Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 14,334.6	$ 13,049.0
Gross Unrealized Gains	19.9	22.0
Gross Unrealized Losses	94.1	60.1
Fair Value	14,260.4	13,010.9
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	87.9	35.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	87.9	35.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	32.6	34.6
Gross Unrealized Gains	1.2	1.4
Gross Unrealized Losses	0.1	0.1
Fair Value	33.7	35.9
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	5.4	5.8
Gross Unrealized Gains	0.3	0.4
Gross Unrealized Losses	0.0	0.0
Fair Value	5.7	6.2
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	429.1	431.5
Gross Unrealized Gains	1.1	1.0
Gross Unrealized Losses	0.7	0.4
Fair Value	429.5	432.1
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,311.9	2,821.5
Gross Unrealized Gains	11.8	11.9
Gross Unrealized Losses	10.2	3.7
Fair Value	3,313.5	2,829.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	5,809.9	5,536.2
Gross Unrealized Gains	1.4	1.3
Gross Unrealized Losses	16.5	6.0
Fair Value	5,794.8	5,531.5
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	48.4	43.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.1
Fair Value	48.4	43.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,273.0	2,788.9
Gross Unrealized Gains	3.6	5.4
Gross Unrealized Losses	17.4	4.1
Fair Value	3,259.2	2,790.2
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	1,152.1	1,175.8
Gross Unrealized Gains	0.5	0.6
Gross Unrealized Losses	0.1	0.5
Fair Value	1,152.5	1,175.9
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	184.3	175.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	49.1	45.2
Fair Value	$ 135.2	$ 130.7