Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Summary of Investment Holdings [Line Items]
AMORTIZED COST	$ 27,127.3	$ 23,659.0
GROSS UNREALIZED GAINS	37.3	13.2
GROSS UNREALIZED LOSSES	302.9	582.4
FAIR VALUE	26,861.7	23,089.8
U.S. Government
Summary of Investment Holdings [Line Items]
AMORTIZED COST	6,102.1	3,681.5
GROSS UNREALIZED GAINS	6.3	2.2
GROSS UNREALIZED LOSSES	18.5	61.5
FAIR VALUE	6,089.9	3,622.2
Obligations of States and Political Subdivisions
Summary of Investment Holdings [Line Items]
AMORTIZED COST	308.7	315.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	15.3	20.0
FAIR VALUE	293.4	295.8
Government Sponsored Agency
Summary of Investment Holdings [Line Items]
AMORTIZED COST	12,743.9	11,744.3
GROSS UNREALIZED GAINS	21.6	9.0
GROSS UNREALIZED LOSSES	132.4	200.3
FAIR VALUE	12,633.1	11,553.0
Non-U.S. Government
Summary of Investment Holdings [Line Items]
AMORTIZED COST	329.3	284.8
GROSS UNREALIZED GAINS	0.1	0.0
GROSS UNREALIZED LOSSES	18.2	20.4
FAIR VALUE	311.2	264.4
Corporate Debt
Summary of Investment Holdings [Line Items]
AMORTIZED COST	225.6	287.5
GROSS UNREALIZED GAINS	0.1	0.1
GROSS UNREALIZED LOSSES	6.0	8.1
FAIR VALUE	219.7	279.5
Covered Bonds
Summary of Investment Holdings [Line Items]
AMORTIZED COST	304.7	356.8
GROSS UNREALIZED GAINS	0.6	0.0
GROSS UNREALIZED LOSSES	5.4	9.7
FAIR VALUE	299.9	347.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Summary of Investment Holdings [Line Items]
AMORTIZED COST	3,810.5	3,013.8
GROSS UNREALIZED GAINS	3.2	0.1
GROSS UNREALIZED LOSSES	60.3	114.0
FAIR VALUE	3,753.4	2,899.9
Other Asset-Backed
Summary of Investment Holdings [Line Items]
AMORTIZED COST	2,646.8	3,061.0
GROSS UNREALIZED GAINS	5.3	1.6
GROSS UNREALIZED LOSSES	26.3	100.0
FAIR VALUE	2,625.8	2,962.6
Commercial Mortgage-Backed
Summary of Investment Holdings [Line Items]
AMORTIZED COST	655.7	913.5
GROSS UNREALIZED GAINS	0.1	0.2
GROSS UNREALIZED LOSSES	20.5	48.4
FAIR VALUE	$ 635.3	$ 865.3