Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 6,370.9	$ 5,248.3
Gross Unrealized Gains	37.9	19.5
Gross Unrealized Losses	32.6	40.3
Fair Value	6,376.2	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	77.1	89.2
Gross Unrealized Gains	4.7	5.2
Gross Unrealized Losses	0.0	0.0
Fair Value	81.8	94.4
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	8.5	9.9
Gross Unrealized Gains	0.6	0.7
Gross Unrealized Losses	0.0	0.0
Fair Value	9.1	10.6
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	150.6
Gross Unrealized Gains	0.4
Gross Unrealized Losses	0.0
Fair Value	151.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	1,771.4	892.4
Gross Unrealized Gains	11.0	0.4
Gross Unrealized Losses	2.3	1.9
Fair Value	1,780.1	890.9
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	974.5	1,118.0
Gross Unrealized Gains	8.2	4.8
Gross Unrealized Losses	0.0	0.5
Fair Value	982.7	1,122.3
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	1,122.6	691.6
Gross Unrealized Gains	0.3	0.1
Gross Unrealized Losses	0.1	0.1
Fair Value	1,122.8	691.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,157.9	2,326.2
Gross Unrealized Gains	12.0	8.3
Gross Unrealized Losses	0.7	0.9
Fair Value	2,169.2	2,333.6
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	100.3	95.0
Gross Unrealized Gains	0.7	0.0
Gross Unrealized Losses	29.5	36.9
Fair Value	$ 71.5	$ 58.1