Components of Net Periodic Pension Costs and Post-Retirement Benefit Costs (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]
Net periodic pension cost	$ 26,007	$ 20,952	$ 18,948
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	11,215	9,277	8,174
Interest cost on benefit obligation	16,796	16,106	15,392
Expected return on assets	(17,114)	(16,068)	(15,025)
Net amortization	15,110	11,637	10,407
Net periodic pension cost	26,007	20,952	18,948
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	392	919	728
Interest cost on benefit obligation	1,020	999	970
Expected return on assets	0	0	0
Amortization of prior service cost	0	0	0
Net amortization	0	(815)	(583)
Net periodic pension cost	$ 1,412	$ 1,103	$ 1,115