Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 5,423	$ 5,863	$ 4,994
Interest cost	10,510	11,687	11,508
Expected return on plan assets	(11,112)	(11,170)	(11,436)
Amortization of prior service credit	(55)	(63)	(129)
Special termination benefits			13
Net loss recognition	5,974	6,030	4,775
Net periodic benefit cost	10,740	12,347	9,725
Other Benefits [Member]
Components of net periodic benefit cost:
Service cost	116	92	142
Interest cost	2,077	2,199	2,305
Amortization of prior service credit	(111)	(116)	(195)
Net loss recognition	842	386	424
Net periodic benefit cost	$ 2,924	$ 2,561	$ 2,676