Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Summary of Investment Holdings [Line Items]
AMORTIZED COST	$ 25,246.1	$ 27,760.0
GROSS UNREALIZED GAINS	9.1	7.7
GROSS UNREALIZED LOSSES	913.1	1,067.8
FAIR VALUE	24,342.1	26,699.9
U.S. Government
Summary of Investment Holdings [Line Items]
AMORTIZED COST	2,981.2	2,837.7
GROSS UNREALIZED GAINS	0.0	2.5
GROSS UNREALIZED LOSSES	101.8	92.8
FAIR VALUE	2,879.4	2,747.4
Obligations of States and Political Subdivisions
Summary of Investment Holdings [Line Items]
AMORTIZED COST	301.5	817.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	19.4	30.2
FAIR VALUE	282.1	787.6
Government Sponsored Agency
Summary of Investment Holdings [Line Items]
AMORTIZED COST	11,988.1	11,892.5
GROSS UNREALIZED GAINS	6.8	4.3
GROSS UNREALIZED LOSSES	338.1	351.6
FAIR VALUE	11,656.8	11,545.2
Non-U.S. Government
Summary of Investment Holdings [Line Items]
AMORTIZED COST	271.0	387.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	24.1	27.6
FAIR VALUE	246.9	360.0
Corporate Debt
Summary of Investment Holdings [Line Items]
AMORTIZED COST	397.9	1,774.3
GROSS UNREALIZED GAINS	0.1	0.2
GROSS UNREALIZED LOSSES	17.1	26.9
FAIR VALUE	380.9	1,747.6
Covered Bonds
Summary of Investment Holdings [Line Items]
AMORTIZED COST	292.2	403.1
GROSS UNREALIZED GAINS	0.1	0.3
GROSS UNREALIZED LOSSES	11.9	14.7
FAIR VALUE	280.4	388.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Summary of Investment Holdings [Line Items]
AMORTIZED COST	3,099.2	2,645.8
GROSS UNREALIZED GAINS	1.0	0.3
GROSS UNREALIZED LOSSES	140.4	166.7
FAIR VALUE	2,959.8	2,479.4
Other Asset-Backed
Summary of Investment Holdings [Line Items]
AMORTIZED COST	4,966.1	5,544.3
GROSS UNREALIZED GAINS	1.0	0.0
GROSS UNREALIZED LOSSES	195.6	288.1
FAIR VALUE	4,771.5	5,256.2
Commercial Mortgage-Backed
Summary of Investment Holdings [Line Items]
AMORTIZED COST	948.9	1,456.9
GROSS UNREALIZED GAINS	0.1	0.1
GROSS UNREALIZED LOSSES	64.7	69.2
FAIR VALUE	$ 884.3	$ 1,387.8