Offsetting of Assets and Liabilities (Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	$ 2,509.4	$ 4,097.2
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,977.1	3,507.8
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	532.3	589.4
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	8.4	0.9
NET AMOUNT	523.9	588.5
Total Derivatives Not Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	697.2	461.0
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	0.0	0.0
NET AMOUNT	697.2	461.0
Total Derivatives
GROSS RECOGNIZED ASSETS	3,206.6	4,558.2
Derivative Assets	1,229.5	1,050.4
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	8.4	0.9
NET AMOUNT	1,221.1	1,049.5
Securities Purchased under Agreements to Resell
GROSS RECOGNIZED ASSETS	1,109.3	1,596.5
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	1,109.3	1,596.5
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	1,109.3	1,596.5
Other assets excluding derivatives and non financial instruments	7,600.0	7,300.0
Foreign Exchange Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	2,303.8	3,799.7
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	1,973.5	3,505.3
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	330.3	294.4
GROSS AMOUNTS NOT OFFSET IN THE BALANCE SHEET	8.4	0.9
NET AMOUNT	321.9	293.5
Interest Rate Swap Contracts | Over the Counter Derivative
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	204.0	295.9
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	3.6	2.5
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	200.4	293.4
NET AMOUNT	200.4	293.4
Interest Rate Swap Contracts | Interest Rate Swaps Exchange Cleared
Total Derivatives Subject to a Master Netting Arrangement
GROSS RECOGNIZED ASSETS	1.6	1.6
GROSS AMOUNTS OFFSET IN THE BALANCE SHEET	0.0	0.0
NET AMOUNTS PRESENTED IN THE BALANCE SHEET	1.6	1.6
NET AMOUNT	$ 1.6	$ 1.6