Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 22,798.6	$ 26,221.7
GROSS UNREALIZED GAINS	4.3	6.9
GROSS UNREALIZED LOSSES	1,860.8	1,755.6
FAIR VALUE	20,942.1	24,473.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,557.9	2,563.9
GROSS UNREALIZED GAINS	0.0	0.5
GROSS UNREALIZED LOSSES	98.7	72.4
FAIR VALUE	2,459.2	2,492.0
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,041.2	9,355.3
GROSS UNREALIZED GAINS	2.0	2.3
GROSS UNREALIZED LOSSES	1,123.0	1,012.4
FAIR VALUE	7,920.2	8,345.2
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,503.3	4,789.1
GROSS UNREALIZED GAINS	0.1	0.2
GROSS UNREALIZED LOSSES	87.3	90.7
FAIR VALUE	2,416.1	4,698.6
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	553.7	646.1
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	20.9	28.2
FAIR VALUE	532.8	617.9
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,023.5	2,208.6
GROSS UNREALIZED GAINS	0.5	0.3
GROSS UNREALIZED LOSSES	96.6	108.3
FAIR VALUE	1,927.4	2,100.6
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	425.0	585.1
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.2	0.7
FAIR VALUE	424.8	584.4
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	4,945.5	5,245.5
GROSS UNREALIZED GAINS	1.0	3.2
GROSS UNREALIZED LOSSES	263.8	294.9
FAIR VALUE	4,682.7	4,953.8
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	121.2	214.2
GROSS UNREALIZED GAINS	0.7	0.4
GROSS UNREALIZED LOSSES	0.2	0.2
FAIR VALUE	121.7	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	0.7	0.8
FAIR VALUE	36.9	36.8
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	589.7	576.3
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	169.4	147.0
FAIR VALUE	$ 420.3	$ 429.3