Income Taxes (Detail) - Temporary Differences Related to Deferred Income Taxes (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred income tax assets:
Post-retirement liability	$ 1,968	$ 1,928
Deferred income	1,637us-gaap_DeferredTaxAssetsDeferredIncome	1,568us-gaap_DeferredTaxAssetsDeferredIncome
Stock based compensation	2,108	2,106
Federal operating loss carryforwards	5,029	12,938
Capital loss carryforward	1,311	926
State tax credits	2,423	3,022
State operating loss carryforwards	4,574	8,277
Other	3,405us-gaap_DeferredTaxAssetsOther	4,049us-gaap_DeferredTaxAssetsOther
Less: valuation allowance	(3,829)	(11,275)
Gross deferred income tax assets	18,626us-gaap_DeferredTaxAssetsGross	23,539us-gaap_DeferredTaxAssetsGross
Deferred income tax liabilities:
Fixed assets	(18,823)	(17,919)
Equity method investment	(1,176)	(391)
Other	0us-gaap_DeferredTaxLiabilitiesOther	(5,229)us-gaap_DeferredTaxLiabilitiesOther
Gross deferred income tax liabilities	19,999us-gaap_DeferredIncomeTaxLiabilities	23,539us-gaap_DeferredIncomeTaxLiabilities
Net deferred income tax liability	$ (1,373)us-gaap_DeferredTaxLiabilities	$ 0us-gaap_DeferredTaxLiabilities

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