |  20.7  |    |     |   Sales of minerals in place (7)   |    |    |  (2.2  |  ) |    |  (0.2  |  ) |    |  (7.2  |  ) |    |  (3.5  |  ) |    |  (21.2  |  ) |     |   Production   |    |    |  (6.7  |  ) |    |  (1.3  |  ) |    |  (32.0  |  ) |    |  (13.3  |  ) |    |  (80.0  |  ) |     |   Proved reserves as of Dec. 31, 2018   |    |    |  39.1  |    |    |  9.8  |    |    |  210.5  |    |    |  84.0  |    |    |  504.1  |    |     |   Revisions of previous estimates (8)   |    |    |  1.4  |    |    |  (1.5  |  ) |    |  (16.9  |  ) |    |  (3.0  |  ) |    |  (18.2  |  ) |     |   Extensions and discoveries (9)   |    |    |  0.9  |    |    |  0.1  |    |    |  1.2  |    |    |  1.1  |    |    |  6.7  |    |     |   Purchase of minerals in place (10)   |    |    |  3.1  |    |    |  17.4  |    |    |  417.6  |    |    |  90.1  |    |    |  540.9  |    |     |   Production   |    |    |  (6.7  |  ) |    |  (1.3  |  ) |    |  (41.3  |  ) |    |  (14.8  |  ) |    |  (89.0  |  ) |     |   Proved reserves as of Dec. 31, 2019   |    |    |  37.8  |    |    |  24.5  |    |    |  571.1  |    |    |  157.4  |    |    |  944.5  |    |     |   |    |    |    |    |    |    |    |    |    |    |    |    |    |    |    |    |     |   Year-end proved developed reserves:   |    |    |    |    |    |    |    |    |    |    |    |    |    |    |    |    |     |   2019   |    |    |  28.0  |    |    |  21.7  |    |    |  504.9  |    |    |  133.8  |    |    |  802.9  |    |     |   2018   |    |    |  31.5  |    |    |  7.8  |    |    |  166.8  |    |    |  67.0  |    |    |  402.2  |    |     |   2017   |    |    |  26.1  |    |    |  7.2  |    |    |  173.5  |    |    |  62.2  |    |    |  373.3  |    |     |   |    |    |    |    |    |    |    |    |    |    |    |    |    |    |    |    |     |   Year-end proved undeveloped reserves:   |    |    |    |    |    |    |    |    |    |    |    |    |    |    |    |    |     | 2019 (11) |    |    |  9.8  |    |    |  2.8  |    |    |  66.2  |    |    |  23.6  |    |    |  141.6  |    |     |   2018   |    |    |  7.6  |    |    |  2.0  |    |    |  43.7  |    |    |  17.0  |    |    |  101.9  |    |     |   2017   |    |    |  8.3  |    |    |  0.6  |    |    |  18.7  |    |    |  12.0  |    |    |  72.0  |    |               |   Volume measurements:   |    |    |     |   MMBbls – million barrels for crude oil, condensate or NGLs   |    |    Bcf – billion cubic feet   |     |   MMBoe – million barrels of oil equivalent   |    |    Bcfe – billion cubic feet of gas equivalent   |                  |    |   The conversion to barrels of oil equivalent and cubic feet equivalent were determined using the energy-equivalent ratio of  six  Mcf of natural gas to  one  barrel of crude oil, condensate or NGLs (totals   may  
not  compute due to rounding). The energy-equivalent ratio does  not  assume price equivalency, and the energy-equivalent prices for crude oil, NGLs and natural gas   may  differ significantly.    |             |    |   Primarily related to upward revisions at our Mississippi Canyon  698  (Big Bend) field, our Fairway field, our Ewing Bank  910  field and our Viosca Knoll  783  (Tahoe/SE Tahoe) field.  Additionally, increases of  3.4  MMBoe were due to price revisions.    |             |    |   Primarily related to extensions and discoveries at our Ship Shoal  349  (Mahogany) field of  3.5  MMBoe and at our Main Pass  286  field of  1.5  MMBoe.    |             |    |   Primarily related to upward revisions at our Mahogany field and our Ship Shoal  028  field.  Additionally, increases of  2.3  MMBoe were due to price revisions.    |             |    |   Primarily related to extensions and discoveries of  1.3  MMBoe at our Viosca Knoll  823  (Virgo) field and  0.7  MMBoe at our Ewing Bank  910  field.    |             |    |   Primarily related to our Ship Shoal  028  field and our Green Canyon  859  field (Heidelberg).    |             |    |    Primarily related to conveyance of interest in properties related to the JV Drilling Program.   |             | ( 8 ) |   Increases primarily related to upward revisions to our Ship Shoal  028  field and our Main Pass  108  field.  Decreases of  10.0  MMBoe were due to price revisions for all proved reserves, which include estimated price revisions of the purchase of minerals in place from the date of purchase to   December 31, 2019. 
   |             |    |   Primarily related to extensions and discoveries of  0.9  MMBoe at our Mississippi Canyon  800  (Gladden) field.    |             |    |    Primarily related to the Mobile Bay Properties and Magnolia acquisitions   |             |    |   We believe that we will be able to develop all but  2.5  MMBoe (approximately  11% ) of the total of  23.6  MMBoe reserves classified as proved undeveloped (“PUDs”) at   December 31, 2019,  within  five  years from the date such reserves were initially recorded.  The lone exceptions are at the Mississippi Canyon  243  field (Matterhorn) and Virgo deepwater fields where future development drilling has been planned as sidetracks of existing wellbores due to conductor slot limitations and rig availability.  Two sidetrack PUD locations,  one  each at Matterhorn and Virgo, will be delayed until an existing well is depleted and available to sidetrack.  We also plan to recomplete and convert an existing producer at Matterhorn to water injection for improved recovery following depletion of existing well.  Based on the latest reserve report, these PUD locations are expected to be developed in  2021  and  2022.     |               Standardized Measure of Discounted Future Net Cash Flows       The following presents the standardized measure of discounted future net cash flows related to our proved oil and natural gas reserves together with changes therein. Future cash inflows represent expected revenues from production of period-end quantities of proved reserves based on the unweighted average of  first -day-of-the-month commodity prices for the periods presented. All prices are adjusted by field for quality, transportation fees, energy content and regional price differentials. Due to the lack of a benchmark price for NGLs, a ratio is computed for each field of the NGLs realized price compared to the crude oil realized price. Then, this ratio is applied to the crude oil price using FASB/SEC guidance. The average commodity prices weighted by field production and after adjustments related to the proved reserves are as follows:          |   |    |     |    |     |   |    |     |    |    |     |    |    |     |    |    |     |    |     |   Oil - per barrel   |    |  $ |  58.11  |    |    |  $ |  65.21  |    |    |  $ |  46.58  |    |    |  $ |  36.28  |    |     |   NGLs per barrel   |    |    |  18.72  |    |    |    |  29.73  |    |    |    |  22.65  |    |    |    |  16.82  |    |     |   Natural gas per Mcf   |    |    |  2.63  |    |    |    |  3.13  |    |    |    |  2.86  |    |    |    |  2.47  |    |            Future production, development costs and ARO are based on costs in effect at the end of each of the respective years with  no  escalations. Estimated future net cash flows, net of future income taxes, have been discounted to their present values based on a  10%  annual discount rate.      The standardized measure of discounted future net cash flows does  not  purport, nor should it be interpreted, to present the fair market value of our oil and natural gas reserves. These estimates reflect proved reserves only and ignore, among other things, future changes in prices and costs, revenues that could result from probable reserves which could become proved reserves in  2019  or later years and the risks inherent in reserve estimates. The standardized measure of discounted future net cash flows relating to our proved oil and natural gas reserves is as follows (in millions):          |   |    |     |    |     |   |    |     |    |    |     |    |    |     |    |      Standardized Measure of Discounted Future Net Cash Flows    |    |    |    |     |    |    |    |     |    |    |    |     |     |   Future cash inflows   |    |  $ |  4,153.8  |    |    |  $ |  3,500.9  |    |    |  $ |  2,328.8  |    |     |   Future costs:   |    |    |    |    |    |    |    |    |    |    |    |    |     |   Production   |    |    |  (1,901.1  |  ) |    |    |  (958.5  |  ) |    |    |  (813.8  |  ) |     |   Development   |    |    |  (297.3  |  ) |    |    |  (272.4  |  ) |    |    |  (157.4  |  ) |     |   Dismantlement and abandonment   |    |    |  (497.4  |  ) |    |    |  (355.9  |  ) |    |    |  (361.9  |  ) |     |   Income taxes   |    |    |  (170.5  |  ) |    |    |  (293.9  |  ) |    |    |  (74.8  |  ) |     |   Future net cash inflows before 10% discount   |    |    |  1,287.5  |    |    |    |  1,620.2  |    |    |    |  920.9  |    |     |   10% annual discount factor   |    |    |  (300.6  |  ) |    |    |  (553.2  |  ) |    |    |  (180.3  |  ) |     |   Total   |    |  $ |  986.9  |    |    |  $ |  1,067.0  |    |    |  $ |  740.6  |    |                     The change in the standardized measure of discounted future net cash flows relating to our proved oil and natural gas reserves is as follows (in millions):          |   |    |     |    |     |   |    |     |    |    |     |    |    |     |    |      Changes in Standardized Measure    |    |    |    |     |    |    |    |     |    |    |    |     |     |   Standardized measure, beginning of year   |    |  $ |  1,067.0  |    |    |  $ |  740.6  |    |    |  $ |  478.3  |    |     |   Increases (decreases):   |    |    |    |    |    |    |    |    |    |    |    |    |     | Sales and transfers of oil and gas produced, net of production costs |    |    |  (315.8  |  ) |    |    |  (398.1  |  ) |    |    |  (315.3  |  ) |     | Net changes in price, net of future production costs |    |    |  (376.4  |  ) |    |    |  571.5  |    |    |    |  288.0  |    |     | Extensions and discoveries, net of future production and development costs |    |    |  27.0  |    |    |    |  53.6  |    |    |    |  119.3  |    |     | Changes in estimated future development costs |    |    |  (6.0  |  ) |    |    |  (114.7  |  ) |    |    |  (38.9  |  ) |     | Previously estimated development costs incurred |    |    |  19.3  |    |    |    |  48.4  |    |    |    |  102.8  |    |     | Revisions of quantity estimates |    |    |  116.4  |    |    |    |  307.6  |    |    |    |  106.4  |    |     | Accretion of discount |    |    |  107.4  |    |    |    |  50.5  |    |    |    |  30.2  |    |     | Net change in income taxes |    |    |  62.9  |    |    |    |  (133.4  |  ) |    |    |  (54.7  |  ) |     | Purchases of reserves in-place |    |    |  298.3  |    |    |    |  27.8  |    |    |    |  —  |    |     | Sales of reserves in-place |    |    |  —  |    |    |    |  (54.1  |  ) |    |    |  —  |    |     | Changes in production rates due to timing and other |    |    |  (13.2  |  ) |    |    |  (32.7  |  ) |    |    |  24.5  |    |     |   Net (decrease) increase   |    |    |  (80.1  |  ) |    |    |  326.4  |    |    |    |  262.3  |    |     |   Standardized measure, end of year   |    |  $ |  986.9  |    |    |  $ |  1,067.0  |    |    |  $ |  740.6  |    |            
 
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