Offsetting of Assets and Liabilities (Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	$ 1,512.1	$ 1,999.7
Gross Amounts Offset in the Balance Sheet	1,171.6	1,357.1
Net Amounts Presented in the Balance Sheet	340.5	642.6
Gross Amounts Not Offset in the Balance Sheet	15.1	12.7
Net Amount	325.4	629.9
Total Derivatives Not Subject to a Master Netting Arrangement
Gross Recognized Assets	379.7	563.8
Gross Amounts Not Offset in the Balance Sheet	0.5	2.7
Net Amount	379.2	561.1
Total Derivatives
Gross Recognized Assets	1,891.8	2,563.5
Derivative Asset	720.2	1,206.4
Gross Amounts Not Offset in the Balance Sheet	15.6	15.4
Net Amount	704.6	1,191.0
Securities Purchased under Agreements to Resell
Gross Recognized Assets	654.5	1,031.2
Net Amounts Presented in the Balance Sheet	654.5	1,031.2
Gross Amounts Not Offset in the Balance Sheet	654.5	1,031.2
Other assets excluding derivatives and non financial instruments	6,600.0	4,600.0
Federal funds sold	6.0	134.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	1,361.8	1,902.3
Gross Amounts Offset in the Balance Sheet	1,168.6	1,308.8
Net Amounts Presented in the Balance Sheet	193.2	593.5
Gross Amounts Not Offset in the Balance Sheet	15.1	12.7
Net Amount	178.1	580.8
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	150.2	71.6
Gross Amounts Offset in the Balance Sheet	3.0	22.6
Net Amounts Presented in the Balance Sheet	147.2	49.0
Net Amount	147.2	49.0
Interest Rate Swap Contracts | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	0.1	24.5
Gross Amounts Offset in the Balance Sheet	0.0	24.4
Net Amounts Presented in the Balance Sheet	0.1	0.1
Net Amount	$ 0.1	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