Change in Accumulated Other Comprehensive Income (Loss) - Components of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 3,500	$ (21,294)
Other comprehensive income before reclassifications	4,552	9,145
Less: Amounts reclassified from accumulated other comprehensive income	(280)	(358)
Amortization of unrealized gain on securities reclassified to held-to-maturity	(171)
Total other comprehensive income	4,101	8,787
Ending Balance	7,601	(12,507)
Net Unrealized (Losses) Gains on Investment Securities Available-for-Sale [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	2,653	(20,822)
Other comprehensive income before reclassifications	4,567	9,145
Less: Amounts reclassified from accumulated other comprehensive income	(280)	(358)
Amortization of unrealized gain on securities reclassified to held-to-maturity	(171)
Total other comprehensive income	4,116	8,787
Ending Balance	6,769	(12,035)
Net Unrealized Losses on Defined Benefit Pension Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	847		(472)
Other comprehensive income before reclassifications	(15)
Total other comprehensive income	(15)
Ending Balance	$ 832	$ (472)	$ (472)