Fair Value - Partnership's Financial Assets at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Cash equivalents:
Cash and cash equivalents	$ 161
Investments segregated under federal regulations:
Total investments segregated under federal regulations	12,151
Investment securities:
Total investment securities	1,302	$ 332
Inventory securities:
Total inventory securities	32	50
Equities [Member]
Investment securities:
Total investment securities	3	1
Inventory securities:
Total inventory securities	19	18
Corporate Bonds and Notes [Member]
Inventory securities:
Total inventory securities	2	3
Government and Agency Obligations [Member]
Investment securities:
Total investment securities	971	3
U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	12,051	3,394
Mutual Funds [Member]
Investment securities:
Total investment securities	327	328
Inventory securities:
Total inventory securities	1	2
State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	10	18
Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	120	326
Investments segregated under federal regulations:
Total investments segregated under federal regulations	100
Investment securities:
Total investment securities	1
Inventory securities:
Total inventory securities		9
Money Market Funds
Cash equivalents:
Cash and cash equivalents	41
Level I [Member]
Cash equivalents:
Cash and cash equivalents	41
Investments segregated under federal regulations:
Total investments segregated under federal regulations	12,051
Investment securities:
Total investment securities	1,301	332
Inventory securities:
Total inventory securities	20	20
Level I [Member] | Equities [Member]
Investment securities:
Total investment securities	3	1
Inventory securities:
Total inventory securities	19	18
Level I [Member] | Government and Agency Obligations [Member]
Investment securities:
Total investment securities	971	3
Level I [Member] | U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	12,051	3,394
Level I [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	327	328
Inventory securities:
Total inventory securities	1	2
Level I [Member] | Money Market Funds
Cash equivalents:
Cash and cash equivalents	41
Level II [Member]
Cash equivalents:
Cash and cash equivalents	120
Investments segregated under federal regulations:
Total investments segregated under federal regulations	100
Investment securities:
Total investment securities	1
Inventory securities:
Total inventory securities	12	30
Level II [Member] | Corporate Bonds and Notes [Member]
Inventory securities:
Total inventory securities	2	3
Level II [Member] | State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	10	18
Level II [Member] | Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	120	326
Investments segregated under federal regulations:
Total investments segregated under federal regulations	100
Investment securities:
Total investment securities	$ 1
Inventory securities:
Total inventory securities		$ 9