Other Comprehensive Income - Reclassification adjustments (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Securities available for sale and transferred securities: Before Tax Amount
Change in net unrealized gain/loss during the period	$ 1,192			$ (2,058)			$ 9,084	$ (6,487)
Reclassification adjustment for net gains included in net income	(11)			(76)			(11)	(76)
Reclassification adjustment for deferred tax on defined benefit postretirement medical plan	0						0
Total other comprehensive income (loss)	1,181			(2,134)			9,073	(6,563)
Securities available for sale and transferred securities: Tax (Expense) Benefit
Change in net unrealized gain/loss during the period	(249)			432			(1,908)	1,362
Reclassification adjustment for net gains included in net income	2			16			2	16
Reclassification adjustment for deferred tax on defined benefit postretirement medical plan	0						82
Total other comprehensive income (loss)	(247)			448			(1,824)	1,378
Securities available for sale and transferred securities: Net of Tax Amount
Change in net unrealized gain/loss during the period	943			(1,626)			7,176	(5,125)
Reclassification adjustment for net gains included in net income	(9)			(60)			(9)	(60)
Reclassification adjustment for deferred tax on defined benefit postretirement medical plan	0						82
Total other comprehensive income (loss)	$ 934	$ 2,598	$ 3,717	$ (1,686)	$ (689)	$ (2,810)	$ 7,249	$ (5,185)