Change in Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 7,601	$ (12,507)	$ 3,500	$ (21,294)
Other comprehensive income (loss) before reclassifications	(6,039)	9,738	(1,487)	18,884
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(296)	(226)	(576)	(585)
Amortization of unrealized gain on securities reclassified to held-to-maturity	(37)		(208)
Total other comprehensive income (loss)	(6,372)	9,512	(2,271)	18,299
Ending Balance	1,229	(2,995)	1,229	(2,995)
Amortization of Defined Benefit Pension Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	832	(472)	847	(472)
Other comprehensive income (loss) before reclassifications	(15)	60	(30)	60
Total other comprehensive income (loss)	(15)	60	(30)	60
Ending Balance	817	(412)	817	(412)
Net Change in Securities Held-to-maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(171)
Amortization of unrealized gain on securities reclassified to held-to-maturity	(37)		(208)
Total other comprehensive income (loss)	(37)		(208)
Ending Balance	(208)		(208)
Net Unrealized Gains (Losses) on Investment Securities Available-for-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	6,940	(12,035)	2,653	(20,822)
Other comprehensive income (loss) before reclassifications	(6,024)	9,678	(1,457)	18,824
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(296)	(226)	(576)	(585)
Total other comprehensive income (loss)	(6,320)	9,452	(2,033)	18,239
Ending Balance	$ 620	$ (2,583)	$ 620	$ (2,583)