Income tax - Deferred Income Tax Assets (Liabilities) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Deferred Income taxes Assets :
Accrued liabilities	$ 121,539	[1]	$ 152,501	[2]
Retirement plans and postretirement benefits	24,312	[1]	52,981	[2]
Share-based compensation	20,468	[1]	34,325	[2]
Receivables and inventories	10,275	[1]	14,268	[2]
Derivative instruments	283	[1]	10,909	[2]
State net operating loss carryforwards	31,824	[1]	30,621	[2]
State tax credit carryforwards	3,007	[1]	8,307	[2]
Valuation allowances	(8,733)	[1]	(7,570)	[2]
Deferred income tax assets	202,975	[1]	296,342	[2]
Property, plant and equipment	(199,004)	[1]	(243,849)	[2]
Intangible assets	(8,751)	[1]	(22,484)	[2]
Other	(145)	[1]	(6,125)	[2]
Deferred income tax liabilities	(207,900)	[1]	(272,458)	[2]
Net deferred income tax asset (liability)	$ (4,925)	[1]	$ 23,884	[2]

[1] Includes $7.5 million of deferred tax assets related to uncertain tax positions.

[2] Includes $11.7 million of deferred tax assets related to uncertain tax positions.