Income Taxes - Significant Components of Temporary Differences with Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Liabilities:
Net unrealized holding gains on securities available-for-sale	$ (2,557)	$ (3,563)
Prepaid pension	(2,770)	(2,865)
Deferred loan income	(3,815)	(2,847)
Depreciation on premises and equipment	(1,239)	(1,226)
Other	(1,766)	(809)
Total Deferred Tax liabilities	(12,147)	(11,310)
Deferred Tax Assets:
Allowance for loan losses	19,446	17,740
Purchase accounting adjustments	365	1,298
Other employee benefits	3,983	2,556
Low income housing partnerships	4,845	4,531
Net adjustment to funded status of pension	10,018	12,425
Impairment of securities	1,318	1,354
State net operating loss carryforwards	3,114	2,670
Other	4,984	6,155
Gross Deferred Tax Assets	48,073	48,729
Less: Valuation allowance	(3,114)	(2,670)
Total Deferred Tax Assets	44,959	46,059
Net Deferred Tax Asset	$ 32,812	$ 34,749