Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 16,754.0	$ 12,284.5
Gross Unrealized Gains	82.9	45.7
Gross Unrealized Losses	90.4	80.9
Fair Value	16,746.5	12,249.3
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	99.0	138.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	99.0	138.8
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	2.6	10.1
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	0.0	0.0
Fair Value	2.7	10.3
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	3.4	4.1
Gross Unrealized Gains	0.3	0.2
Gross Unrealized Losses	0.0	0.0
Fair Value	3.7	4.3
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	530.3	405.1
Gross Unrealized Gains	5.5	1.4
Gross Unrealized Losses	0.0	0.3
Fair Value	535.8	406.2
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,361.7	3,006.7
Gross Unrealized Gains	25.4	16.1
Gross Unrealized Losses	1.0	2.4
Fair Value	3,386.1	3,020.4
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	726.3	262.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	726.3	262.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,390.5	3,285.4
Gross Unrealized Gains	42.8	21.7
Gross Unrealized Losses	0.3	2.1
Fair Value	3,433.0	3,305.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	645.8	804.3
Gross Unrealized Gains	0.7	0.7
Gross Unrealized Losses	0.2	0.3
Fair Value	646.3	804.7
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	442.6	291.1
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	88.7	73.3
Fair Value	353.9	217.9
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	7,551.8	4,076.0
Gross Unrealized Gains	8.1	5.3
Gross Unrealized Losses	0.2	2.5
Fair Value	$ 7,559.7	$ 4,078.8