Income Taxes - Schedule of Net Deferred Tax Asset and Liability (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Tax effect of temporary differences relating to:
Inventories	$ 5,859	$ 4,587
Accrued pension cost	11,036	8,430
Accrued OPEB cost	21,454	20,445
Accrued employee benefits	5,825	5,604
Accrued insurance	1,066	1,389
Other accrued liabilities	226	555
Other deductible differences	236	210
Gross deferred tax assets	45,702	41,220
Reclassification, principally netting by tax jurisdiction	(32,862)	(29,026)
Net deferred tax asset	12,840	12,194
Less current deferred tax asset	12,840	(12,194)
Tax effect of temporary differences relating to:
Property and equipment	(20,246)	(18,612)
Pension asset	(40,323)	(28,197)
Gross deferred tax liabilities	(60,569)	(46,809)
Reclassification, principally netting by tax jurisdiction	32,862	29,026
Net deferred tax Liability	(27,707)	(17,783)
Noncurrent deferred tax liability	$ (27,707)	$ (17,783)