Accumulated Other Comprehensive (Loss) Income - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 564,704	$ 528,119
Other comprehensive income (loss)	4,235	(5,250)
Ending balance	575,107	533,061
Net Change in Unrealized Gains on Available-for- Sale Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(6,229)	(2,861)
Other comprehensive income (loss)	4,219	(5,296)
Ending balance	(2,010)	(8,157)
Net Change in Unfunded Pension Liability
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,284)	(1,553)
Other comprehensive income (loss)	16	46
Ending balance	(1,268)	(1,507)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(7,513)	(4,414)
Other comprehensive income (loss)	4,235	(5,250)
Ending balance	$ (3,278)	$ (9,664)