Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Current
Deferred compensation	$ 6,192	$ 8,607
Current portion of asset retirement obligation	961	2,011
Other	8,896	1,423
Total current	16,049	12,041
Non-current
Net operating loss carryforward	56,402	63,568
Deferred compensation	72,904	64,176
Equity compensation	23,363	20,576
AMT credits and other credits	2,761	3,505
Non-current portion of asset retirement obligation	56,764	30,358
Cumulative unrealized mark-to-market (gain) loss	(262)	2,373
Other	1,379	1,500
Valuation allowance	(9,052)	(4,534)
Total non-current	204,259	181,522
Current
Net unrealized gain in AOCI related to hedge derivatives	(49,124)	(68,636)
Other	(2,004)
Cumulative unrealized mark-to-market gain	(2,845)
Total current	(53,973)	(68,636)
Non-current
Depreciation, depletion and investments	(894,031)	(862,597)
Net unrealized gain in AOCI related to hedge derivatives	(4,522)	(29,415)
Other	(4,008)
Total non-current	(902,561)	(892,012)
Net deferred tax liability	$ (736,226)	$ (767,085)