Financial Instruments Subject to Off-Balance Sheet Credit Risk or Concentration Risk ( Summary of the Fair Value of Client Securities Available to Utilize as Collateral and Amounts Pledged) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Securities Financing Transaction [Line Items]
Fair value of client securities available to be pledged	$ 21,516	$ 22,385
Fair value of client securities pledged	5,193	4,729
Securities Lending to Other Broker-Dealers [Member]
Securities Financing Transaction [Line Items]
Fair value of client securities pledged	1,626	1,935
Fulfillment of Client Short Sales [Member]
Securities Financing Transaction [Line Items]
Fair value of client securities pledged	2,048	1,344
Fulfillment of Requirements with the Options Clearing Corporation [Member]
Securities Financing Transaction [Line Items]
Fair value of client securities pledged	1,519	1,450
Fully-Paid Client Securities [Member]
Securities Financing Transaction [Line Items]
Fair value of client securities pledged	$ 58	$ 43