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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]
Total before tax	$ 3,031	$ 4,608	$ 5,605	$ 9,151
Tax	(1,061)	(1,611)	(1,962)	(3,202)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]
Total after tax	(208)	(1,906)	(10,784)	2,716
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized investment gains (losses) on available for sale assets [Member]
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]
Realized (gains) losses	(577)	(5,912)	(17,196)	(3,351)
Amortization of (discount) premium	(2,317)	(1,579)	(4,544)	(3,461)
Total before tax	(2,894)	(7,491)	(21,740)	(6,812)
Tax	1,013	2,588	7,610	3,579
Total after tax	(1,881)	(4,903)	(14,130)	(3,233)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Pension adjustments [Member]
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]
Amortization of prior service cost	528	567	1,056	1,138
Amortization of actuarial gain (loss)	2,046	4,041	4,092	8,013
Total before tax	2,574	4,608	5,148	9,151
Tax	(901)	(1,611)	(1,802)	(3,202)
Total after tax	$ 1,673	$ 2,997	$ 3,346	$ 5,949