Other Comprehensive Income (Loss) - Tax effects of components in other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components of Other Comprehensive Income (Loss), Before Tax
Net (losses) gains arising during the period	$ (5,623)	$ 2,689	$ (17,261)	$ 3,340
Accretion of unrealized loss on securities reclassified as held-to-maturity	80	0	(16)	0
Reclassification adjustment for gains included in net income	0	0	116	0
Total other comprehensive (loss) income, before tax	(5,543)	2,689	(17,161)	3,340
Unrealized gain (loss) on swap contract, before tax	297	(57)	678	(48)
Employee benefit plans, Before tax
Amortization of prior service cost included in net income	0	0	148	0
Reclassification adjustment of actuarial net (loss) gain included in net income	0	0	673	0
Change in funded status of retirement obligations	0	0	(819)	0
Total employee benefit plans, before tax	0	0	2	0
Total other comprehensive (loss) income	(5,246)	2,632	(16,481)	3,292
Components of Other Comprehensive Income (Loss), Tax Effect
Net (losses) gains arising during the period	2,763	(950)	4,433	(1,206)
Accretion of unrealized loss on securities reclassified as held-to-maturity	(83)	0	14	0
Reclassification adjustment for gains included in net income	0	0	(29)	0
Total other comprehensive (loss) income, tax	2,680	(950)	4,418	(1,206)
Unrealized gain (loss) on swap contract, Tax	(133)	20	(172)	17
Employee benefit plans, Tax Effect
Amortization of prior service cost included in net income	0	0	(30)	0
Reclassification adjustment of actuarial net (loss) gain included in net income	0	(2)	(146)	(5)
Change in funded status of retirement obligations	2,323	72	2,085	(74)
Total employee benefit plans, tax	2,323	70	1,909	(79)
Total other comprehensive (loss) income	4,870	(860)	6,155	(1,268)
Components of Other Comprehensive Income (Loss), After Tax
Net (losses) gains arising during the period	(2,860)	1,739	(12,828)	2,134
Accretion of unrealized gain on securities reclassified as held-to-maturity	(3)	0	(2)	0
Reclassification adjustment for gains included in net income	0	0	87	0
Total other comprehensive (loss) income, net of tax	(2,863)	1,739	(12,743)	2,134
Unrealized gain (loss) on swap contract, Net of tax	164	(37)	506	(31)
Employee benefit plans, After Tax
Amortization of prior service cost included in net income	0	0	118	0
Reclassification adjustment of actuarial net (loss) gain included in net income	0	(2)	527	(5)
Change in funded status of retirement obligations	2,323	72	1,266	(74)
Total employee benefit plans, net of tax	2,323	70	1,911	(79)
Total other comprehensive (loss) income	$ (376)	$ 1,772	$ (10,326)	$ 2,024