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, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]            
Future cash inflows       $ 5,920,328 $ 3,180,005 $ 2,227,463
Production       (1,692,871) (974,667) (827,555)
Development and net abandonment       (680,948) (384,117) (239,100)
Future net inflows before income taxes       3,546,509 1,821,221 1,160,808
Future income taxes       (166,985) (1,602) 0
Future net cash flows       3,379,524 1,819,619 1,160,808
10% discount factor       (1,822,842) (1,009,787) (589,918)
Standardized measure of discounted future net cash flows $ 809,832 $ 570,890 $ 579,542 $ 1,556,682 $ 809,832 $ 570,890
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 809,832 570,890 579,542      
Sales and transfers, net of production costs (294,172) (150,628) (110,476)      
Net change in sales and transfer prices, net of production costs 176,234 (103,136) (286,660)      
Net change due to purchases and sales of in place reserves 129,454 260,859 37,616      
Extensions, discoveries, and improved recovery, net of future production and development costs incurred 635,000 180,228 184,469      
Changes in future development cost 36,983 82,320 108,216      
Revisions of quantity estimates (79,325) (35,938) (12,625)      
Accretion of discount 80,983 57,091 62,968      
Net change in income taxes (20,073) 16 35,407      
Changes in production rates, timing and other 81,766 (51,870) (27,567)      
Aggregate change 746,850 238,942 (8,652)      
Standardized measure at the end of the period $ 1,556,682 $ 809,832 $ 570,890