Offsetting of Assets and Liabilities (Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	$ 2,152.5	$ 4,097.2
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	939.5	3,507.8
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	1,213.0	589.4
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	2.8	0.9
NET AMOUNT	1,210.2	588.5
Total Derivatives Not Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	851.7	461.0
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.0	0.0
NET AMOUNT	851.7	461.0
Total Derivatives
GROSS RECOGNIZED ASSETS	3,004.2	4,558.2
Derivative Assets	2,064.7	1,050.4
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	2.8	0.9
NET AMOUNT	2,061.9	1,049.5
Securities Purchased under Agreements to Resell
GROSS RECOGNIZED ASSETS	1,372.8	1,596.5
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	500.0
Securities Purchased under Agreements to Resell	872.8	1,596.5
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	872.8	1,596.5
Other assets excluding derivatives and non financial instruments	7,700.0	7,300.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	1,957.7	3,799.7
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	936.9	3,505.3
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	1,020.8	294.4
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	2.8	0.9
NET AMOUNT	1,018.0	293.5
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	192.6	295.9
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	2.6	2.5
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	190.0	293.4
NET AMOUNT	190.0	293.4
Interest Rate Swap Contracts | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	2.2	1.6
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	0.0	0.0
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	2.2	1.6
NET AMOUNT	$ 2.2	$ 1.6