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Supplemental Information on Oil and Natural Gas Producing Activities - Standardized Measure of Discounted Future Cash Flows (Unaudited) (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Extractive Industries [Abstract]        
Future cash inflows from production $ 6,387,944 $ 21,022,320 $ 19,937,484  
Future production costs (2,731,542) (6,499,366) (6,843,713)  
Future development costs (838,945) (1,810,201) (2,546,680)  
Future income tax expenses (901) (3,223,740) (2,283,541)  
Undiscounted future net cash flows 2,816,556 9,489,013 8,263,550  
10% annual discount (1,501,994) (5,401,261) (4,245,939)  
Standardized measure of discounted future net cash flows $ 1,314,562 $ 4,087,752 $ 4,017,611 $ 5,840,368