Other Comprehensive Income (Loss) - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Before-Tax Amount
Net unrealized gains/losses / Other comprehensive income (loss)	$ 296	$ 14,272	$ 29,824	$ 30,449
Tax (Expense) Benefit
Net unrealized gains/losses / Other comprehensive income (loss)	(73)	(3,496)	(7,308)	(7,458)
Net of Tax
Other comprehensive income	223	10,776	22,516	22,991
Net unrealized gain/loss
Before-Tax Amount
Change in net unrealized gain/loss during the period	(339)	14,140	28,963	29,892
Reclassification adjustment for net realized gain on sale of available-for-sale debt securities included in net income	0	(271)	(429)	(271)
Net unrealized gains/losses / Other comprehensive income (loss)	(339)	13,869	28,534	29,621
Tax (Expense) Benefit
Change in net unrealized gain/loss during the period	82	(3,464)	(7,097)	(7,322)
Reclassification adjustment for net realized gain on sale of available-for-sale debt securities included in net income	0	67	105	67
Net unrealized gains/losses / Other comprehensive income (loss)	82	(3,397)	(6,992)	(7,255)
Net of Tax
Change in net unrealized gain/loss during the period	(257)	10,676	21,866	22,570
Reclassification adjustment for net realized gain on sale of available-for-sale debt securities included in net income	0	(204)	(324)	(204)
Other comprehensive income	(257)	10,472	21,542	22,366
Amortization of net retirement plan actuarial gain/loss
Before-Tax Amount
Reclassification adjustment for net realized gain on sale of available-for-sale debt securities included in net income	581	400	1,183	821
Tax (Expense) Benefit
Reclassification adjustment for net realized gain on sale of available-for-sale debt securities included in net income	(142)	(98)	(290)	(201)
Net of Tax
Reclassification adjustment for net realized gain on sale of available-for-sale debt securities included in net income	439	302	893	620
Amortization of net retirement plan prior service cost
Before-Tax Amount
Reclassification adjustment for net realized gain on sale of available-for-sale debt securities included in net income	54	3	107	7
Tax (Expense) Benefit
Reclassification adjustment for net realized gain on sale of available-for-sale debt securities included in net income	(13)	(1)	(26)	(2)
Net of Tax
Reclassification adjustment for net realized gain on sale of available-for-sale debt securities included in net income	41	2	81	5
Employee benefit plans
Before-Tax Amount
Net unrealized gains/losses / Other comprehensive income (loss)	635	403	1,290	828
Tax (Expense) Benefit
Net unrealized gains/losses / Other comprehensive income (loss)	(155)	(99)	(316)	(203)
Net of Tax
Other comprehensive income	$ 480	$ 304	$ 974	$ 625