Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 8,716.0	$ 5,248.3
Gross Unrealized Gains	49.1	19.5
Gross Unrealized Losses	32.0	40.3
Fair Value	8,733.1	5,227.5
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	8.0	26.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	8.0	26.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	76.6	89.2
Gross Unrealized Gains	3.7	5.2
Gross Unrealized Losses	0.0	0.0
Fair Value	80.3	94.4
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	8.1	9.9
Gross Unrealized Gains	0.6	0.7
Gross Unrealized Losses	0.0	0.0
Fair Value	8.7	10.6
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	149.3
Gross Unrealized Gains	0.4
Gross Unrealized Losses	0.0
Fair Value	149.7
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	1,860.7	892.4
Gross Unrealized Gains	16.9	0.4
Gross Unrealized Losses	1.1	1.9
Fair Value	1,876.5	890.9
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	3,468.4	1,118.0
Gross Unrealized Gains	13.5	4.8
Gross Unrealized Losses	0.1	0.5
Fair Value	3,481.8	1,122.3
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	750.4	691.6
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0.1	0.1
Fair Value	750.4	691.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,282.9	2,326.2
Gross Unrealized Gains	13.8	8.3
Gross Unrealized Losses	0.2	0.9
Fair Value	2,296.5	2,333.6
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	111.6	95.0
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	30.5	36.9
Fair Value	$ 81.2	$ 58.1