Summary of Principal Components of Net Deferred Income Tax Liability (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax liabilities:
Property and equipment	$ 311,002	$ 429,568
Fair value of derivative instruments	5,268	3,006
Convertible notes	0	60
Total deferred tax liabilities	316,270	432,634
Deferred tax assets:
Pension and postretirement benefits	2,864	3,046
Share-based compensation	10,760	8,838
Net operating loss (NOL) carryforwards	102,407	150,953
Other	15,788	10,642
Deferred Tax Assets, Gross, Total	131,819	173,479
Less: Valuation allowance	(26,686)	(19,492)
Total deferred tax assets	105,133	153,987
Net deferred tax liability	$ 211,137	$ 278,647