Supplemental Information on Oil and Natural Gas Producing Activities - Estimate of Changes in Standardized Measure of Discounted Future Net Cash Flows from Proved Reserves (Unaudited) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Present value, beginning balance	$ 4,087,752	$ 4,017,611	$ 5,840,368
Changes during the year
Revenues less production and other costs	(383,293)	(1,043,060)	(1,271,559)
Net changes in prices, production and other costs	(3,813,465)	331,694	271,566
Development costs incurred	217,596	364,262	474,275
Net changes in future development costs	273,437	(341,183)	(207,729)
Extensions and discoveries	230,055	1,785,963	1,406,102
Revisions of previous quantity estimates	(1,354,778)	(77,688)	(296,418)
Accretion of discount	512,483	477,458	711,385
Net change in income taxes	1,426,333	(256,371)	477,328
Purchases of reserves in-place	18,429	50,958	1,628
Sales of reserves in-place	0	(1,058,330)	(3,172,187)
Timing differences and other	100,013	(163,562)	(217,148)
Net change for the year	(2,773,190)	70,141	(1,822,757)
Present value, ending balance	$ 1,314,562	$ 4,087,752	$ 4,017,611