Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Components of Other Comprehensive Income (Loss) [Abstract]
Net unrealized investment (losses) gains on securities without an allowance for credit losses	$ (154,555)	$ 169,135	$ 220,139
Income tax benefit (expense)	32,456	(35,519)	(46,229)
Net of taxes	(122,099)	133,616	173,910
Net changes in benefit plan assets and obligations	31,613	13,288	29,129
Income tax benefit (expense)	(6,638)	(2,791)	(6,117)
Net of taxes	24,975	10,497	23,012
Total other comprehensive income (loss)	(122,942)	182,423	249,268
Total income tax benefit (expense)	25,818	(38,310)	(52,346)
Other comprehensive income (loss), net of tax	(97,124)	144,113	196,922
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications	801,777	559,263	847,977
Income tax benefit (expense)	(166,794)	(113,170)	(174,214)
Net income	634,983	446,093	673,763
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	4,698,986	4,309,234
Balance, end of year	4,861,382	4,698,986	4,309,234
Accumulated other comprehensive income (loss)
Components of Other Comprehensive Income (Loss) [Abstract]
Other comprehensive income (loss), net of tax	(97,124)	144,113	196,922
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	216,821	72,708	(124,214)
Other comprehensive income (loss) before reclassifications	(96,590)	142,448	193,400
Less: Amounts reclassified from AOCL	534	(1,665)	(3,522)
Balance, end of year	119,697	216,821	72,708
Net unrealized gains and losses on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	272,137	138,521	(35,389)
Other comprehensive income (loss) before reclassifications	(113,839)	144,566	176,784
Less: Amounts reclassified from AOCL	8,260	10,950	2,874
Balance, end of year	150,038	272,137	138,521
Net benefit plan assets and obligations recognized in shareholders' equity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(55,316)	(65,813)	(88,825)
Other comprehensive income (loss) before reclassifications	17,249	(2,118)	16,616
Less: Amounts reclassified from AOCL	(7,726)	(12,615)	(6,396)
Balance, end of year	(30,341)	(55,316)	(65,813)
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications	676	(2,106)	(4,460)
Income tax benefit (expense)	(142)	441	938
Net income	534	(1,665)	(3,522)
Reclassification from Accumulated Other Comprehensive Income | Net unrealized gains and losses on available-for-sale securities
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications	10,455	13,862	3,637
Income tax benefit (expense)	(2,195)	(2,912)	(763)
Net income	8,260	10,950	2,874
Reclassification from Accumulated Other Comprehensive Income | Net benefit plan assets and obligations recognized in shareholders' equity
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications	(9,779)	(15,968)	(8,097)
Income tax benefit (expense)	2,053	3,353	1,701
Net income	$ (7,726)	$ (12,615)	$ (6,396)