Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 5,846.5	$ 4,170.8
Gross Unrealized Gains	21.7	26.4
Gross Unrealized Losses	35.6	21.1
Fair Value	5,832.6	4,176.1
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	97.2	121.9
Gross Unrealized Gains	5.8	7.4
Gross Unrealized Losses	0.0	0.0
Fair Value	103.0	129.3
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	10.6	18.4
Gross Unrealized Gains	0.7	1.1
Gross Unrealized Losses	0.0	0.0
Fair Value	11.3	19.5
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	408.9
Gross Unrealized Gains	0.3
Gross Unrealized Losses	0.6
Fair Value	408.6
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	1,187.1	1,281.6
Gross Unrealized Gains	6.6	6.6
Gross Unrealized Losses	0.4	0.4
Fair Value	1,193.3	1,287.8
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	1,732.5	924.3
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Losses	0.2	0.1
Fair Value	1,732.5	924.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,314.6
Gross Unrealized Gains	8.1
Gross Unrealized Losses	0.3
Fair Value	2,322.4
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	95.6	78.8
Gross Unrealized Gains	0.0	0.3
Gross Unrealized Losses	34.1	20.1
Fair Value	$ 61.5	59.0
Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total		1,745.8
Gross Unrealized Gains		10.9
Gross Unrealized Losses		0.5
Fair Value		$ 1,756.2