Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 22,708.6	$ 26,221.7
GROSS UNREALIZED GAINS	10.6	6.9
GROSS UNREALIZED LOSSES	1,347.2	1,755.6
FAIR VALUE	21,372.0	24,473.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,549.9	2,563.9
GROSS UNREALIZED GAINS	4.4	0.5
GROSS UNREALIZED LOSSES	14.8	72.4
FAIR VALUE	2,539.5	2,492.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	8,826.8	9,355.3
GROSS UNREALIZED GAINS	2.6	2.3
GROSS UNREALIZED LOSSES	823.8	1,012.4
FAIR VALUE	8,005.6	8,345.2
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,361.3	4,789.1
GROSS UNREALIZED GAINS	0.2	0.2
GROSS UNREALIZED LOSSES	65.2	90.7
FAIR VALUE	2,296.3	4,698.6
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	491.1	646.1
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	14.7	28.2
FAIR VALUE	476.4	617.9
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,124.8	2,208.6
GROSS UNREALIZED GAINS	0.3	0.3
GROSS UNREALIZED LOSSES	72.7	108.3
FAIR VALUE	2,052.4	2,100.6
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	542.6	585.1
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.3	0.7
FAIR VALUE	542.3	584.4
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	5,062.1	5,245.5
GROSS UNREALIZED GAINS	2.6	3.2
GROSS UNREALIZED LOSSES	197.6	294.9
FAIR VALUE	4,867.1	4,953.8
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	111.6	214.2
GROSS UNREALIZED GAINS	0.5	0.4
GROSS UNREALIZED LOSSES	0.2	0.2
FAIR VALUE	111.9	214.4
Commercial Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37.6	37.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	1.0	0.8
FAIR VALUE	36.6	36.8
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	600.8	576.3
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	156.9	147.0
FAIR VALUE	$ 443.9	$ 429.3