Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 11,487.8	$ 14,354.0
Gross Unrealized Gains	64.0	18.7
Gross Unrealized Losses	80.4	105.7
Fair Value	11,471.4	14,267.0
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	113.8	101.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	113.8	101.6
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	16.4	18.9
Gross Unrealized Gains	0.3	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	4.1	4.5
Gross Unrealized Gains	0.3	0.2
Gross Unrealized Losses	0.0	0.0
Fair Value	4.4	4.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	3,859.3	6,488.2
Gross Unrealized Gains	7.4	2.1
Gross Unrealized Losses	1.1	8.7
Fair Value	3,865.6	6,481.6
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	401.8	472.9
Gross Unrealized Gains	1.9	0.4
Gross Unrealized Losses	0.2	1.8
Fair Value	403.5	471.5
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,701.8	2,877.6
Gross Unrealized Gains	23.2	9.6
Gross Unrealized Losses	1.1	9.3
Fair Value	2,723.9	2,877.9
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	131.8	45.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	131.8	45.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,309.2	2,966.8
Gross Unrealized Gains	30.1	5.8
Gross Unrealized Losses	0.9	12.3
Fair Value	3,338.4	2,960.3
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	669.8	1,146.4
Gross Unrealized Gains	0.8	0.0
Gross Unrealized Losses	1.1	4.0
Fair Value	669.5	1,142.4
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	279.8	232.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	76.0	69.6
Fair Value	$ 203.8	$ 162.4