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 |