Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 10,351.2	$ 14,354.0
Gross Unrealized Gains	56.2	18.7
Gross Unrealized Losses	82.0	105.7
Fair Value	10,325.4	14,267.0
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	117.7	101.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	117.7	101.6
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	16.2	18.9
Gross Unrealized Gains	0.5	0.6
Gross Unrealized Losses	0.0	0.0
Fair Value	16.7	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	376.0	472.9
Gross Unrealized Gains	1.3	0.4
Gross Unrealized Losses	0.2	1.8
Fair Value	377.1	471.5
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,591.8	2,877.6
Gross Unrealized Gains	22.9	9.6
Gross Unrealized Losses	2.0	9.3
Fair Value	2,612.7	2,877.9
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	2,981.9	6,488.2
Gross Unrealized Gains	7.0	2.1
Gross Unrealized Losses	3.9	8.7
Fair Value	2,985.0	6,481.6
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	37.8	45.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	37.8	45.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,302.6	2,966.8
Gross Unrealized Gains	23.9	5.8
Gross Unrealized Losses	3.1	12.3
Fair Value	3,323.4	2,960.3
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	652.7	1,146.4
Gross Unrealized Gains	0.4	0.0
Gross Unrealized Losses	0.6	4.0
Fair Value	652.5	1,142.4
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	270.2	232.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	72.2	69.6
Fair Value	$ 198.0	$ 162.4