Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Investment Holdings [Line Items]
AMORTIZED COST	$ 25,464.4	$ 27,760.0
GROSS UNREALIZED GAINS	12.8	7.7
GROSS UNREALIZED LOSSES	849.7	1,067.8
FAIR VALUE	24,627.5	26,699.9
U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	2,931.1	2,837.7
GROSS UNREALIZED GAINS	4.8	2.5
GROSS UNREALIZED LOSSES	76.0	92.8
FAIR VALUE	2,859.9	2,747.4
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
AMORTIZED COST	318.1	817.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	26.2	30.2
FAIR VALUE	291.9	787.6
Government Sponsored Agency
Investment Holdings [Line Items]
AMORTIZED COST	11,686.8	11,892.5
GROSS UNREALIZED GAINS	5.1	4.3
GROSS UNREALIZED LOSSES	258.5	351.6
FAIR VALUE	11,433.4	11,545.2
Non-U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	382.0	387.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	20.6	27.6
FAIR VALUE	361.4	360.0
Corporate Debt
Investment Holdings [Line Items]
AMORTIZED COST	652.6	1,774.3
GROSS UNREALIZED GAINS	0.3	0.2
GROSS UNREALIZED LOSSES	18.4	26.9
FAIR VALUE	634.5	1,747.6
Covered Bonds
Investment Holdings [Line Items]
AMORTIZED COST	356.1	403.1
GROSS UNREALIZED GAINS	0.2	0.3
GROSS UNREALIZED LOSSES	11.9	14.7
FAIR VALUE	344.4	388.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
AMORTIZED COST	2,918.9	2,645.8
GROSS UNREALIZED GAINS	1.9	0.3
GROSS UNREALIZED LOSSES	133.1	166.7
FAIR VALUE	2,787.7	2,479.4
Other Asset-Backed
Investment Holdings [Line Items]
AMORTIZED COST	5,247.6	5,544.3
GROSS UNREALIZED GAINS	0.5	0.0
GROSS UNREALIZED LOSSES	237.7	288.1
FAIR VALUE	5,010.4	5,256.2
Commercial Mortgage-Backed
Investment Holdings [Line Items]
AMORTIZED COST	971.2	1,456.9
GROSS UNREALIZED GAINS	0.0	0.1
GROSS UNREALIZED LOSSES	67.3	69.2
FAIR VALUE	$ 903.9	$ 1,387.8