Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	$ 182,517	$ 91,523	$ 92,031	$ 196,233
Income tax (expense) benefit	(38,329)	(19,220)	(19,327)	(41,209)
Other comprehensive (loss) income, net of tax	144,188	72,303	72,704	155,024
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	16,483	211,211	204,722	402,147
Income tax (expense) benefit	(2,436)	(43,433)	(40,870)	(82,428)
Net income	14,047	167,778	163,852	319,719
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			4,309,234
Other comprehensive income before reclassifications			69,272
Less: Amounts reclassified from AOCI			(3,432)
Balance, end of period	4,383,355	4,035,665	4,383,355	4,035,665
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	(7,665)	(260)	(4,345)	(5,028)
Income tax (expense) benefit	1,610	55	913	1,057
Net income	(6,055)	(205)	(3,432)	(3,971)
Accumulated other comprehensive income (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive (loss) income, net of tax	144,188	72,303	72,704	155,024
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	1,224	(41,493)	72,708	(124,214)
Balance, end of period	145,412	30,810	145,412	30,810
Net unrealized gains and (losses) on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	181,242	89,562	89,362	192,183
Income tax (expense) benefit	(38,061)	(18,808)	(18,766)	(40,358)
Other comprehensive (loss) income, net of tax	143,181	70,754	70,596	151,825
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			138,521
Other comprehensive income before reclassifications			69,272
Less: Amounts reclassified from AOCI			(1,324)
Balance, end of period	209,117		209,117
Net unrealized gains and (losses) on available-for-sale securities | Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	(6,390)	1,701	(1,676)	(978)
Income tax (expense) benefit	1,342	(357)	352	206
Net income	(5,048)	1,344	(1,324)	(772)
Net benefit plan assets and (obligations) recognized in shareholders' equity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive income	1,275	1,961	2,669	4,050
Income tax (expense) benefit	(268)	(412)	(561)	(851)
Other comprehensive (loss) income, net of tax	1,007	1,549	2,108	3,199
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(65,813)
Other comprehensive income before reclassifications			0
Less: Amounts reclassified from AOCI			(2,108)
Balance, end of period	(63,705)		(63,705)
Net benefit plan assets and (obligations) recognized in shareholders' equity | Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	(1,275)	(1,961)	(2,669)	(4,050)
Income tax (expense) benefit	268	412	561	851
Net income	$ (1,007)	$ (1,549)	$ (2,108)	$ (3,199)