Other Comprehensive Income (Loss) - Schedule of Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Before-Tax Amount
Net unrealized gains/losses / Other comprehensive (loss) income	$ 25,815	$ (85,244)	$ 43,258	$ (248,031)
Tax (Expense) Benefit
Net unrealized gains/losses / Other comprehensive (loss) income	(6,324)	20,876	(10,598)	60,744
Net of Tax
Change in net unrealized gain (loss) during the period	(28,273)	(64,873)	(20,932)	(188,727)
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income (loss)	47,764	505	53,592	1,440
Other comprehensive income (loss)	19,491	(64,368)	32,660	(187,287)
Net unrealized gain/loss
Before-Tax Amount
Change in net unrealized gain (loss) during the period	(37,449)	(85,912)	(27,725)	(249,937)
Reclassification adjustment for net realized gain (loss) on sale of available-for-sale debt securities included in net income (loss)	62,932	49	69,984	49
Net unrealized gains/losses / Other comprehensive (loss) income	25,483	(85,863)	42,259	(249,888)
Tax (Expense) Benefit
Change in net unrealized gain (loss) during the period	9,176	21,039	6,793	61,210
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income (loss)	(15,419)	(12)	(17,146)	(12)
Net unrealized gains/losses / Other comprehensive (loss) income	(6,243)	21,027	(10,353)	61,198
Net of Tax
Change in net unrealized gain (loss) during the period	(28,273)	(64,873)	(20,932)	(188,727)
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income (loss)	47,513	37	52,838	37
Other comprehensive income (loss)	19,240	(64,836)	31,906	(188,690)
Amortization of net retirement plan actuarial gain/loss
Before-Tax Amount
Reclassification adjustment for net realized gain (loss) on sale of available-for-sale debt securities included in net income (loss)	279	565	837	1,695
Tax (Expense) Benefit
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income (loss)	(68)	(138)	(205)	(415)
Net of Tax
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income (loss)	211	427	632	1,280
Amortization of net retirement plan prior service cost
Before-Tax Amount
Reclassification adjustment for net realized gain (loss) on sale of available-for-sale debt securities included in net income (loss)	53	54	162	162
Tax (Expense) Benefit
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income (loss)	(13)	(13)	(40)	(39)
Net of Tax
Reclassification adjustment for net realized loss on sale of available-for-sale debt securities included in net income (loss)	40	41	122	123
Employee benefit plans
Before-Tax Amount
Net unrealized gains/losses / Other comprehensive (loss) income	332	619	999	1,857
Tax (Expense) Benefit
Net unrealized gains/losses / Other comprehensive (loss) income	(81)	(151)	(245)	(454)
Net of Tax
Other comprehensive income (loss)	$ 251	$ 468	$ 754	$ 1,403