Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension
Pension and Other Postretirement Benefits Disclosure [Line Items]
Service cost	$ 4,064	$ 3,600
Interest cost	5,749	5,941
Expected return on assets	(6,663)	(5,970)
Prior service cost (credit)	112	122
Actuarial loss (gain)	3,835	3,317
Net periodic benefit cost (credit)	7,097	7,010
Postretirement Benefit
Pension and Other Postretirement Benefits Disclosure [Line Items]
Service cost	65	66
Interest cost	248	315
Prior service cost (credit)	(814)	(814)
Actuarial loss (gain)		(64)
Net periodic benefit cost (credit)	$ (501)	$ (497)