Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 10,811.0	$ 8,921.1
Gross Unrealized Gains	24.2	29.2
Gross Unrealized Losses	56.5	45.2
Fair Value	10,778.7	8,905.1
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	23.9	15.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	23.9	15.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	44.8	63.6
Gross Unrealized Gains	2.0	2.7
Gross Unrealized Losses	0.0	0.0
Fair Value	46.8	66.3
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	6.3	7.4
Gross Unrealized Gains	0.4	0.5
Gross Unrealized Losses	0.0	0.0
Fair Value	6.7	7.9
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	327.1	231.2
Gross Unrealized Gains	0.8	0.2
Gross Unrealized Losses	0.0	0.4
Fair Value	327.9	231.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,777.4	2,051.6
Gross Unrealized Gains	13.1	10.1
Gross Unrealized Losses	3.5	3.7
Fair Value	2,787.0	2,058.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	3,609.2	3,517.5
Gross Unrealized Gains	1.5	4.9
Gross Unrealized Losses	8.1	2.3
Fair Value	3,602.6	3,520.1
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	153.4	606.0
Gross Unrealized Gains	0.0	0.2
Gross Unrealized Losses	0.1	0.1
Fair Value	153.3	606.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,650.4	2,154.7
Gross Unrealized Gains	5.7	10.5
Gross Unrealized Losses	6.8	2.8
Fair Value	2,649.3	2,162.4
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	1,030.4	143.4
Gross Unrealized Gains	0.7	0.1
Gross Unrealized Losses	0.7	0.0
Fair Value	1,030.4	143.5
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	188.1	130.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	37.3	35.9
Fair Value	$ 150.8	$ 94.8