Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 12,143.0	$ 14,354.0
Gross Unrealized Gains	35.2	18.7
Gross Unrealized Losses	80.5	105.7
Fair Value	12,097.7	14,267.0
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	119.6	101.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	119.6	101.6
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	16.0	18.9
Gross Unrealized Gains	0.6	0.6
Gross Unrealized Losses	0.0	0.0
Fair Value	16.6	19.5
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	4.3	4.5
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	0.0	0.0
Fair Value	4.5	4.7
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	451.6	472.9
Gross Unrealized Gains	0.4	0.4
Gross Unrealized Losses	0.3	1.8
Fair Value	451.7	471.5
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,617.8	2,877.6
Gross Unrealized Gains	15.5	9.6
Gross Unrealized Losses	4.8	9.3
Fair Value	2,628.5	2,877.9
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	4,338.1	6,488.2
Gross Unrealized Gains	4.7	2.1
Gross Unrealized Losses	5.0	8.7
Fair Value	4,337.8	6,481.6
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	75.8	45.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	75.8	45.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,170.2	2,966.8
Gross Unrealized Gains	13.5	5.8
Gross Unrealized Losses	4.6	12.3
Fair Value	3,179.1	2,960.3
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	1,099.6	1,146.4
Gross Unrealized Gains	0.3	0.0
Gross Unrealized Losses	1.2	4.0
Fair Value	1,098.7	1,142.4
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	250.0	232.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	64.6	69.6
Fair Value	$ 185.4	$ 162.4