Other Comprehensive Income (Loss) - Tax effects of components in other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of Other Comprehensive Income (Loss), Before Tax
Net (losses) gains arising during the period	$ (5,992)	$ 5,079	$ (23,253)	$ 8,419
Accretion of unrealized gain (loss) on securities reclassified as held-to-maturity	1	(402)	(17)	(402)
Reclassification adjustment for gains included in net income	0	0	116	0
Total other comprehensive (loss) income, before tax	(5,991)	4,677	(23,154)	8,017
Unrealized gain (loss) on swap contract, before tax	1,554	143	2,232	95
Employee benefit plans, Before tax
Amortization of prior service cost included in net income	0	0	125	0
Reclassification adjustment of actuarial net gain included in net income	0	0	663	1,998
Change in funded status of retirement obligations	0	23,115	(789)	21,117
Total employee benefit plans, before tax	0	23,115	(1)	23,115
Total other comprehensive (loss) income	(4,437)	27,935	(20,923)	31,227
Components of Other Comprehensive Income (Loss), Tax Effect
Net (losses) gains arising during the period	634	(1,813)	5,067	(3,020)
Accretion of unrealized gain (loss) on securities reclassified as held-to-maturity	(12)	144	5	144
Reclassification adjustment for gains included in net income	3	0	(26)	0
Total other comprehensive (loss) income, tax	625	(1,669)	5,046	(2,876)
Unrealized gain (loss) on swap contract, Tax	(314)	(50)	(486)	(33)
Employee benefit plans, Tax Effect
Amortization of prior service cost included in net income	0	0	(27)	(1)
Reclassification adjustment of actuarial net gain included in net income	0	25	(144)	(619)
Change in funded status of retirement obligations	(1,908)	(8,283)	174	(7,716)
Total employee benefit plans, tax	(1,908)	(8,258)	3	(8,336)
Total other comprehensive (loss) income	(1,597)	(9,977)	4,563	(11,245)
Components of Other Comprehensive Income (Loss), After Tax
Net (losses) gains arising during the period	(5,358)	3,266	(18,186)	5,399
Accretion of unrealized gain on securities reclassified as held-to-maturity	(11)	(258)	(12)	(258)
Reclassification adjustment for gains included in net income	3	0	90	0
Total other comprehensive (loss) income, net of tax	(5,366)	3,008	(18,108)	5,141
Unrealized gain (loss) on swap contract, Net of tax	1,240	93	1,746	62
Employee benefit plans, After Tax
Amortization of prior service cost included in net income	0	0	98	(1)
Reclassification adjustment of actuarial net gain included in net income	0	25	519	1,379
Change in funded status of retirement obligations	(1,908)	14,832	(615)	13,401
Total employee benefit plans, net of tax	(1,908)	14,857	2	14,779
Total other comprehensive (loss) income	$ (6,034)	$ 17,958	$ (16,360)	$ 19,982