Other Comprehensive Income (Loss) - Roll-Forward for AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	$ 4,642,740	$ 4,861,382
Balance, end of year	5,072,017	4,642,740	$ 4,861,382
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(481,511)	119,697	216,821
Other comprehensive income (loss) before reclassifications	132,769	(622,230)	(96,590)
Less: Amounts reclassified from AOCI	(32,461)	(21,022)	534
Balance, end of year	(316,281)	(481,511)	119,697
Net unrealized gains and losses on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(408,496)	150,038	272,137
Other comprehensive income (loss) before reclassifications	120,139	(566,324)	(113,839)
Less: Amounts reclassified from AOCI	(21,409)	(7,790)	8,260
Balance, end of year	(266,948)	(408,496)	150,038
Net benefit plan assets and obligations recognized in shareholders' equity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(73,015)	(30,341)	(55,316)
Other comprehensive income (loss) before reclassifications	12,630	(55,906)	17,249
Less: Amounts reclassified from AOCI	(11,052)	(13,232)	(7,726)
Balance, end of year	$ (49,333)	$ (73,015)	$ (30,341)