Offsetting of Assets and Liabilities (Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	$ 2,717.2	$ 2,003.1
Gross amounts offset, total derivatives subject to a master netting arrangement	2,463.3	1,621.4
Net Amounts Presented in the Balance Sheet	253.9	381.7
Gross Amounts Not Offset in the Balance Sheet	3.2
Net Amount	250.7	381.7
Total Derivatives Not Subject to a Master Netting Arrangement
Gross Recognized Liabilities	496.0	825.9
Gross Amounts Not Offset in the Balance Sheet	0.2
Net Amount	495.8	825.9
Offsetting Derivative Liabilities [Abstract]
Gross Recognized Liabilities	3,213.2	2,829.0
Derivative Liabilities	749.9	1,207.6
Gross Amounts Not Offset in the Balance Sheet	3.4
Net Amount	746.5	1,207.6
Securities Sold under Agreements to Repurchase
Gross Recognized Liabilities	981.3	834.0
Net Amounts Presented in the Balance Sheet	981.3	834.0
Gross Amounts Not Offset in the Balance Sheet	981.3	834.0
Other liabilities	2,400.0	2,400.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	2,567.1	1,889.2
Gross amounts offset, total derivatives subject to a master netting arrangement	1,436.9	1,397.7
Net Amounts Presented in the Balance Sheet	1,130.2	491.5
Gross Amounts Not Offset in the Balance Sheet	3.2
Net Amount	1,127.0	491.5
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	104.0	69.2
Gross amounts offset, total derivatives subject to a master netting arrangement	14.3	14.2
Net Amounts Presented in the Balance Sheet	89.7	55.0
Net Amount	89.7	55.0
Interest Rate Swap Contracts | Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	9.4	14.3
Gross amounts offset, total derivatives subject to a master netting arrangement	8.2	10.1
Net Amounts Presented in the Balance Sheet	1.2	4.2
Net Amount	1.2	4.2
Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	36.7	30.4
Net Amounts Presented in the Balance Sheet	36.7	30.4
Net Amount	36.7	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	18.3	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	$ 985.6	$ 189.0