Accumulated Other Comprehensive (Loss) Income - Components of Accumulated 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
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 528,802
Other comprehensive income (loss)	$ (2,211)	$ 164	(8,988)	$ 1,009
Ending balance	552,103		552,103
Net Change in Unrealized Gains on Available-for- Sale Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(9,669)	(433)	(2,861)	(1,231)
Other comprehensive income (loss)	(2,319)	149	(9,127)	947
Ending balance	(11,988)	(284)	(11,988)	(284)
Net Change in Unfunded Pension Liability
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,522)	(1,131)	(1,553)	(1,178)
Other comprehensive income (loss)	108	15	139	62
Ending balance	(1,414)	(1,116)	(1,414)	(1,116)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(11,191)	(1,564)	(4,414)	(2,409)
Other comprehensive income (loss)			(8,988)
Ending balance	$ (13,402)	$ (1,400)	$ (13,402)	$ (1,400)