Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Summary of Investment Holdings [Line Items]
AMORTIZED COST	$ 25,941.9	$ 23,659.0
GROSS UNREALIZED GAINS	29.7	13.2
GROSS UNREALIZED LOSSES	332.7	582.4
FAIR VALUE	25,638.9	23,089.8
U.S. Government
Summary of Investment Holdings [Line Items]
AMORTIZED COST	6,121.6	3,681.5
GROSS UNREALIZED GAINS	11.0	2.2
GROSS UNREALIZED LOSSES	20.3	61.5
FAIR VALUE	6,112.3	3,622.2
Obligations of States and Political Subdivisions
Summary of Investment Holdings [Line Items]
AMORTIZED COST	309.6	315.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	14.6	20.0
FAIR VALUE	295.0	295.8
Government Sponsored Agency
Summary of Investment Holdings [Line Items]
AMORTIZED COST	12,501.9	11,744.3
GROSS UNREALIZED GAINS	12.5	9.0
GROSS UNREALIZED LOSSES	163.0	200.3
FAIR VALUE	12,351.4	11,553.0
Non-U.S. Government
Summary of Investment Holdings [Line Items]
AMORTIZED COST	325.0	284.8
GROSS UNREALIZED GAINS	0.1	0.0
GROSS UNREALIZED LOSSES	17.5	20.4
FAIR VALUE	307.6	264.4
Corporate Debt
Summary of Investment Holdings [Line Items]
AMORTIZED COST	256.5	287.5
GROSS UNREALIZED GAINS	0.2	0.1
GROSS UNREALIZED LOSSES	6.5	8.1
FAIR VALUE	250.2	279.5
Covered Bonds
Summary of Investment Holdings [Line Items]
AMORTIZED COST	302.7	356.8
GROSS UNREALIZED GAINS	0.3	0.0
GROSS UNREALIZED LOSSES	6.2	9.7
FAIR VALUE	296.8	347.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Summary of Investment Holdings [Line Items]
AMORTIZED COST	2,912.6	3,013.8
GROSS UNREALIZED GAINS	3.8	0.1
GROSS UNREALIZED LOSSES	55.6	114.0
FAIR VALUE	2,860.8	2,899.9
Other Asset-Backed
Summary of Investment Holdings [Line Items]
AMORTIZED COST	2,552.4	3,061.0
GROSS UNREALIZED GAINS	1.6	1.6
GROSS UNREALIZED LOSSES	31.0	100.0
FAIR VALUE	2,523.0	2,962.6
Commercial Mortgage-Backed
Summary of Investment Holdings [Line Items]
AMORTIZED COST	659.6	913.5
GROSS UNREALIZED GAINS	0.2	0.2
GROSS UNREALIZED LOSSES	18.0	48.4
FAIR VALUE	$ 641.8	$ 865.3