Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Summary of Investment Holdings [Line Items]
AMORTIZED COST	$ 28,513.1	$ 23,659.0
GROSS UNREALIZED GAINS	44.3	13.2
GROSS UNREALIZED LOSSES	245.5	582.4
FAIR VALUE	28,311.9	23,089.8
U.S. Government
Summary of Investment Holdings [Line Items]
AMORTIZED COST	6,497.4	3,681.5
GROSS UNREALIZED GAINS	3.4	2.2
GROSS UNREALIZED LOSSES	19.1	61.5
FAIR VALUE	6,481.7	3,622.2
Obligations of States and Political Subdivisions
Summary of Investment Holdings [Line Items]
AMORTIZED COST	321.6	315.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	15.6	20.0
FAIR VALUE	306.0	295.8
Government Sponsored Agency
Summary of Investment Holdings [Line Items]
AMORTIZED COST	13,749.0	11,744.3
GROSS UNREALIZED GAINS	19.0	9.0
GROSS UNREALIZED LOSSES	114.3	200.3
FAIR VALUE	13,653.7	11,553.0
Non-U.S. Government
Summary of Investment Holdings [Line Items]
AMORTIZED COST	341.5	284.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	14.4	20.4
FAIR VALUE	327.1	264.4
Corporate Debt
Summary of Investment Holdings [Line Items]
AMORTIZED COST	205.0	287.5
GROSS UNREALIZED GAINS	0.1	0.1
GROSS UNREALIZED LOSSES	4.0	8.1
FAIR VALUE	201.1	279.5
Covered Bonds
Summary of Investment Holdings [Line Items]
AMORTIZED COST	242.2	356.8
GROSS UNREALIZED GAINS	0.6	0.0
GROSS UNREALIZED LOSSES	3.1	9.7
FAIR VALUE	239.7	347.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Summary of Investment Holdings [Line Items]
AMORTIZED COST	4,019.7	3,013.8
GROSS UNREALIZED GAINS	14.0	0.1
GROSS UNREALIZED LOSSES	45.6	114.0
FAIR VALUE	3,988.1	2,899.9
Other Asset-Backed
Summary of Investment Holdings [Line Items]
AMORTIZED COST	2,494.5	3,061.0
GROSS UNREALIZED GAINS	7.1	1.6
GROSS UNREALIZED LOSSES	14.2	100.0
FAIR VALUE	2,487.4	2,962.6
Commercial Mortgage-Backed
Summary of Investment Holdings [Line Items]
AMORTIZED COST	642.2	913.5
GROSS UNREALIZED GAINS	0.1	0.2
GROSS UNREALIZED LOSSES	15.2	48.4
FAIR VALUE	$ 627.1	$ 865.3