Benefit Plans - Net Periodic Pension Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,253	$ 5,951	$ 5,544
Interest cost	6,429	6,147	5,709
Expected return on plan assets	(8,937)	(6,698)	(6,214)
Amortization of prior service cost	818	707	643
Amortization of net loss	2,377	1,525	1,098
Total net periodic pension expense	$ 7,940	$ 7,632	$ 6,780