Supplemental Information on Oil and Natural Gas Producing Activities - Results of Operations from Oil and Natural Gas Producing Activities (Unaudited) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Extractive Industries [Abstract]
Revenues	$ 1,420,879		$ 1,820,278		$ 1,759,282
Expenses
Production costs	377,819		548,719		524,364
Depreciation and depletion	434,295		567,732		568,029
Accretion of asset retirement obligations	9,092		36,777		28,996
Total expenses	821,206		1,153,228		1,121,389
Income before income taxes	599,673		667,050		637,893
Benefit of income taxes	(3,933)	[1]	(7,471)	[1]	(437,595)	[1]
Results of operations for oil and natural gas producing activities (excluding corporate overhead and interest costs)	$ 603,606		$ 674,521		$ 1,075,488

[1] Reflects the Company’s effective tax rate, including the partial valuation allowance releases.