Income Taxes - Schedule of Significant Components of Temporary Differences with Deferred Tax Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred Tax Liabilities:
Net unrealized holding gains on securities available-for-sale	$ (3,783)	$ 0
Prepaid pension	(3,472)	(3,730)
Deferred loan income	(2,165)	(1,614)
Purchase accounting adjustments	(631)	(801)
Depreciation on premises and equipment	(1,590)	(1,061)
Other	(812)	(823)
Total Deferred Tax liabilities	(12,453)	(8,029)
Deferred Tax Assets:
Net unrealized holding losses on securities available-for-sale	0	361
Allowance for loan losses	17,567	18,890
Other employee benefits	2,453	2,369
Low income housing partnerships	4,049	3,147
Net adjustment to funded status of pension	11,089	6,495
Impairment of securities	1,313	1,313
Delinquent interest on nonaccrual loans	0	1,626
State net operating loss carryforwards	2,249	1,828
Other	4,668	3,950
Gross Deferred Tax Assets	43,388	39,979
Less: Valuation allowance	(2,249)	(2,199)
Total Deferred Tax Assets	41,139	37,780
Net Deferred Tax Asset	$ 28,686	$ 29,751