Offsetting of Assets and Liabilities (Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	$ 2,043.8	$ 2,003.1
Gross amounts offset, total derivatives subject to a master netting arrangement	1,631.5	1,621.4
Net Amounts Presented in the Balance Sheet	412.3	381.7
Gross Amounts Not Offset in the Balance Sheet	3.7
Net Amount	408.6	381.7
Total Derivatives Not Subject to a Master Netting Arrangement
Gross Recognized Liabilities	304.1	825.9
Gross Amounts Not Offset in the Balance Sheet	0.0
Net Amount	304.1	825.9
Offsetting Derivative Liabilities [Abstract]
Gross Recognized Liabilities	2,347.9	2,829.0
Derivative Liabilities	716.4	1,207.6
Gross Amounts Not Offset in the Balance Sheet	3.7
Net Amount	712.7	1,207.6
Securities Sold under Agreements to Repurchase
Gross Recognized Liabilities	46.9	834.0
Net Amounts Presented in the Balance Sheet	46.9	834.0
Gross Amounts Not Offset in the Balance Sheet	46.9	834.0
Other liabilities	2,700.0	2,400.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	1,875.8	1,889.2
Gross amounts offset, total derivatives subject to a master netting arrangement	1,193.6	1,397.7
Net Amounts Presented in the Balance Sheet	682.2	491.5
Gross Amounts Not Offset in the Balance Sheet	3.7
Net Amount	678.5	491.5
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	118.9	69.2
Gross amounts offset, total derivatives subject to a master netting arrangement	11.4	14.2
Net Amounts Presented in the Balance Sheet	107.5	55.0
Net Amount	107.5	55.0
Interest Rate Swap Contracts | Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	12.0	14.3
Gross amounts offset, total derivatives subject to a master netting arrangement	10.1	10.1
Net Amounts Presented in the Balance Sheet	1.9	4.2
Net Amount	1.9	4.2
Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	37.1	30.4
Net Amounts Presented in the Balance Sheet	37.1	30.4
Net Amount	37.1	30.4
Cross Product Collateral Adjustment
Total Derivatives Subject to a Master Netting Arrangement
Gross amounts offset, total derivatives subject to a master netting arrangement	17.1	10.4
Cross Product Collateral Adjustment
Total Derivatives Subject to a Master Netting Arrangement
Gross amounts offset, total derivatives subject to a master netting arrangement	$ 399.3	$ 189.0