Other Comprehensive Income (Loss) - Schedule of Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Before-Tax Amount
Net unrealized gains/losses / Other comprehensive income (loss)	$ (9,552)	$ (3,728)	$ (30,490)	$ 26,098
Tax (Expense) Benefit
Net unrealized gains/losses / Other comprehensive income (loss)	2,340	912	7,470	(6,398)
Net of Tax
Change in net unrealized gain/loss during the period	(7,852)	(3,303)	(24,609)	18,563
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income	640	487	1,589	1,137
Other comprehensive (loss) income	(7,212)	(2,816)	(23,020)	19,700
Net unrealized gain/loss
Before-Tax Amount
Change in net unrealized gain/loss during the period	(10,400)	(4,374)	(32,595)	24,592
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income	54	0	(275)	(429)
Net unrealized gains/losses / Other comprehensive income (loss)	(10,346)	(4,374)	(32,870)	24,163
Tax (Expense) Benefit
Change in net unrealized gain/loss during the period	2,548	1,071	7,986	(6,029)
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income	(13)	0	67	105
Net unrealized gains/losses / Other comprehensive income (loss)	2,535	1,071	8,053	(5,924)
Net of Tax
Change in net unrealized gain/loss during the period	(7,852)	(3,303)	(24,609)	18,563
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income	41	0	(208)	(324)
Other comprehensive (loss) income	(7,811)	(3,303)	(24,817)	18,239
Amortization of net retirement plan actuarial gain/loss
Before-Tax Amount
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income	738	593	2,213	1,775
Tax (Expense) Benefit
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income	(181)	(146)	(542)	(435)
Net of Tax
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income	557	447	1,671	1,340
Amortization of net retirement plan prior service cost
Before-Tax Amount
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income	56	53	167	160
Tax (Expense) Benefit
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income	(14)	(13)	(41)	(39)
Net of Tax
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income	42	40	126	121
Employee benefit plans
Before-Tax Amount
Net unrealized gains/losses / Other comprehensive income (loss)	794	646	2,380	1,935
Tax (Expense) Benefit
Net unrealized gains/losses / Other comprehensive income (loss)	(195)	(159)	(583)	(474)
Net of Tax
Other comprehensive (loss) income	$ 599	$ 487	$ 1,797	$ 1,461