Pledged Assets, Accepted Collateral and Restricted Assets (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Financial Instruments Owned and Pledged as Collateral [Line Items]
Cash, held in foreign currency	$ 1,000.0		$ 1,000.0		$ 722.2
Restricted cash	519.8		519.8		575.2
Average deposits maintained to meet Federal Reserve Bank reserve requirements	0.0	$ 0.0	0.0	$ 0.0
Debt Securities And Loans
Financial Instruments Owned and Pledged as Collateral [Line Items]
Collateral that may be repledged or sold	$ 1,200.0		$ 1,200.0		$ 1,100.0