Summary of Reclassification out of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Other Comprehensive Income Loss Reclassification Adjustment [Line Items]
Realized (gains) losses	$ 2,561	$ (5,050)
Amortization of (discount) premium	(1,882)	(96)
Total before tax	679
Tax	991
Amortization of prior service cost	571
Amortization of actuarial gain (loss)	3,972
Total before tax	4,543	3,506
Tax	(1,591)	(1,227)
Total reclassifications, net of tax	4,622
Available-for-sale Assets [Member]
Other Comprehensive Income Loss Reclassification Adjustment [Line Items]
Total reclassifications, net of tax	1,670
Pension Adjustments [Member]
Other Comprehensive Income Loss Reclassification Adjustment [Line Items]
Total reclassifications, net of tax	$ 2,952