Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Investment Holdings [Line Items]
AMORTIZED COST	$ 23,659.0	$ 27,760.0
GROSS UNREALIZED GAINS	13.2	7.7
GROSS UNREALIZED LOSSES	582.4	1,067.8
FAIR VALUE	23,089.8	26,699.9
U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	3,681.5	2,837.7
GROSS UNREALIZED GAINS	2.2	2.5
GROSS UNREALIZED LOSSES	61.5	92.8
FAIR VALUE	3,622.2	2,747.4
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
AMORTIZED COST	315.8	817.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	20.0	30.2
FAIR VALUE	295.8	787.6
Government Sponsored Agency
Investment Holdings [Line Items]
AMORTIZED COST	11,744.3	11,892.5
GROSS UNREALIZED GAINS	9.0	4.3
GROSS UNREALIZED LOSSES	200.3	351.6
FAIR VALUE	11,553.0	11,545.2
Non-U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	284.8	387.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	20.4	27.6
FAIR VALUE	264.4	360.0
Corporate Debt
Investment Holdings [Line Items]
AMORTIZED COST	287.5	1,774.3
GROSS UNREALIZED GAINS	0.1	0.2
GROSS UNREALIZED LOSSES	8.1	26.9
FAIR VALUE	279.5	1,747.6
Covered Bonds
Investment Holdings [Line Items]
AMORTIZED COST	356.8	403.1
GROSS UNREALIZED GAINS	0.0	0.3
GROSS UNREALIZED LOSSES	9.7	14.7
FAIR VALUE	347.1	388.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
AMORTIZED COST	3,013.8	2,645.8
GROSS UNREALIZED GAINS	0.1	0.3
GROSS UNREALIZED LOSSES	114.0	166.7
FAIR VALUE	2,899.9	2,479.4
Other Asset-Backed
Investment Holdings [Line Items]
AMORTIZED COST	3,061.0	5,544.3
GROSS UNREALIZED GAINS	1.6	0.0
GROSS UNREALIZED LOSSES	100.0	288.1
FAIR VALUE	2,962.6	5,256.2
Commercial Mortgage-Backed
Investment Holdings [Line Items]
AMORTIZED COST	913.5	1,456.9
GROSS UNREALIZED GAINS	0.2	0.1
GROSS UNREALIZED LOSSES	48.4	69.2
FAIR VALUE	$ 865.3	$ 1,387.8