Offsetting of Assets and Liabilities (Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	$ 2,090.8	$ 2,203.3
Gross Amounts Offset in the Balance Sheet	1,622.8	1,860.0
Net Amounts Presented in the Balance Sheet	468.0	343.3
Gross Amounts Not Offset in the Balance Sheet	10.2	2.1
Net Amount	457.8	341.2
Total Derivatives Not Subject to a Master Netting Arrangement
Gross Recognized Assets	518.7	450.8
Gross Amounts Not Offset in the Balance Sheet	2.2	2.5
Net Amount	516.5	448.3
Total Derivatives
Gross Recognized Assets	2,609.5	2,654.1
Derivative Asset	986.7	794.1
Gross Amounts Not Offset in the Balance Sheet	12.4	4.6
Net Amount	974.3	789.5
Securities Purchased under Agreements to Resell
Gross Recognized Assets	1,458.5	1,303.3
Net Amounts Presented in the Balance Sheet	1,458.5	1,303.3
Gross Amounts Not Offset in the Balance Sheet	1,458.5	1,303.3
Other assets excluding derivatives and non financial instruments	4,800.0	3,900.0
Federal funds sold	43.0	21.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	1,984.4	2,106.3
Gross Amounts Offset in the Balance Sheet	1,193.6	1,397.7
Net Amounts Presented in the Balance Sheet	790.8	708.6
Gross Amounts Not Offset in the Balance Sheet	10.2	2.1
Net Amount	780.6	706.5
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	96.3	86.9
Gross Amounts Offset in the Balance Sheet	11.4	14.2
Net Amounts Presented in the Balance Sheet	84.9	72.7
Net Amount	84.9	72.7
Interest Rate Swap Contracts | Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	10.1	10.1
Gross Amounts Offset in the Balance Sheet	10.1	10.1
Cross Product Collateral Adjustment
Total Derivatives Subject to a Master Netting Arrangement
Gross Amounts Offset in the Balance Sheet	17.1	10.4
Cross Product Collateral Adjustment
Total Derivatives Subject to a Master Netting Arrangement
Gross Amounts Offset in the Balance Sheet	$ 390.6	$ 427.6