Oil and Natural Gas Operations (Unaudited) (Standardized Measure of Discounted Future Net Cash Flows) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Standardized Measure [Abstract]
Future gross revenues					$ 11,714,729	$ 20,971,672	$ 19,509,305
Future production costs					4,353,974	7,532,273	6,136,709
Future development costs					1,961,661	1,784,738	1,896,602
Future income tax expense					1,065,887	3,440,582	3,209,697
Future net cash flows					4,333,207	8,214,079	8,266,297
Discount at 10% per annum					2,299,859	3,994,423	4,248,456
Standardized measure of discounted future net cash flows relating to proved oil and natural gas reserves		$ 4,219,656	$ 4,017,841	$ 3,699,319	$ 2,033,348	$ 4,219,656	$ 4,017,841
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward]
Balance at beginning of year		4,219,656	4,017,841	3,699,319
Revisions to reserves proved in prior years:
Net changes in prices, production costs and future development costs		(2,861,591)	(1,147,028)	566,838
Net changes due to revisions in quantity estimates		(404,708)	(1,285,394)	(81,762)
Development costs incurred, previously estimated		350,560	337,198	299,432
Accretion of discount		421,966	401,784	369,932
Changes in timing and other	[1]	(903,975)	987,652	(179,502)
Total revisions		(3,397,748)	(705,788)	974,938
New field discoveries and extensions, net of future production and development costs		776,315	2,321,028	376,326
Sales of oil and gas produced, net of production costs		(514,380)	(1,054,553)	(1,014,593)
Purchases		8	4,241	4,690
Sales		(372,039)	(21,092)	(24,876)
Net change in income taxes		1,321,536	(342,021)	2,037
Net change in standardized measure of discounted future net cash flows		(2,186,308)	201,815	318,522
Balance at end of year		$ 2,033,348	$ 4,219,656	$ 4,017,841

[1] Amount represents changes in production timing and other. In 2015, the production timing is significantly affected by changes related to the delay of the drilling program. For 2014, the production timing is significantly affected by changes related to the acceleration of the horizontal drilling program and the delay of the vertical drilling program.