Offsetting of Assets and Liabilities (Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	$ 2,011.7	$ 1,999.7
Gross Amounts Offset in the Balance Sheet	1,114.9	1,357.1
Net Amounts Presented in the Balance Sheet	896.8	642.6
Gross Amounts Not Offset in the Balance Sheet	23.6	12.7
Net Amount	873.2	629.9
Total Derivatives Not Subject to a Master Netting Arrangement
Gross Recognized Assets	454.9	563.8
Gross Amounts Not Offset in the Balance Sheet	0.0	2.7
Net Amount	454.9	561.1
Total Derivatives
Gross Recognized Assets	2,466.6	2,563.5
Derivative Asset	1,351.7	1,206.4
Gross Amounts Not Offset in the Balance Sheet	23.6	15.4
Net Amount	1,328.1	1,191.0
Securities Purchased under Agreements to Resell
Gross Recognized Assets	525.0	1,031.2
Net Amounts Presented in the Balance Sheet	525.0	1,031.2
Gross Amounts Not Offset in the Balance Sheet	525.0	1,031.2
Other assets excluding derivatives and non financial instruments	6,600.0	4,600.0
Federal funds sold	10.0	134.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	1,834.2	1,902.3
Gross Amounts Offset in the Balance Sheet	1,112.7	1,308.8
Net Amounts Presented in the Balance Sheet	721.5	593.5
Gross Amounts Not Offset in the Balance Sheet	23.6	12.7
Net Amount	697.9	580.8
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	177.1	71.6
Gross Amounts Offset in the Balance Sheet	2.2	22.6
Net Amounts Presented in the Balance Sheet	174.9	49.0
Net Amount	174.9	49.0
Interest Rate Swap Contracts | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	0.4	24.5
Gross Amounts Offset in the Balance Sheet	0.0	24.4
Net Amounts Presented in the Balance Sheet	0.4	0.1
Net Amount	$ 0.4	0.1
Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets		1.3
Gross Amounts Offset in the Balance Sheet		1.3
Net Amounts Presented in the Balance Sheet		0.0
Net Amount		$ 0.0