Pledged and Restricted Assets (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Financial Instruments Owned and Pledged as Collateral [Line Items]
Security owned and pledged as collateral, fair value		$ 54,800.0	$ 47,500.0
Securities eligible to be repledged or sold		589.0	33.5
Average deposits maintained to meet Federal Reserve Bank reserve requirements	$ 500.0
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Security owned and pledged as collateral, fair value		$ 7,500.0	$ 5,700.0