Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Deferred compensation	$ 9,286	$ 9,128
Total current	25,536	57,190
Deferred compensation	64,656	91,094
Valuation allowance	(16,599)	(14,781)
Total non-current	360,264	247,743
Deferred tax liabilities:
Total current	(141,123)	(5,776)
Total non-current	(1,357,759)	(1,019,723)
Net deferred tax liability	(1,113,082)	(720,566)
Current
Deferred tax assets:
Portion of asset retirement obligation	5,745	1,854
Cumulative mark-to-market loss	584	15,193
Net operating loss carryforward		23,079
Other	9,921	7,936
Deferred Tax Assets Non Current
Deferred tax assets:
Portion of asset retirement obligation	104,063	86,126
Cumulative mark-to-market loss	65
Net operating loss carryforward	176,812	57,266
Other	987	1,116
Equity compensation	25,833	22,800
AMT credits and other credits	4,447	4,122
Current
Deferred tax liabilities:
Net gain in AOCI related to hedge derivatives		(3,987)
Other	(2,350)	(1,789)
Cumulative mark-to-market gain	(138,773)
Deferred Tax Liabilities Non Current
Deferred tax liabilities:
Other	(310)	(2,542)
Cumulative mark-to-market gain	(15,410)	(6,424)
Depreciation, depletion and investments	$ (1,342,039)	$ (1,010,757)