Other Comprehensive Income (Loss) - Summary of Each Component of Accumulated Other Comprehensive Income (Loss), Net Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	$ (25,901)	$ (29,203)	$ (41,665)	$ 6,682
Other comprehensive income before reclassification, net of tax	(196)	(2,121)	15,590	(35,428)
Amounts reclassified from accumulated other compressive income, net of tax	(213)	0	(235)	(2,578)
Net current period other comprehensive income, net of tax	(409)	(2,121)	15,355	(38,006)
Ending balance	(26,310)	(31,324)	(26,310)	(31,324)
Unrealized gains/losses on Available for Sale Securities [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	(27,600)	(30,803)	(43,364)	5,082
Other comprehensive income before reclassification, net of tax	(196)	(2,121)	15,590	(35,428)
Amounts reclassified from accumulated other compressive income, net of tax	(213)	0	(235)	(2,578)
Net current period other comprehensive income, net of tax	(409)	(2,121)	15,355	(38,006)
Ending balance	(28,009)	(32,924)	(28,009)	(32,924)
Postretirement Benefits [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Beginning balance	1,699	1,600	1,699	1,600
Amounts reclassified from accumulated other compressive income, net of tax	0	0	0	0
Ending balance	$ 1,699	$ 1,600	$ 1,699	$ 1,600