Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,110	$ 3,724
Interest cost	6,648	6,909
Expected return on plan assets	(13,114)	(10,512)
Amortization of prior service cost (credit)	428	476
Amortization of actuarial losses	2,987	3,088
Net periodic benefit cost	1,059	3,685
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	201	227
Interest cost	474	569
Amortization of prior service cost (credit)	(669)	(654)
Amortization of actuarial losses	54	89
Net periodic benefit cost	$ 60	$ 231