Income Taxes - Tax Effects of Significant Temporary Differences Representing Net Deferred Tax Asset and Liability (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Current deferred tax liabilities:
Marketable securities	$ (1,262)	$ (7,503)
Derivative financial instruments	(4,078)	(161)
Net current deferred tax liability	(5,340)	(7,664)
Noncurrent deferred tax liabilities:
Property and equipment	(247,062)	(253,580)
Other assets	8,319	8,772
Net operating loss carryforwards	95,180	47,320
Alternative minimum tax carryforward	19,080	19,093
Valuation allowance on net operating loss carryforwards	(23,009)	(19,630)
Other	(2,409)	(3,680)
Net noncurrent deferred tax liability	(149,901)	(201,705)
Net deferred tax liability	$ (155,241)	$ (209,369)