Components of Post-Retirement Benefit Costs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,928	$ 2,187	$ 8,309	$ 6,865
Interest cost	4,100	4,022	12,397	12,073
Expected return on assets	(4,242)	(4,124)	(12,479)	(12,050)
Prior service cost	516	519	1,547	1,556
Net actuarial (gain)/loss	3,434	2,371	9,392	7,161
Net periodic benefit cost	6,736	4,975	19,166	15,605
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	81	184	325	650
Interest cost	254	250	767	752
Expected return on assets	0	0	0	0
Prior service cost	0	0	0	0
Net actuarial (gain)/loss	0	(203)	0	(607)
Net periodic benefit cost	$ 335	$ 231	$ 1,092	$ 795