Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 17,136.4	$ 17,791.1
Gross Unrealized Gains	66.6	84.3
Gross Unrealized Losses	103.0	78.0
Fair Value	17,100.0	17,797.4
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	100.0	90.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	100.0	90.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	1.0	2.1
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	0.0	0.0
Fair Value	1.0	2.2
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	53.6	3.0
Gross Unrealized Gains	0.3	0.3
Gross Unrealized Losses	0.9	0.0
Fair Value	53.0	3.3
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	6,539.3	8,336.6
Gross Unrealized Gains	6.1	7.3
Gross Unrealized Losses	0.2	0.2
Fair Value	6,545.2	8,343.7
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	916.7	588.0
Gross Unrealized Gains	5.0	6.5
Gross Unrealized Losses	1.6	0.1
Fair Value	920.1	594.4
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,046.8	3,184.6
Gross Unrealized Gains	19.9	24.6
Gross Unrealized Losses	1.7	0.3
Fair Value	3,065.0	3,208.9
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	644.1	807.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	644.1	807.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	4,672.6	3,648.0
Gross Unrealized Gains	33.1	43.5
Gross Unrealized Losses	23.3	0.9
Fair Value	4,682.4	3,690.6
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	706.2	677.0
Gross Unrealized Gains	1.2	0.9
Gross Unrealized Losses	0.3	0.0
Fair Value	707.1	677.9
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	456.1	454.6
Gross Unrealized Gains	1.0	1.1
Gross Unrealized Losses	75.0	76.5
Fair Value	$ 382.1	$ 379.2