Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other comprehensive (loss) income, net of tax:
Unrealized net holding (losses) arising during the period , pre-tax amounts	$ (6,915)	$ (8,618)	$ (3,159)
Reclassification adjustment for net (gains) losses related to securities available for sale included in net income, pre-tax amounts	(1,869)	644	(3,087)
Reclassification adjustment for an impairment write-down of equity security, pre-tax amounts	1,312	0	0
Unrealized gain on derivatives (cash flow hedges), pre-tax amounts	609	2,901	0
Amortization of unrealized net gains and losses related to the reclassification of available for sale investment securities to held to maturity, pre-tax amounts	875	1,094	1,311
Amortization of prior service cost and actuarial gains, pre-tax amounts	1,852	2,370	2,239
Decrease (increase) in unrecognized actuarial loss, pre-tax amounts	$ 2,401	$ 3,154	$ (6,144)