Retirement Plans - Components of Net Periodic Benefits Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost	$ 52	$ 48
Interest cost	327	361
Amortization of prior service cost		(13)
Amortization of net loss	459	176
Amortization of transition obligation		1
Net periodic benefit cost	838	573
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost	3,969	3,249	11,905	9,743
Interest cost	4,151	4,301	12,457	12,904
Expected return on plan assets	(6,982)	(6,350)	(20,946)	(19,049)
Amortization of net loss	1,605	1,645	4,813	4,936
Net periodic benefit cost	2,743	2,845	8,229	8,534
Other Post-retirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost			162	144
Interest cost			987	1,083
Amortization of prior service cost				(41)
Amortization of net loss			1,320	530
Amortization of transition obligation				4
Net periodic benefit cost			$ 2,469	$ 1,720