Other Comprehensive Income (Loss) - Reclassification adjustments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Securities available for sale and transferred securities: Before Tax Amount
Unrealized holding gains (losses) on available-for-sale securities arising during the period	$ 14,431	$ 8,754	$ (3,356)
Reclassification adjustment for (gains) losses realized in income	(1,464)	(24)	(173)
Net gain on defined benefit postretirement medical plan realized during the period	195	(310)	560
Reclassification adjustment for net amortization and deferral on defined benefit postretirement medical plan (included in compensation and benefits)	13	14	18
Total other comprehensive income	13,175	8,434	(2,951)
Securities available for sale and transferred securities: Tax (Expense) Benefit
Change in net unrealized gain/(loss) during the period	3,030	1,839	(706)
Reclassification adjustment for net gains included in net income	(307)	(5)	(36)
Net gain on defined benefit postretirement medical plan realized during the period	41	(146)	200
Reclassification adjustment for net amortization and deferral on defined benefit postretirement medical plan (included in compensation and benefits)	2	3	3
Total other comprehensive income	2,766	1,691	(539)
Securities available for sale and transferred securities:
Change in net unrealized gain/(loss) during the period	11,401	6,915	(2,650)
Reclassification adjustment for net gains included in net income	(1,157)	(19)	(137)
Net gain on defined benefit postretirement medical plan realized during the period	154	(164)	360
Reclassification adjustment for net amortization and deferral on defined benefit postretirement medical plan (included in compensation and benefits)	11	11	15
Total other comprehensive income (loss)	$ 10,409	$ 6,743	$ (2,412)