Pledged and Restricted Assets (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2021	Dec. 31, 2020
Financial Instruments Owned and Pledged as Collateral [Line Items]
Security owned and pledged as collateral, fair value		$ 45,900,000,000	$ 47,500,000,000
Securities eligible to be repledged or sold		93,400,000	33,500,000
Collateral securities repledged, delivered, or used		0	0
Average deposits maintained to meet Federal Reserve Bank reserve requirements	$ 1,700,000,000
Collateral Requirements
Financial Instruments Owned and Pledged as Collateral [Line Items]
Security owned and pledged as collateral, fair value		$ 1,600,000,000	$ 5,700,000,000