Change in Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 1,229	$ (2,995)	$ 3,500	$ (21,294)
Other comprehensive income (loss) before reclassifications	6,163	(773)	4,676	18,111
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(47)	(23)	(623)	(608)
Amortization of unrealized gain on securities reclassified to held-to-maturity	(104)		(312)
Net current-period other comprehensive income (loss)	6,012	(796)	3,741	17,503
Ending Balance	7,241	(3,791)	7,241	(3,791)
Net Change in Defined Benefit Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	817	(412)	847	(472)
Other comprehensive income (loss) before reclassifications	(15)	5	(45)	65
Net current-period other comprehensive income (loss)	(15)	5	(45)	65
Ending Balance	802	(407)	802	(407)
Net Change in Securities Held-to-maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(208)
Amortization of unrealized gain on securities reclassified to held-to-maturity	(104)		(312)
Net current-period other comprehensive income (loss)	(104)		(312)
Ending Balance	(312)		(312)
Net Change in Securities Available-For-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	620	(2,583)	2,653	(20,822)
Other comprehensive income (loss) before reclassifications	6,178	(778)	4,721	18,046
Less: Amounts reclassified from accumulated other comprehensive income (loss)	(47)	(23)	(623)	(608)
Net current-period other comprehensive income (loss)	6,131	(801)	4,098	17,438
Ending Balance	$ 6,751	$ (3,384)	$ 6,751	$ (3,384)