Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 9,247.9	$ 8,921.1
Gross Unrealized Gains	25.4	29.2
Gross Unrealized Losses	47.7	45.2
Fair Value	9,225.6	8,905.1
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	18.0	15.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	18.0	15.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	47.5	63.6
Gross Unrealized Gains	2.3	2.7
Gross Unrealized Losses	0.0	0.0
Fair Value	49.8	66.3
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	6.7	7.4
Gross Unrealized Gains	0.4	0.5
Gross Unrealized Losses	0.0	0.0
Fair Value	7.1	7.9
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	267.6	231.2
Gross Unrealized Gains	0.5	0.2
Gross Unrealized Losses	0.2	0.4
Fair Value	267.9	231.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,327.5	2,051.6
Gross Unrealized Gains	9.5	10.1
Gross Unrealized Losses	3.1	3.7
Fair Value	2,333.9	2,058.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	3,523.3	3,517.5
Gross Unrealized Gains	2.4	4.9
Gross Unrealized Losses	3.9	2.3
Fair Value	3,521.8	3,520.1
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	148.2	606.0
Gross Unrealized Gains	0.6	0.2
Gross Unrealized Losses	0.6	0.1
Fair Value	148.2	606.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,583.9	2,154.7
Gross Unrealized Gains	7.6	10.5
Gross Unrealized Losses	3.6	2.8
Fair Value	2,587.9	2,162.4
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	186.6	143.4
Gross Unrealized Gains	2.1	0.1
Gross Unrealized Losses	0.0	0.0
Fair Value	188.7	143.5
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	138.6	130.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	36.3	35.9
Fair Value	$ 102.3	$ 94.8