Supplemental Information about Changes to Accumulated Other Comprehensive Income - Summary of Reclassification out of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]
Realized (gains) losses	$ (4,520)	$ (6,900)	$ (7,871)	$ (16,609)
Amortization of (discount) premium	(1,478)	1,348	(4,939)	1,224
Total before tax	4,605	3,569	13,756	10,678
Tax	(1,613)	(1,250)	(4,815)	(3,737)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]
Total after tax	(890)		1,827
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized gains (losses) on available for sale assets [Member]
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]
Realized (gains) losses	(4,520)		(7,871)
Amortization of (discount) premium	(1,478)		(4,939)
Total before tax	(5,998)		(12,810)
Tax	2,116		5,696
Total after tax	(3,882)		(7,114)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension adjustments [Member]
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]
Amortization of prior service cost	569		1,707
Amortization of actuarial gain (loss)	4,036		12,049
Total before tax	4,605		13,756
Tax	(1,613)		(4,815)
Total after tax	$ 2,992		$ 8,941