Accumulated Other Comprehensive (Loss) Income - Schedule of Pretax and After-tax Components of Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, net amount			$ (50.0)	$ (481.0)
Amounts reclassified from AOCI to earnings, Net Amount			13.0	6.0
Other comprehensive loss on securities available for sale, gross amount	$ (139.0)	$ (212.0)	(51.0)	(627.0)
Other comprehensive loss on securities available for sale, gross amount, tax	36.0	52.0	14.0	152.0
Other comprehensive loss, net of tax	(103.0)	(160.0)	(37.0)	(475.0)
Unrealized loss on securities available for sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, gross amount	(134.0)	(216.0)	(74.0)	(635.0)
AOCI activity before reclassification, tax	35.0	53.0	20.0	154.0
AOCI activity before reclassification, net amount	(99.0)	(163.0)	(54.0)	(481.0)
Other comprehensive loss on securities available for sale, gross amount	(135.0)	(216.0)	(57.0)	(635.0)
Other comprehensive loss on securities available for sale, gross amount, tax	35.0	53.0	15.0	154.0
Other comprehensive loss, net of tax	(100.0)	(163.0)	(42.0)	(481.0)
Unrealized loss on securities available for sale \| Provision for credit losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from AOCI to earnings, gross amount	(1.0)	0.0	17.0	0.0
Amounts reclassified from AOCI to earnings, tax	0.0	0.0	(5.0)	0.0
Amounts reclassified from AOCI to earnings, Net Amount	(1.0)	0.0	12.0	0.0
Unrealized loss on securities available for sale transferred to held to maturity
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 to earnings, Net Amount			1.0	1.0
Other comprehensive loss on securities available for sale, gross amount	1.0	0.0	1.0	1.0
Other comprehensive loss on securities available for sale, gross amount, tax	0.0	0.0	0.0	0.0
Other comprehensive loss, net of tax	1.0	0.0	1.0	1.0
Unrealized loss on securities available for sale transferred to held to maturity \| Interest on investment securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from AOCI to earnings, gross amount	1.0	0.0	1.0	1.0
Amounts reclassified from AOCI to earnings, tax	0.0	0.0	0.0	0.0
Amounts reclassified from AOCI to earnings, Net Amount	1.0	0.0	1.0	1.0
Defined benefit pension items
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI activity before reclassification, gross amount	(5.0)	0.0	5.0	0.0
AOCI activity before reclassification, tax	1.0	0.0	(1.0)	0.0
AOCI activity before reclassification, net amount	(4.0)	0.0	4.0	0.0
Amounts reclassified from AOCI to earnings, Net Amount			0.0	5.0
Other comprehensive loss on securities available for sale, gross amount	(5.0)	4.0	5.0	7.0
Other comprehensive loss on securities available for sale, gross amount, tax	1.0	(1.0)	(1.0)	(2.0)
Other comprehensive loss, net of tax	(4.0)	3.0	4.0	5.0
Defined benefit pension items \| Other noninterest expense
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified from AOCI to earnings, gross amount	0.0	4.0	0.0	7.0
Amounts reclassified from AOCI to earnings, tax	0.0	(1.0)	0.0	(2.0)
Amounts reclassified from AOCI to earnings, Net Amount	$ 0.0	$ 3.0	$ 0.0	$ 5.0

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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