Annual report pursuant to Section 13 and 15(d)

Supplemental Oil and Gas Disclosures - Change in Standard Measure of Discounted Future Net Cash Flows (Details)

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Supplemental Oil and Gas Disclosures - Change in Standard Measure of Discounted Future Net Cash Flows (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Changes in Standardized Measure      
Changes in Standardized Measure, beginning of year $ 478.3 $ 613.9 $ 1,702.8
Sales and transfers of oil and gas produced, net of production costs (315.3) (218.6) (289.1)
Net changes in price, net of future production costs 288.0 (275.2) (1,455.6)
Extensions and discoveries, net of future production and development costs 119.3   65.3
Changes in estimated future development costs (38.9) (32.5) (8.5)
Previously estimated development costs incurred 102.8 114.5 158.9
Revisions of quantity estimates 106.4 190.1 137.9
Accretion of discount 30.2 52.6 150.6
Net change in income taxes (54.7)   600.8
Purchases of reserves in-place     6.0
Sales of reserves in-place     (401.4)
Changes in production rates due to timing and other 24.5 33.5 (53.8)
Net increase (decrease) in standardized measure 262.3 (135.6) (1,088.9)
Changes in Standardized measure, end of year $ 740.6 $ 478.3 $ 613.9