Accumulated Other Comprehensive (Loss) Income - Components of Accumulated 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
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 528,802
Other comprehensive income (loss)	$ (1,527)	$ 426	(6,777)	$ 845
Ending balance	543,180		543,180
Net Change in Unrealized Gains on Available-for- Sale Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(8,157)	(844)	(2,861)	(1,231)
Other comprehensive income (loss)	(1,512)	411	(6,808)	798
Ending balance	(9,669)	(433)	(9,669)	(433)
Net Change in Unfunded Pension Liability
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,507)	(1,146)	(1,553)	(1,178)
Other comprehensive income (loss)	(15)	15	31	47
Ending balance	(1,522)	(1,131)	(1,522)	(1,131)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(9,664)	(1,990)	(4,414)	(2,409)
Other comprehensive income (loss)			(6,777)
Ending balance	$ (11,191)	$ (1,564)	$ (11,191)	$ (1,564)