Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	$ 57,869	$ 182,517	$ (60,176)	$ 92,031
Income tax benefit (expense)	(12,152)	(38,329)	12,637	(19,327)
Other comprehensive income (loss), net of tax	45,717	144,188	(47,539)	72,704
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income tax benefit (expense)	(40,817)	(2,436)	(80,413)	(40,870)
Total reclassifications, net of tax	153,051	14,047	303,072	163,852
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			4,698,986
Other comprehensive income before reclassifications			(44,005)
Less: Amounts reclassified from AOCI			3,534
Balance, end of period	4,914,707	4,383,355	4,914,707	4,383,355
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications before tax	1,638	(7,665)	4,473	(4,345)
Income tax benefit (expense)	(344)	1,610	(939)	913
Total reclassifications, net of tax	1,294	(6,055)	3,534	(3,432)
Accumulated other comprehensive income (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive income (loss), net of tax	45,717	144,188	(47,539)	72,704
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	123,565	1,224	216,821	72,708
Balance, end of period	169,282	145,412	169,282	145,412
Net unrealized gains and (losses) on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	57,030	181,242	(62,120)	89,362
Income tax benefit (expense)	(11,976)	(38,061)	13,045	(18,766)
Other comprehensive income (loss), net of tax	45,054	143,181	(49,075)	70,596
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			272,137
Other comprehensive income before reclassifications			(44,005)
Less: Amounts reclassified from AOCI			5,070
Balance, end of period	223,062		223,062
Net unrealized gains and (losses) on available-for-sale securities | Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications before tax	2,477	(6,390)	6,417	(1,676)
Income tax benefit (expense)	(520)	1,342	(1,347)	352
Total reclassifications, net of tax	1,957	(5,048)	5,070	(1,324)
Net benefit plan assets and (obligations) recognized in shareholders' equity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income (loss)	839	1,275	1,944	2,669
Income tax benefit (expense)	(176)	(268)	(408)	(561)
Other comprehensive income (loss), net of tax	663	1,007	1,536	2,108
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(55,316)
Other comprehensive income before reclassifications			0
Less: Amounts reclassified from AOCI			(1,536)
Balance, end of period	(53,780)		(53,780)
Net benefit plan assets and (obligations) recognized in shareholders' equity | Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications before tax	(839)	(1,275)	(1,944)	(2,669)
Income tax benefit (expense)	176	268	408	561
Total reclassifications, net of tax	$ (663)	$ (1,007)	$ (1,536)	$ (2,108)