Fair Value - Partnership's Financial Assets and Liabilities at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Cash equivalents:
Cash and cash equivalents	$ 313	$ 161
Investments segregated under federal regulations:
Total investments segregated under federal regulations	14,308	12,168
Investment securities:
Total investment securities	852	1,302
Inventory securities:
Total inventory securities	38	32
Other assets:
Client fractional share ownership assets	710	534
Accounts payable, accrued expenses and other:
Client fractional share redemption obligations	710	534
Equities [Member]
Investment securities:
Total investment securities	3	3
Inventory securities:
Total inventory securities	18	19
Corporate Bonds and Notes [Member]
Inventory securities:
Total inventory securities	3	2
Government and Agency Obligations [Member]
Investment securities:
Total investment securities	413	971
U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	13,908	12,068
Mutual Funds [Member]
Investment securities:
Total investment securities	366	327
Inventory securities:
Total inventory securities	2	1
Municipal Obligations [Member]
Inventory securities:
Total inventory securities	9	10
Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	266	120
Investments segregated under federal regulations:
Total investments segregated under federal regulations	400	100
Investment securities:
Total investment securities	70	1
Inventory securities:
Total inventory securities	6
Money Market Funds
Cash equivalents:
Cash and cash equivalents	47	41
Level I [Member]
Cash equivalents:
Cash and cash equivalents	47	41
Investments segregated under federal regulations:
Total investments segregated under federal regulations	13,908	12,068
Investment securities:
Total investment securities	782	1,301
Inventory securities:
Total inventory securities	20	20
Other assets:
Client fractional share ownership assets	710	534
Accounts payable, accrued expenses and other:
Client fractional share redemption obligations	710	534
Level I [Member] | Equities [Member]
Investment securities:
Total investment securities	3	3
Inventory securities:
Total inventory securities	18	19
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	413	971
Level I [Member] | U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	13,908	12,068
Level I [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	366	327
Inventory securities:
Total inventory securities	2	1
Level I [Member] | Municipal Obligations [Member]
Inventory securities:
Total inventory securities	0	0
Level I [Member] | Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	0	0
Investments segregated under federal regulations:
Total investments segregated under federal regulations	0	0
Investment securities:
Total investment securities	0	0
Inventory securities:
Total inventory securities	0
Level I [Member] | Money Market Funds
Cash equivalents:
Cash and cash equivalents	47	41
Level II [Member]
Cash equivalents:
Cash and cash equivalents	266	120
Investments segregated under federal regulations:
Total investments segregated under federal regulations	400	100
Investment securities:
Total investment securities	70	1
Inventory securities:
Total inventory securities	18	12
Other assets:
Client fractional share ownership assets	0	0
Accounts payable, accrued expenses and other:
Client fractional share 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	3	2
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	9	10
Level II [Member] | Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	266	120
Investments segregated under federal regulations:
Total investments segregated under federal regulations	400	100
Investment securities:
Total investment securities	70	1
Inventory securities:
Total inventory securities	6
Level II [Member] | Money Market Funds
Cash equivalents:
Cash and cash equivalents	$ 0	$ 0