Consolidated Statements of Comprehensive Income - AOCI - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Other comprehensive (loss) income	$ 7,874	$ (12,521)	$ (12,534)
Accumulated Unrealized Loss on Securities Available- for-Sale
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(22,645)	(10,271)	(2,950)
Other comprehensive (loss) income	8,669	(12,374)	(7,321)
Ending balance	(13,976)	(22,645)	(10,271)
Accumulated Unrealized Loss on Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(1,176)	(2,468)	(1,138)
Other comprehensive (loss) income	(2,587)	1,292	(1,330)
Ending balance	(3,763)	(1,176)	(2,468)
Unrecognized Net Pension Costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(17,089)	(15,650)	(11,767)
Other comprehensive (loss) income	1,792	(1,439)	(3,883)
Ending balance	(15,297)	(17,089)	(15,650)
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(40,910)	(28,389)	(15,855)
Other comprehensive (loss) income	7,874	(12,521)	(12,534)
Ending balance	$ (33,036)	$ (40,910)	$ (28,389)