Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of Other Comprehensive Income (Loss) [Abstract]
Net unrealized investment gains (losses) on securities without an allowance for credit losses									$ 169,135	$ 220,139	$ (81,834)
Income tax (expense) benefit									(35,519)	(46,229)	17,188
Net of taxes									133,616	173,910	(64,646)
Net changes in benefit plan assets and obligations									13,288	29,129	(19,958)
Income tax (expense) benefit									(2,791)	(6,117)	4,191
Net of taxes									10,497	23,012	(15,767)
Total other comprehensive income (loss)									182,423	249,268	(101,792)
Total income tax expense, net									(38,310)	(52,346)	21,379
Other comprehensive income (loss), net of tax									144,113	196,922	(80,413)
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									559,263	847,977	844,150
Income tax (expense) benefit	$ (38,782)	$ (33,518)	$ (2,436)	$ (38,434)	$ (45,599)	$ (46,186)	$ (43,433)	$ (38,996)	(113,170)	(174,214)	(174,053)
Net income	151,430	$ 130,811	$ 14,047	149,805	177,110	$ 176,934	$ 167,778	151,941	446,093	673,763	670,097
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				4,309,234				3,581,891	4,309,234	3,581,891
Balance, end of year	4,698,986				4,309,234				4,698,986	4,309,234	3,581,891
Accumulated other comprehensive income (loss)
Components of Other Comprehensive Income (Loss) [Abstract]
Other comprehensive income (loss), net of tax									144,113	196,922	(80,413)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				72,708				(124,214)	72,708	(124,214)	(43,783)
Cumulative effect of adopting the accounting standard update for financial instruments											(18)
Other comprehensive income (loss) before reclassifications									142,448	193,400	(87,736)
Less: Amounts reclassified from AOCL									(1,665)	(3,522)	(7,323)
Balance, end of year	216,821				72,708				216,821	72,708	(124,214)
Net unrealized gains and losses on available-for-sale securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				138,521				(35,389)	138,521	(35,389)	29,275
Cumulative effect of adopting the accounting standard update for financial instruments											(18)
Other comprehensive income (loss) before reclassifications									144,566	176,784	(70,206)
Less: Amounts reclassified from AOCL									10,950	2,874	(5,560)
Balance, end of year	272,137				138,521				272,137	138,521	(35,389)
Net benefit plan assets and obligations recognized in shareholders' equity
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year				$ (65,813)				$ (88,825)	(65,813)	(88,825)	(73,058)
Cumulative effect of adopting the accounting standard update for financial instruments											0
Other comprehensive income (loss) before reclassifications									(2,118)	16,616	(17,530)
Less: Amounts reclassified from AOCL									(12,615)	(6,396)	(1,763)
Balance, end of year	$ (55,316)				$ (65,813)				(55,316)	(65,813)	(88,825)
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications									(2,106)	(4,460)	(9,269)
Income tax (expense) benefit									441	938	1,946
Net income									(1,665)	(3,522)	(7,323)
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									13,862	3,637	(7,037)
Income tax (expense) benefit									(2,912)	(763)	1,477
Net income									10,950	2,874	(5,560)
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									(15,968)	(8,097)	(2,232)
Income tax (expense) benefit									3,353	1,701	469
Net income									$ (12,615)	$ (6,396)	$ (1,763)