Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 14,225.1	$ 13,049.0
Gross Unrealized Gains	21.3	22.0
Gross Unrealized Losses	88.6	60.1
Fair Value	14,157.8	13,010.9
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	115.7	35.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	115.7	35.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	32.6	34.6
Gross Unrealized Gains	1.1	1.4
Gross Unrealized Losses	0.0	0.1
Fair Value	33.7	35.9
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	5.1	5.8
Gross Unrealized Gains	0.3	0.4
Gross Unrealized Losses	0.0	0.0
Fair Value	5.4	6.2
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	413.3	431.5
Gross Unrealized Gains	0.7	1.0
Gross Unrealized Losses	0.6	0.4
Fair Value	413.4	432.1
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,228.7	2,821.5
Gross Unrealized Gains	12.5	11.9
Gross Unrealized Losses	9.6	3.7
Fair Value	3,231.6	2,829.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	5,781.5	5,536.2
Gross Unrealized Gains	1.5	1.3
Gross Unrealized Losses	10.3	6.0
Fair Value	5,772.7	5,531.5
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	52.0	43.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.1
Fair Value	52.0	43.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,267.7	2,788.9
Gross Unrealized Gains	4.9	5.4
Gross Unrealized Losses	14.1	4.1
Fair Value	3,258.5	2,790.2
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	1,137.3	1,175.8
Gross Unrealized Gains	0.3	0.6
Gross Unrealized Losses	0.2	0.5
Fair Value	1,137.4	1,175.9
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	191.2	175.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	53.8	45.2
Fair Value	$ 137.4	$ 130.7