Financial Instruments Subject to Off-Balance Sheet Credit Risk (Summary of the Fair Value of Client Securities Available to Utilize as Collateral and Amounts Pledged) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Securities Financing Transaction [Line Items]
Fair value of client securities available to be pledged	$ 22,896	$ 26,685
Total collateral pledged	8,001	5,481
Fulfillment of Requirements with the Options Clearing Corporation [Member]
Securities Financing Transaction [Line Items]
Total collateral pledged	4,622	2,171
Fulfillment of Client Short Sales [Member]
Securities Financing Transaction [Line Items]
Total collateral pledged	2,102	2,293
Securities Lending to Other Broker-Dealers [Member]
Securities Financing Transaction [Line Items]
Total collateral pledged	1,277	1,017
Fully Paid Client Securities [Member]
Securities Financing Transaction [Line Items]
Total collateral pledged	$ 104	$ 142