Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Summary of Investment Holdings [Line Items]
AMORTIZED COST	$ 25,188.4	$ 27,760.0
GROSS UNREALIZED GAINS	7.3	7.7
GROSS UNREALIZED LOSSES	931.3	1,067.8
FAIR VALUE	24,264.4	26,699.9
U.S. Government
Summary of Investment Holdings [Line Items]
AMORTIZED COST	2,942.3	2,837.7
GROSS UNREALIZED GAINS	0.6	2.5
GROSS UNREALIZED LOSSES	99.0	92.8
FAIR VALUE	2,843.9	2,747.4
Obligations of States and Political Subdivisions
Summary of Investment Holdings [Line Items]
AMORTIZED COST	310.2	817.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	21.0	30.2
FAIR VALUE	289.2	787.6
Government Sponsored Agency
Summary of Investment Holdings [Line Items]
AMORTIZED COST	11,656.4	11,892.5
GROSS UNREALIZED GAINS	3.9	4.3
GROSS UNREALIZED LOSSES	311.3	351.6
FAIR VALUE	11,349.0	11,545.2
Non-U.S. Government
Summary of Investment Holdings [Line Items]
AMORTIZED COST	277.8	387.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	26.3	27.6
FAIR VALUE	251.5	360.0
Corporate Debt
Summary of Investment Holdings [Line Items]
AMORTIZED COST	479.7	1,774.3
GROSS UNREALIZED GAINS	0.3	0.2
GROSS UNREALIZED LOSSES	19.1	26.9
FAIR VALUE	460.9	1,747.6
Covered Bonds
Summary of Investment Holdings [Line Items]
AMORTIZED COST	353.6	403.1
GROSS UNREALIZED GAINS	0.2	0.3
GROSS UNREALIZED LOSSES	12.8	14.7
FAIR VALUE	341.0	388.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Summary of Investment Holdings [Line Items]
AMORTIZED COST	3,068.0	2,645.8
GROSS UNREALIZED GAINS	1.4	0.3
GROSS UNREALIZED LOSSES	151.4	166.7
FAIR VALUE	2,918.0	2,479.4
Other Asset-Backed
Summary of Investment Holdings [Line Items]
AMORTIZED COST	5,135.0	5,544.3
GROSS UNREALIZED GAINS	0.8	0.0
GROSS UNREALIZED LOSSES	226.2	288.1
FAIR VALUE	4,909.6	5,256.2
Commercial Mortgage-Backed
Summary of Investment Holdings [Line Items]
AMORTIZED COST	965.4	1,456.9
GROSS UNREALIZED GAINS	0.1	0.1
GROSS UNREALIZED LOSSES	64.2	69.2
FAIR VALUE	$ 901.3	$ 1,387.8