Components of Post-Retirement Benefit Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	$ (571)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3,765	2,691
Interest cost	4,264	4,152
Expected return on assets	(4,356)	(4,120)
Prior service cost	571	516
Net actuarial (gain)/loss	4,063	2,966
Net periodic benefit cost	8,307	6,205
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	85	124
Interest cost	260	257
Expected return on assets	0	0
Prior service cost	0	0
Net actuarial (gain)/loss	0	0
Net periodic benefit cost	$ 345	$ 381