Employee Benefits - Components of Net Periodic Benefit Cost and Changes in Plan Assets and Benefit Obligations Recognized in OCI (Detail) (Postretirement Benefit Plan [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 1,357	$ 1,178	$ 1,378
Interest cost on projected benefit obligation	5,656	5,861	6,468
Net amortization and deferrals	(1,770)	(2,552)	(2,232)
Expense	5,243	4,487	5,614
Net actuarial loss (gain)	(10,921)	16,909
Recognized actuarial loss	(2,951)	(2,167)
Recognized prior service credit	4,721	4,719
Total recognized in OCI (before tax effects)	$ (9,151)	$ 19,461