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, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows [Abstract]            
Future cash inflows       $ 11,794,080 $ 5,920,328 $ 3,180,005
Production       (2,923,959) (1,692,871) (974,667)
Development and net abandonment       (1,429,787) (680,948) (384,117)
Future net inflows before income taxes       7,440,334 3,546,509 1,821,221
Future income taxes       (782,470) (166,985) (1,602)
Future net cash flows       6,657,864 3,379,524 1,819,619
10% discount factor       (3,716,571) (1,822,842) (1,009,787)
Standardized measure of discounted future net cash flows $ 1,556,682 $ 809,832 $ 570,890 $ 2,941,293 $ 1,556,682 $ 809,832
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Roll Forward]            
Standardized measure at the beginning of the period 1,556,682 809,832 570,890      
Sales and transfers, net of production costs (481,306) (294,172) (150,628)      
Net change in sales and transfer prices, net of production costs 222,802 176,234 (103,136)      
Net change due to purchases and sales of in place reserves 554,697 129,454 260,859      
Extensions, discoveries, and improved recovery, net of future production and development costs incurred 1,093,773 635,000 180,228      
Changes in future development cost 40,483 36,983 82,320      
Revisions of quantity estimates (167,096) (79,325) (35,938)      
Accretion of discount 157,676 80,983 57,091      
Net change in income taxes (187,841) (20,073) 16      
Changes in production rates, timing and other 151,423 81,766 (51,870)      
Aggregate change 1,384,611 746,850 238,942      
Standardized measure at the end of period $ 2,941,293 $ 1,556,682 $ 809,832