Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	$ 28,310	$ 41,736	$ 120,341	$ 237,969
Income tax (expense) benefit	(5,945)	(8,764)	(25,272)	(49,973)
Other comprehensive (loss) income, net of tax	22,365	32,972	95,069	187,996
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications before tax	164,329	223,120	369,051	625,267
Income tax (expense) benefit	(33,518)	(46,186)	(74,388)	(128,614)
Total reclassifications, net of tax	130,811	176,934	294,663	496,653
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			4,309,234
Other comprehensive income before reclassifications			98,757
Less: Amounts reclassified from AOCI			3,688
Balance, end of period	4,513,622	4,159,617	4,513,622	4,159,617
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Total reclassifications before tax	9,013	748	4,668	(4,280)
Income tax (expense) benefit	(1,893)	(158)	(980)	899
Total reclassifications, net of tax	7,120	590	3,688	(3,381)
Accumulated other comprehensive income (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive (loss) income, net of tax	22,365	32,972	95,069	187,996
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	145,412	30,810	72,708	(124,214)
Balance, end of period	167,777	63,782	167,777	63,782
Net unrealized gains and (losses) on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	42,674	39,712	132,036	231,895
Income tax (expense) benefit	(8,962)	(8,340)	(27,728)	(48,698)
Other comprehensive (loss) income, net of tax	33,712	31,372	104,308	183,197
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			138,521
Other comprehensive income before reclassifications			111,160
Less: Amounts reclassified from AOCI			6,852
Balance, end of period	242,829		242,829
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	10,349	2,772	8,673	1,794
Income tax (expense) benefit	(2,173)	(582)	(1,821)	(376)
Total reclassifications, net of tax	8,176	2,190	6,852	1,418
Net benefit plan assets and (obligations) recognized in shareholders' equity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	(14,364)	2,024	(11,695)	6,074
Income tax (expense) benefit	3,017	(424)	2,456	(1,275)
Other comprehensive (loss) income, net of tax	(11,347)	1,600	(9,239)	4,799
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(65,813)
Other comprehensive income before reclassifications			(12,403)
Less: Amounts reclassified from AOCI			(3,164)
Balance, end of period	(75,052)		(75,052)
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	(1,336)	(2,024)	(4,005)	(6,074)
Income tax (expense) benefit	280	424	841	1,275
Total reclassifications, net of tax	$ (1,056)	$ (1,600)	$ (3,164)	$ (4,799)