Retirement Plans - Components of Net Periodic Benefits Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Other Post-retirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost	$ 55	$ 48
Interest cost	330	361
Amortization of prior service cost		(14)
Amortization of net loss	38	177
Amortization of transition obligation		1
Net periodic benefit cost	423	573
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost	3,929	3,247
Interest cost	3,944	4,302
Expected return on plan assets	(6,263)	(6,297)
Amortization of net loss	1,745	1,645
Amortization of transition obligation	0	0
Net periodic benefit cost	$ 3,355	$ 2,897