Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 12,635.7	$ 12,284.5
Gross Unrealized Gains	60.3	45.7
Gross Unrealized Losses	163.0	80.9
Fair Value	12,533.0	12,249.3
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	104.9	138.8
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	105.0	138.8
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	6.9	10.1
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	0.0	0.0
Fair Value	7.1	10.3
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	3.9	4.1
Gross Unrealized Gains	0.3	0.2
Gross Unrealized Losses	0.0	0.0
Fair Value	4.2	4.3
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	3,696.0	3,006.7
Gross Unrealized Gains	7.5	16.1
Gross Unrealized Losses	0.3	2.4
Fair Value	3,703.2	3,020.4
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	520.9	4,076.0
Gross Unrealized Gains	1.1	5.3
Gross Unrealized Losses	8.4	2.5
Fair Value	513.6	4,078.8
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,116.9	405.1
Gross Unrealized Gains	15.5	1.4
Gross Unrealized Losses	14.2	0.3
Fair Value	3,118.2	406.2
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	1,041.9	262.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	1,041.9	262.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,060.0	3,285.4
Gross Unrealized Gains	35.6	21.7
Gross Unrealized Losses	3.3	2.1
Fair Value	3,092.3	3,305.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	783.3	804.3
Gross Unrealized Gains	0.0	0.7
Gross Unrealized Losses	8.1	0.3
Fair Value	775.2	804.7
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	301.0	291.1
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	128.7	73.3
Fair Value	$ 172.3	$ 217.9