Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive (loss) income	$ (90,486)	$ 104,710
Income tax (expense) benefit	19,002	(21,989)
Other comprehensive (loss) income, net of tax	(71,484)	82,721
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	188,239	190,936
Income tax (expense) benefit	(38,434)	(38,995)
Net income	149,805	151,941
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	4,309,234
Other comprehensive income before reclassifications	(68,861)
Less: Amounts reclassified from AOCI	2,623
Balance, end of period	4,241,828	3,816,183
Reclassification from Accumulated Other Comprehensive Income
Amounts Reclassified From Accumulated Other Comprehensive Income [Abstract]
Income before tax	3,320	(4,768)
Income tax (expense) benefit	(697)	1,002
Net income	2,623	(3,766)
Accumulated other comprehensive income (loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive (loss) income, net of tax	(71,484)	82,721
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	72,708	(124,214)
Balance, end of period	1,224	(41,493)
Net unrealized gains and (losses) on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive (loss) income	(91,880)	102,621
Income tax (expense) benefit	19,295	(21,550)
Other comprehensive (loss) income, net of tax	(72,585)	81,071
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period	138,521
Other comprehensive income before reclassifications	(68,861)
Less: Amounts reclassified from AOCI	3,724
Balance, end of period	65,936
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	4,714	(2,679)
Income tax (expense) benefit	(990)	563
Net income	3,724	(2,116)
Net benefit plan assets and (obligations) recognized in shareholders' equity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total other comprehensive (loss) income	1,394	2,089
Income tax (expense) benefit	(293)	(439)
Other comprehensive (loss) income, net of tax	1,101	1,650
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	(1,101)
Balance, end of period	(64,712)
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	(1,394)	(2,089)
Income tax (expense) benefit	293	439
Net income	$ (1,101)	$ (1,650)