Change in Accumulated Other Comprehensive Income - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 696	$ 3,500
Other comprehensive income before reclassifications	10,572	4,567
Less: Amounts reclassified from accumulated other comprehensive income	186	(466)
Total other comprehensive (loss) income	10,758	4,101
Ending Balance	11,454	7,601
Net Change in Investment Securities Available-For-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(1,887)	446
Other comprehensive income before reclassifications	10,572	4,567
Less: Amounts reclassified from accumulated other comprehensive income	(189)	(280)
Total other comprehensive (loss) income	10,383	4,287
Ending Balance	8,496	4,733
Net Change in Securities Held-to-maturity [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	1,795	2,207
Less: Amounts reclassified from accumulated other comprehensive income	(103)	(171)
Total other comprehensive (loss) income	(103)	(171)
Ending Balance	1,692	2,036
Net Change in Defined Benefit Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	788	847
Less: Amounts reclassified from accumulated other comprehensive income	478	(15)
Total other comprehensive (loss) income	478	(15)
Ending Balance	$ 1,266	$ 832