Other Comprehensive Income - Reclassification adjustments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Securities available for sale and transferred securities:
Change in net unrealized gain/loss during the period	$ 9,458	$ 4,603
Total other comprehensive loss	9,458	4,603
Change in net unrealized gain/loss during the period	7,473	3,635
Reclassification adjustment for deferred tax on defined benefit postretirement medical plan		82
Total other comprehensive income (loss)	7,473	3,717
Securities available for sale and transferred securities: Tax (Expense) Benefit
Change in net unrealized gain/loss during the period	(1,985)	(968)
Reclassification adjustment for deferred tax on defined benefit postretirement medical plan		82
Total other comprehensive income	(1,985)	(886)
Securities available for sale and transferred securities: Before Tax Amount
Change in net unrealized gain/loss during the period	9,458	4,603
Total other comprehensive loss	$ 9,458	$ 4,603