Accumulated Other Comprehensive Income (Loss) - Changes in components of AOCI - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	$ (8,211)	$ (3,871)	$ (377)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(2,535)	(5,004)	(4,617)
Amounts reclassified from accumulated other comprehensive income (loss)	319	664	1,123
Other comprehensive income (loss), net of tax	(2,216)	(4,340)	(3,494)
Balance at the end of the period	(10,427)	(8,211)	(3,871)
Amortization of defined benefit pension
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	(6,195)	(6,015)	(5,315)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(365)	(749)	(1,332)
Amounts reclassified from accumulated other comprehensive income (loss)	562	569	632
Other comprehensive income (loss), net of tax	197	(180)	(700)
Balance at the end of the period	(5,998)	(6,195)	(6,015)
Unrealized gains and losses on securities
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	(1,708)	2,588	5,467
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(2,157)	(4,221)	(3,181)
Amounts reclassified from accumulated other comprehensive income (loss)	(413)	(75)	302
Other comprehensive income (loss), net of tax	(2,570)	(4,296)	(2,879)
Balance at the end of the period	(4,278)	(1,708)	2,588
Gains on sales of available for sale securities
Accumulated Other Comprehensive Income (Loss)
Balance at the beginning of the period	(308)	(444)	(529)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(13)	(34)	(104)
Amounts reclassified from accumulated other comprehensive income (loss)	170	170	189
Other comprehensive income (loss), net of tax	157	136	85
Balance at the end of the period	$ (151)	$ (308)	$ (444)