Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 21,223.5	$ 23,564.5
GROSS UNREALIZED GAINS	10.8	34.5
GROSS UNREALIZED LOSSES	835.2	283.6
FAIR VALUE	20,399.1	23,315.4
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	49.0	47.0
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	49.0	47.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		0.8
GROSS UNREALIZED GAINS		0.0
GROSS UNREALIZED LOSSES		0.0
FAIR VALUE		0.8
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,079.8	5,927.6
GROSS UNREALIZED GAINS	0.6	1.1
GROSS UNREALIZED LOSSES	400.9	106.1
FAIR VALUE	5,679.5	5,822.6
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,950.9	5,773.3
GROSS UNREALIZED GAINS	0.8	3.9
GROSS UNREALIZED LOSSES	50.6	9.8
FAIR VALUE	2,901.1	5,767.4
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	932.2	901.8
GROSS UNREALIZED GAINS	0.3	2.5
GROSS UNREALIZED LOSSES	19.0	6.5
FAIR VALUE	913.5	897.8
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,941.9	2,942.4
GROSS UNREALIZED GAINS	2.6	8.3
GROSS UNREALIZED LOSSES	64.4	9.6
FAIR VALUE	2,880.1	2,941.1
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	892.1	674.7
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	892.1	674.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,223.6	6,098.0
GROSS UNREALIZED GAINS	5.2	14.3
GROSS UNREALIZED LOSSES	208.2	80.0
FAIR VALUE	6,020.6	6,032.3
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	608.4	682.6
GROSS UNREALIZED GAINS	0.4	1.0
GROSS UNREALIZED LOSSES	1.0	0.0
FAIR VALUE	607.8	683.6
Other
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	545.6	516.3
GROSS UNREALIZED GAINS	0.9	3.4
GROSS UNREALIZED LOSSES	91.1	71.6
FAIR VALUE	$ 455.4	$ 448.1