Annual report pursuant to Section 13 and 15(d)

Supplemental Information on Oil and Natural Gas Properties (unaudited) (Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserves Disclosure) (Details)

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Supplemental Information on Oil and Natural Gas Properties (unaudited) (Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserves Disclosure) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Future Net Cash Flows [Abstract]            
Future cash inflows       $ 3,180,005 $ 2,227,463 $ 2,492,178
Production       (974,667) (827,555) (873,469)
Development and net abandonment       (384,117) (239,100) (288,081)
Future net inflows before income taxes       1,821,221 1,160,808 1,330,628
Future income taxes       (1,602)   (164,490)
Future net cash flows       1,819,619 1,160,808 1,166,138
10% discount factor       (1,009,787) (589,918) (586,596)
Standardized measure of discounted future net cash flows $ 570,890 $ 579,542 $ 283,946 $ 809,832 $ 570,890 $ 579,542
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]            
Standardized measure at the beginning of the period 570,890 579,542 283,946      
Sales and transfers, net of production costs (150,628) (110,476) (120,518)      
Net change in sales and transfer prices, net of production costs (103,136) (286,660) (156,066)      
Net change due to purchases and sales of in place reserves 260,859 37,616 111,331      
Extensions, discoveries, and improved recovery, net of future production and development costs incurred 180,228 184,469 299,192      
Changes in future development cost 82,320 108,216 186,605      
Revisions of quantity estimates (35,938) (12,625) (7,673)      
Accretion of discount 57,091 62,968 30,114      
Net change in income taxes 16 35,407 (32,940)      
Changes in production rates, timing and other (51,870) (27,567) (14,449)      
Aggregate change 238,942 (8,652) 295,596      
Standardized measure at the end of the period $ 809,832 $ 570,890 $ 579,542