Summary of Principal Components of Net Deferred Income Tax Liability (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Deferred tax liabilities:
Property and equipment	$ 429,568	$ 352,431
Fair value of derivative instruments	3,006	2,215
Convertible notes	60	6,143
Total deferred tax liabilities	432,634	360,789
Deferred tax assets:
Pension and postretirement benefits	3,046	3,951
Share-based compensation	8,838	7,602
Net operating loss carryforwards	150,953	27,915
Other	10,642	5,230
Deferred Tax Assets, Gross, Total	173,479	44,698
Less: Valuation allowance	(19,492)	(19,063)
Total deferred tax assets	153,987	25,635
Net deferred tax liability	$ 278,647	$ 335,154