Offsetting of Assets and Liabilities (Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	$ 1,285.0	$ 1,947.0
Gross Amounts Offset in the Balance Sheet	1,123.7	1,796.3
Net Amounts Presented in the Balance Sheet	161.3	150.7
Gross Amounts Not Offset in the Balance Sheet	0.1
Net Amount	161.2	150.7
Total Derivatives Not Subject to a Master Netting Arrangement
Gross Recognized Liabilities	321.0	441.4
Gross Amounts Not Offset in the Balance Sheet	0.0
Net Amount	321.0	441.4
Offsetting Derivative Liabilities [Abstract]
Gross Recognized Liabilities	1,606.0	2,388.4
Derivative Liabilities	482.3	592.1
Gross Amounts Not Offset in the Balance Sheet	0.1
Net Amount	482.2	592.1
Securities Sold under Agreements to Repurchase
Gross Recognized Liabilities	39.7	168.3
Net Amounts Presented in the Balance Sheet	39.7	168.3
Gross Amounts Not Offset in the Balance Sheet	39.7	168.3
Other liabilities	3,100.0	2,500.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	1,169.5	1,821.0
Gross Amounts Offset in the Balance Sheet	1,054.9	1,751.7
Net Amounts Presented in the Balance Sheet	114.6	69.3
Gross Amounts Not Offset in the Balance Sheet	0.1
Net Amount	114.5	69.3
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	38.8	68.8
Gross Amounts Offset in the Balance Sheet	27.8	19.0
Net Amounts Presented in the Balance Sheet	11.0	49.8
Net Amount	11.0	49.8
Interest Rate Swap Contracts | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	40.8	24.4
Gross Amounts Offset in the Balance Sheet	40.8	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	35.9	32.8
Gross Amounts Offset in the Balance Sheet	0.2	1.2
Net Amounts Presented in the Balance Sheet	35.7	31.6
Net Amount	$ 35.7	$ 31.6