Summary of Principal Components of Net Deferred Income Tax Liability (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax liabilities:
Property and equipment	$ 6,347	$ 248,164
Fair value of derivative instruments	57,707	0
Total deferred tax liabilities	64,054us-gaap_DeferredTaxLiabilities	248,164us-gaap_DeferredTaxLiabilities
Deferred tax assets:
Fair value of derivative instruments	0	1,665
Pension and postretirement benefits	2,370	2,248
Share-based compensation	7,171	6,907
Net operating loss (“NOL”) carryforwards	102,098	115,355
Deferred gains	33,704	0
Other	19,875us-gaap_DeferredTaxAssetsOther	18,029us-gaap_DeferredTaxAssetsOther
Deferred Tax Assets, Gross, Total	165,218us-gaap_DeferredTaxAssetsGross	144,204us-gaap_DeferredTaxAssetsGross
Less: Valuation allowance	(105,615)	(35,727)
Total deferred tax assets	59,603us-gaap_DeferredTaxAssetsNet	108,477us-gaap_DeferredTaxAssetsNet
Net deferred tax liability	$ 4,451us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 139,687us-gaap_DeferredTaxAssetsLiabilitiesNet

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from a gain reported for tax purposes on sale and leaseback transactions in accordance with enacted tax laws.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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