Fair Value - Partnership's Financial Assets at Fair Value (Detail) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Cash equivalents:
Total cash equivalents	$ 287	$ 313
Investments segregated under federal regulations:
Total investments segregated under federal regulations	12,653	14,308
Investment securities:
Total investment securities	1,103	852
Inventory securities:
Total inventory securities	91	38
Other Assets [Abstract]
Client Fractional Shares Ownership Assets	593	710
Accounts Payable and Accrued Liabilities [Abstract]
Client Fractional Shares Redemption Obligations	593	710
Equities [Member]
Investment securities:
Total investment securities	7	3
Inventory securities:
Total inventory securities	24	18
Corporate Bonds and Notes [Member]
Inventory securities:
Total inventory securities	20	3
Government and Agency Obligations [Member]
Investment securities:
Total investment securities	807	413
U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	12,253	13,908
Mutual Funds [Member]
Investment securities:
Total investment securities	289	366
Inventory securities:
Total inventory securities	8	2
Municipal Obligations [Member]
Inventory securities:
Total inventory securities	22	9
Certificates of Deposit [Member]
Cash equivalents:
Total cash equivalents	246	266
Investments segregated under federal regulations:
Total investments segregated under federal regulations	400	400
Investment securities:
Total investment securities		70
Inventory securities:
Total inventory securities	17	6
Money Market Fund [Member]
Cash equivalents:
Total cash equivalents	41	47
Level I [Member]
Cash equivalents:
Total cash equivalents	41	47
Investments segregated under federal regulations:
Total investments segregated under federal regulations	12,253	13,908
Investment securities:
Total investment securities	1,103	782
Inventory securities:
Total inventory securities	32	20
Other Assets [Abstract]
Client Fractional Shares Ownership Assets	593	710
Accounts Payable and Accrued Liabilities [Abstract]
Client Fractional Shares Redemption Obligations	593	710
Level I [Member] | Equities [Member]
Investment securities:
Total investment securities	7	3
Inventory securities:
Total inventory securities	24	18
Level I [Member] | Corporate Bonds and Notes [Member]
Inventory securities:
Total inventory securities	0	0
Level I [Member] | Government and Agency Obligations [Member]
Investment securities:
Total investment securities	807	413
Level I [Member] | U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	12,253	13,908
Level I [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	289	366
Inventory securities:
Total inventory securities	8	2
Level I [Member] | Municipal Obligations [Member]
Inventory securities:
Total inventory securities	0	0
Level I [Member] | Certificates of Deposit [Member]
Cash equivalents:
Total cash equivalents	0	0
Investments segregated under federal regulations:
Total investments segregated under federal regulations	0	0
Investment securities:
Total investment securities		0
Inventory securities:
Total inventory securities	0	0
Level I [Member] | Money Market Fund [Member]
Cash equivalents:
Total cash equivalents	41	47
Level II [Member]
Cash equivalents:
Total cash equivalents	246	266
Investments segregated under federal regulations:
Total investments segregated under federal regulations	400	400
Investment securities:
Total investment securities	0	70
Inventory securities:
Total inventory securities	59	18
Other Assets [Abstract]
Client Fractional Shares Ownership Assets	0	0
Accounts Payable and Accrued Liabilities [Abstract]
Client Fractional Shares Redemption Obligations	0	0
Level II [Member] | Equities [Member]
Investment securities:
Total investment securities	0	0
Inventory securities:
Total inventory securities	0	0
Level II [Member] | Corporate Bonds and Notes [Member]
Inventory securities:
Total inventory securities	20	3
Level II [Member] | Government and Agency Obligations [Member]
Investment securities:
Total investment securities	0	0
Level II [Member] | U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	0	0
Level II [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	0	0
Inventory securities:
Total inventory securities	0	0
Level II [Member] | Municipal Obligations [Member]
Inventory securities:
Total inventory securities	22	9
Level II [Member] | Certificates of Deposit [Member]
Cash equivalents:
Total cash equivalents	246	266
Investments segregated under federal regulations:
Total investments segregated under federal regulations	400	400
Investment securities:
Total investment securities		70
Inventory securities:
Total inventory securities	17	6
Level II [Member] | Money Market Fund [Member]
Cash equivalents:
Total cash equivalents	$ 0	$ 0