Offsetting of Assets and Liabilities (Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	$ 1,849.6	$ 1,947.0
Gross Amounts Offset in the Balance Sheet	1,507.8	1,796.3
Net Amounts Presented in the Balance Sheet	341.8	150.7
Gross Amounts Not Offset in the Balance Sheet	0.1
Net Amount	341.7	150.7
Total Derivatives Not Subject to a Master Netting Arrangement
Gross Recognized Liabilities	377.0	441.4
Gross Amounts Not Offset in the Balance Sheet	0.0
Net Amount	377.0	441.4
Offsetting Derivative Liabilities [Abstract]
Gross Recognized Liabilities	2,226.6	2,388.4
Derivative Liabilities	718.8	592.1
Gross Amounts Not Offset in the Balance Sheet	0.1
Net Amount	718.7	592.1
Securities Sold under Agreements to Repurchase
Gross Recognized Liabilities	265.7	168.3
Net Amounts Presented in the Balance Sheet	265.7	168.3
Gross Amounts Not Offset in the Balance Sheet	265.7	168.3
Other liabilities	3,300.0	2,500.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	1,705.5	1,821.0
Gross Amounts Offset in the Balance Sheet	1,451.6	1,751.7
Net Amounts Presented in the Balance Sheet	253.9	69.3
Gross Amounts Not Offset in the Balance Sheet	0.1
Net Amount	253.8	69.3
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	106.4	68.8
Gross Amounts Offset in the Balance Sheet	56.2	19.0
Net Amounts Presented in the Balance Sheet	50.2	49.8
Net Amount	50.2	49.8
Interest Rate Swap Contracts | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities		24.4
Gross Amounts Offset in the Balance Sheet		24.4
Net Amounts Presented in the Balance Sheet		0.0
Net Amount		0.0
Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Liabilities	37.7	32.8
Gross Amounts Offset in the Balance Sheet	0.0	1.2
Net Amounts Presented in the Balance Sheet	37.7	31.6
Net Amount	$ 37.7	$ 31.6