Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 17,791.1	$ 12,284.5
GROSS UNREALIZED GAINS	84.3	45.7
GROSS UNREALIZED LOSSES	78.0	80.9
FAIR VALUE	17,797.4	12,249.3
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	90.0	138.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	90.0	138.8
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	2.1	10.1
GROSS UNREALIZED GAINS	0.1	0.2
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	2.2	10.3
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	3.0	4.1
GROSS UNREALIZED GAINS	0.3	0.2
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	3.3	4.3
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	8,336.6	4,076.0
GROSS UNREALIZED GAINS	7.3	5.3
GROSS UNREALIZED LOSSES	0.2	2.5
FAIR VALUE	8,343.7	4,078.8
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	588.0	405.1
GROSS UNREALIZED GAINS	6.5	1.4
GROSS UNREALIZED LOSSES	0.1	0.3
FAIR VALUE	594.4	406.2
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	3,184.6	3,006.7
GROSS UNREALIZED GAINS	24.6	16.1
GROSS UNREALIZED LOSSES	0.3	2.4
FAIR VALUE	3,208.9	3,020.4
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	807.2	262.9
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	807.2	262.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	3,648.0	3,285.4
GROSS UNREALIZED GAINS	43.5	21.7
GROSS UNREALIZED LOSSES	0.9	2.1
FAIR VALUE	3,690.6	3,305.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	677.0	804.3
GROSS UNREALIZED GAINS	0.9	0.7
GROSS UNREALIZED LOSSES	0.0	0.3
FAIR VALUE	677.9	804.7
Other
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	454.6	291.1
GROSS UNREALIZED GAINS	1.1	0.1
GROSS UNREALIZED LOSSES	76.5	73.3
FAIR VALUE	$ 379.2	$ 217.9