Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 8,938.4	$ 8,921.1
Gross Unrealized Gains	33.8	29.2
Gross Unrealized Losses	38.8	45.2
Fair Value	8,933.4	8,905.1
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	6.0	15.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	6.0	15.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	49.0	63.6
Gross Unrealized Gains	2.6	2.7
Gross Unrealized Losses	0.0	0.0
Fair Value	51.6	66.3
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	6.8	7.4
Gross Unrealized Gains	0.5	0.5
Gross Unrealized Losses	0.0	0.0
Fair Value	7.3	7.9
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	252.6	231.2
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	0.4	0.4
Fair Value	252.3	231.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,295.6	2,051.6
Gross Unrealized Gains	11.6	10.1
Gross Unrealized Losses	2.7	3.7
Fair Value	2,304.5	2,058.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	3,282.6	3,517.5
Gross Unrealized Gains	6.6	4.9
Gross Unrealized Losses	0.4	2.3
Fair Value	3,288.8	3,520.1
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	306.5	606.0
Gross Unrealized Gains	0.0	0.2
Gross Unrealized Losses	0.0	0.1
Fair Value	306.5	606.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,422.3	2,154.7
Gross Unrealized Gains	10.8	10.5
Gross Unrealized Losses	1.3	2.8
Fair Value	2,431.8	2,162.4
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	186.4	143.4
Gross Unrealized Gains	1.6	0.1
Gross Unrealized Losses	0.0	0.0
Fair Value	188.0	143.5
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	130.6	130.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	34.0	35.9
Fair Value	$ 96.6	$ 94.8