Offsetting of Assets and Liabilities (Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	$ 2,512.9	$ 2,203.3
Gross Amounts Offset in the Balance Sheet	1,775.8	1,860.0
Net Amounts Presented in the Balance Sheet	737.1	343.3
Gross Amounts Not Offset in the Balance Sheet	143.8	2.1
Net Amount	593.3	341.2
Total Derivatives Not Subject to a Master Netting Arrangement
Gross Recognized Assets	896.5	450.8
Gross Amounts Not Offset in the Balance Sheet	18.9	2.5
Net Amount	877.6	448.3
Total Derivatives
Gross Recognized Assets	3,409.4	2,654.1
Derivative Asset	1,633.6	794.1
Gross Amounts Not Offset in the Balance Sheet	162.7	4.6
Net Amount	1,470.9	789.5
Securities Purchased under Agreements to Resell
Gross Recognized Assets	1,417.8	1,303.3
Net Amounts Presented in the Balance Sheet	1,417.8	1,303.3
Gross Amounts Not Offset in the Balance Sheet	1,417.8	1,303.3
Other assets excluding derivatives and non financial instruments	4,700.0	3,900.0
Federal funds sold	10.0	21.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	2,412.2	2,106.3
Gross Amounts Offset in the Balance Sheet	1,436.9	1,397.7
Net Amounts Presented in the Balance Sheet	975.3	708.6
Gross Amounts Not Offset in the Balance Sheet	143.8	2.1
Net Amount	831.5	706.5
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	92.4	86.9
Gross Amounts Offset in the Balance Sheet	14.3	14.2
Net Amounts Presented in the Balance Sheet	78.1	72.7
Net Amount	78.1	72.7
Interest Rate Swap Contracts | Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	8.2	10.1
Gross Amounts Offset in the Balance Sheet	8.2	10.1
Other Financial Derivatives
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	0.1
Gross Amounts Offset in the Balance Sheet	0.0
Net Amounts Presented in the Balance Sheet	0.1
Net Amount	0.1
Cross Product Collateral Adjustment
Total Derivatives Subject to a Master Netting Arrangement
Gross Amounts Offset in the Balance Sheet	18.3	10.4
Cross Product Collateral Adjustment
Total Derivatives Subject to a Master Netting Arrangement
Gross Amounts Offset in the Balance Sheet	$ 298.1	$ 427.6