Other Comprehensive Income - Reclassification adjustments (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Securities available for sale and transferred securities: Before Tax Amount
Change in net unrealized gain/loss during the period	$ 720			$ 1,192			$ 13,338	$ 9,084
Reclassification adjustment for net gains (losses) included in net income	(1,466)			(11)			(1,464)	(11)
Total other comprehensive gain (loss)	(746)			1,181			11,874	9,073
Securities available for sale and transferred securities: Tax (Expense) Benefit
Change in net unrealized gain/loss during the period	(150)			(249)			(2,800)	(1,908)
Reclassification adjustment for net gains (losses) included in net income	308			2			307	2
Total other comprehensive income gain (loss)	158			(247)			(2,493)	(1,824)
Reclassification adjustment for deferred tax on defined benefit postretirement medical plan								82
Securities available for sale and transferred securities:
Change in net unrealized gain/loss during the period	570			943			10,538	7,176
Reclassification adjustment for net gains (losses) included in net income	(1,158)			(9)			(1,157)	(9)
Total other comprehensive income (loss)	$ (588)	$ 2,496	$ 7,473	$ 934	$ 2,598	$ 3,717	$ 9,381	7,249
Reclassification adjustment for deferred tax on defined benefit postretirement medical plan								$ 82