Annual report pursuant to Section 13 and 15(d)

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

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Supplemental Oil and Gas Disclosures - Standardized Measure of Discounted Future Net Cash Flows (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Standardized Measure of Discounted Future Net Cash Flows        
Future cash inflows $ 2,328,800,000 $ 1,818,400,000 $ 2,296,700,000  
Production (813,800,000) (691,500,000) (840,100,000)  
Development (157,400,000) (141,100,000) (161,400,000)  
Dismantlement and abandonment (361,900,000) (427,700,000) (471,800,000)  
Income taxes (74,800,000) [1] 0 0  
Future net cash inflows before 10% discount 920,900,000 558,100,000 823,400,000  
10% annual discount factor (180,300,000) (79,800,000) (209,500,000)  
Standardized measure of discounted future net cash flows $ 740,600,000 $ 478,300,000 $ 613,900,000 $ 1,702,800,000
[1] No future income taxes were estimated for 2016 and 2015 as our tax position had sufficient tax basis to offset estimated future taxes. State income taxes were disregarded due to immateriality.