Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 14,445.4	$ 13,049.0
Gross Unrealized Gains	12.4	22.0
Gross Unrealized Losses	102.7	60.1
Fair Value	14,355.1	13,010.9
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	83.7	35.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	83.7	35.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	33.0	34.6
Gross Unrealized Gains	0.7	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	5.0	5.8
Gross Unrealized Gains	0.2	0.4
Gross Unrealized Losses	0.0	0.0
Fair Value	5.2	6.2
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	517.6	431.5
Gross Unrealized Gains	0.7	1.0
Gross Unrealized Losses	0.7	0.4
Fair Value	517.6	432.1
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,110.3	2,821.5
Gross Unrealized Gains	7.7	11.9
Gross Unrealized Losses	11.5	3.7
Fair Value	3,106.5	2,829.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	5,961.5	5,536.2
Gross Unrealized Gains	0.8	1.3
Gross Unrealized Losses	12.5	6.0
Fair Value	5,949.8	5,531.5
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	60.1	43.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.1
Fair Value	60.1	43.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	3,262.3	2,788.9
Gross Unrealized Gains	2.1	5.4
Gross Unrealized Losses	19.7	4.1
Fair Value	3,244.7	2,790.2
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
Total	1,199.5	1,175.8
Gross Unrealized Gains	0.2	0.6
Gross Unrealized Losses	1.2	0.5
Fair Value	1,198.5	1,175.9
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	212.4	175.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	57.1	45.2
Fair Value	$ 155.3	$ 130.7