Oil and Natural Gas Operations (Unaudited) (Results of Operations) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reserve Quantities [Line Items]
Gross revenues	[1]	$ 878,554	$ 1,679,213	$ 1,206,293
Production (lifting costs)		285,760	376,495	351,541
Exploration expense		14,877	28,090	14,036
Depreciation, depletion and amortization including asset impairments		1,880,190	960,539	463,606
Accretion expense		7,108	7,608	6,995
Income tax expense (benefit)		(469,362)	99,469	128,773
Results of operations from producing activities		(840,019)	207,012	241,342
Commodity contracts
Reserve Quantities [Line Items]
Open non-cash mark-to-market gains (losses) on derivative instruments		$ (281,752)	$ 315,445	$ (47,832)

[1] The years ended December 31, 2015, 2014 and 2013 gross revenues include a pre-tax non-cash mark-to-market loss on derivatives of $281.8 million, a pre-tax non-cash mark-to-market gain on derivatives of $315.4 million and a pre-tax non-cash mark-to-market loss on derivatives of $47.8 million, respectively.