Changes in Cumulative Other Comprehensive Loss by Component - Changes in Cumulative Other Comprehensive Loss by Component (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount of gain recognized in other comprehensive loss, tax portion	$ 1,942	$ (1,337)	$ 856	$ 605
Amount of gain reclassified from Cumulative Other Comprehensive Loss	885		2,185
Income tax benefit (expense) on derivative instruments	234		672
Other comprehensive income, pre-tax	6,880		2,760
Other comprehensive loss, tax	1,445		567
Amortization of prior service credit	141	142	424	425
Amortization of actuarial loss	(9,147)	(13,191)	(27,438)	(39,558)
Income tax provision on postretirement benefit plans	3,075	4,744	9,267	14,236
Derivatives Designated as Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives (Effective Portion)	5,808	(2,316)	4,119	2,926
Amount of gain recognized in other comprehensive loss, tax portion	$ 2,176		$ 1,528