Accumulated Other Comprehensive Income (Loss) - Schedule of Pretax and After-tax Components of Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, net amount			$ (481.0)	$ (25.0)
Amounts reclassified from AOCI			6.0	(9.0)
Gross Unrealized	$ (212.0)	$ 15.0	(627.0)	(44.0)
Income Taxes	52.0	(3.0)	152.0	10.0
Other comprehensive (loss) income, net of tax	(160.0)	12.0	(475.0)	(34.0)
Defined benefit pension items
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, gross amount	0.0	0.0	0.0	0.0
AOCI activity before reclassification, tax	0.0	0.0	0.0	0.0
AOCI activity before reclassification, net amount	0.0	0.0	0.0	0.0
Amounts reclassified from AOCI			5.0	10.0
Gross Unrealized	4.0	6.0	7.0	13.0
Income Taxes	(1.0)	(1.0)	(2.0)	(3.0)
Other comprehensive (loss) income, net of tax	3.0	5.0	5.0	10.0
Defined benefit pension items | Other noninterest expense
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification out of AOCI, gross amount	4.0	6.0	7.0	13.0
Reclassification out of AOCI, tax	(1.0)	(1.0)	(2.0)	(3.0)
Amounts reclassified from AOCI	3.0	5.0	5.0	10.0
Unrealized loss on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, gross amount	(216.0)	25.0
AOCI activity before reclassification, tax	53.0	(6.0)
AOCI activity before reclassification, net amount	(163.0)	19.0
Gross Unrealized	(216.0)	9.0
Income Taxes	53.0	(2.0)
Other comprehensive (loss) income, net of tax	(163.0)	7.0
Unrealized loss on securities available for sale | Realized gain on sales of investment securities available for sale, net
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification out of AOCI, gross amount	0.0	(16.0)
Reclassification out of AOCI, tax	0.0	4.0
Amounts reclassified from AOCI	$ 0.0	$ (12.0)
Unrealized loss on securities available for sale transferred to securities held to maturity
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, gross amount			0.0	0.0
AOCI activity before reclassification, tax			0.0	0.0
AOCI activity before reclassification, net amount			0.0	0.0
Amounts reclassified from AOCI			1.0	0.0
Gross Unrealized			1.0	0.0
Income Taxes			0.0	0.0
Other comprehensive (loss) income, net of tax			1.0	0.0
Unrealized loss on securities available for sale transferred to securities held to maturity | Interest on investment securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification out of AOCI, gross amount			1.0	0.0
Reclassification out of AOCI, tax			0.0	0.0
Amounts reclassified from AOCI			1.0	0.0
Unrealized (loss) gain on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, gross amount			(635.0)	(32.0)
AOCI activity before reclassification, tax			154.0	7.0
AOCI activity before reclassification, net amount			(481.0)	(25.0)
Amounts reclassified from AOCI			0.0	(19.0)
Gross Unrealized			(635.0)	(57.0)
Income Taxes			154.0	13.0
Other comprehensive (loss) income, net of tax			(481.0)	(44.0)
Unrealized (loss) gain on securities available for sale | Realized gain on sales of investment securities available for sale, net
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification out of AOCI, gross amount			0.0	(25.0)
Reclassification out of AOCI, tax			0.0	6.0
Amounts reclassified from AOCI			$ 0.0	$ (19.0)