Schedule of Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension Plans, Defined Benefit
Pension and Other Postretirement Benefits Disclosure [Line Items]
Service cost	$ 3,174	$ 2,437	$ 6,740	$ 5,630
Interest cost	5,743	5,027	12,193	11,617
Expected return on assets	(5,802)	(5,298)	(12,318)	(12,243)
Prior service cost (credit)	113	116	240	267
Actuarial loss (gain)	3,016	1,368	6,403	3,162
Settlement charge	524		524	14
Net periodic benefit cost (credit)	6,768	3,650	13,782	8,447
Other Postretirement Benefit Plans, Defined Benefit
Pension and Other Postretirement Benefits Disclosure [Line Items]
Service cost	59	34	114	175
Interest cost	322	214	617	1,113
Prior service cost (credit)	(849)	(167)	(1,628)	(871)
Actuarial loss (gain)	(74)	(8)	(142)	(43)
Net periodic benefit cost (credit)	$ (542)	$ 73	$ (1,039)	$ 374