Other Changes in Plan Assets and Benefit Obligations Recognized in AOCI (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) loss	$ 8,352	$ 12,640	$ (7,994)
Net prior service (credit)	(2,845)	(8)	(37)
Net transition obligation	0	(1)	1
Amortization	(1,664)	(1,212)	(2,030)
Total recognized in AOCI	3,843	11,419	(10,060)
Total recognized in net periodic benefit cost and AOCI	20,910	24,492	3,209
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) loss	537	(548)	1,020
Net prior service (credit)	0	0	0
Net transition obligation	0	0	0
Amortization	(189)	(265)	(281)
Total recognized in AOCI	348	(813)	739
Total recognized in net periodic benefit cost and AOCI	$ 1,151	$ 115	$ 1,746