Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 15,241.9	$ 12,284.5
Gross Unrealized Gains	80.2	45.7
Gross Unrealized Losses	98.7	80.9
Fair Value	15,223.4	12,249.3
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	98.0	138.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	98.0	138.8
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	7.0	10.1
Gross Unrealized Gains	0.1	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	3.6	4.1
Gross Unrealized Gains	0.3	0.2
Gross Unrealized Losses	0.0	0.0
Fair Value	3.9	4.3
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	509.6	405.1
Gross Unrealized Gains	4.9	1.4
Gross Unrealized Losses	1.0	0.3
Fair Value	513.5	406.2
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,198.0	3,006.7
Gross Unrealized Gains	21.6	16.1
Gross Unrealized Losses	2.0	2.4
Fair Value	3,217.6	3,020.4
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	1,176.3	262.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.1	0.0
Fair Value	1,176.2	262.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,999.3	3,285.4
Gross Unrealized Gains	44.3	21.7
Gross Unrealized Losses	0.5	2.1
Fair Value	3,043.1	3,305.0
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	696.2	804.3
Gross Unrealized Gains	0.2	0.7
Gross Unrealized Losses	2.6	0.3
Fair Value	693.8	804.7
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	419.3	291.1
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	92.3	73.3
Fair Value	327.0	217.9
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	6,134.6	4,076.0
Gross Unrealized Gains	8.8	5.3
Gross Unrealized Losses	0.2	2.5
Fair Value	$ 6,143.2	$ 4,078.8