Offsetting of Assets and Liabilities (Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	$ 1,416.4	$ 1,947.0
Gross Amounts Offset in the Balance Sheet	1,248.6	1,796.3
Net Amounts Presented in the Balance Sheet	167.8	150.7
Gross Amounts Not Offset in the Balance Sheet	0.1
Net Amount	167.7	150.7
Total Derivatives Not Subject to a Master Netting Arrangement
Gross Recognized Liabilities	399.1	441.4
Gross Amounts Not Offset in the Balance Sheet	0.0
Net Amount	399.1	441.4
Offsetting Derivative Liabilities [Abstract]
Gross Recognized Liabilities	1,815.5	2,388.4
Derivative Liabilities	566.9	592.1
Gross Amounts Not Offset in the Balance Sheet	0.1
Net Amount	566.8	592.1
Securities Sold under Agreements to Repurchase
Gross Recognized Liabilities	125.2	168.3
Net Amounts Presented in the Balance Sheet	125.2	168.3
Gross Amounts Not Offset in the Balance Sheet	125.2	168.3
Other liabilities	2,700.0	2,500.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	1,283.5	1,821.0
Gross Amounts Offset in the Balance Sheet	1,191.1	1,751.7
Net Amounts Presented in the Balance Sheet	92.4	69.3
Gross Amounts Not Offset in the Balance Sheet	0.1
Net Amount	92.3	69.3
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	97.6	68.8
Gross Amounts Offset in the Balance Sheet	56.7	19.0
Net Amounts Presented in the Balance Sheet	40.9	49.8
Net Amount	40.9	49.8
Interest Rate Swap Contracts | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	0.0	24.4
Gross Amounts Offset in the Balance Sheet	0.0	24.4
Net Amounts Presented in the Balance Sheet	0.0	0.0
Net Amount	0.0	0.0
Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	35.3	32.8
Gross Amounts Offset in the Balance Sheet	0.8	1.2
Net Amounts Presented in the Balance Sheet	34.5	31.6
Net Amount	$ 34.5	$ 31.6