Offsetting of Assets and Liabilities (Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	$ 1,232.5	$ 1,999.7
Gross Amounts Offset in the Balance Sheet	979.9	1,357.1
Net Amounts Presented in the Balance Sheet	252.6	642.6
Gross Amounts Not Offset in the Balance Sheet	30.4	12.7
Net Amount	222.2	629.9
Total Derivatives Not Subject to a Master Netting Arrangement
Gross Recognized Assets	514.1	563.8
Gross Amounts Not Offset in the Balance Sheet	0.0	2.7
Net Amount	514.1	561.1
Total Derivatives
Gross Recognized Assets	1,746.6	2,563.5
Derivative Asset	766.7	1,206.4
Gross Amounts Not Offset in the Balance Sheet	30.4	15.4
Net Amount	736.3	1,191.0
Securities Purchased under Agreements to Resell
Gross Recognized Assets	515.2	1,031.2
Net Amounts Presented in the Balance Sheet	515.2	1,031.2
Gross Amounts Not Offset in the Balance Sheet	515.2	1,031.2
Other assets excluding derivatives and non financial instruments	5,300.0	4,600.0
Federal funds sold	37.0	134.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	1,127.8	1,902.3
Gross Amounts Offset in the Balance Sheet	926.8	1,308.8
Net Amounts Presented in the Balance Sheet	201.0	593.5
Gross Amounts Not Offset in the Balance Sheet	30.4	12.7
Net Amount	170.6	580.8
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	60.5	71.6
Gross Amounts Offset in the Balance Sheet	12.3	22.6
Net Amounts Presented in the Balance Sheet	48.2	49.0
Net Amount	48.2	49.0
Interest Rate Swap Contracts | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
Gross Recognized Assets	44.2	24.5
Gross Amounts Offset in the Balance Sheet	40.8	24.4
Net Amounts Presented in the Balance Sheet	3.4	0.1
Net Amount	$ 3.4	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