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	Sep. 30, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Fair value of client securities available to be pledged	$ 28,806	$ 25,905
Total collateral pledged	5,831	5,075
Fulfillment of Requirements with the Options Clearing Corporation [Member]
Securities Financing Transaction [Line Items]
Total collateral pledged	3,036	2,280
Fulfillment of Client Short Sales [Member]
Securities Financing Transaction [Line Items]
Total collateral pledged	1,923	2,011
Securities Lending to Other Broker-Dealers [Member]
Securities Financing Transaction [Line Items]
Total collateral pledged	872	784
Fully Paid Client Securities [Member]
Securities Financing Transaction [Line Items]
Total collateral pledged	$ 88	$ 78