Fair Value - Partnership's Financial Assets at Fair Value (Detail) - USD ($) $ in Millions | Mar. 31, 2017 | Dec. 31, 2016 |
Investments segregated under federal regulations: | | |
Total investments segregated under federal regulations | $ 4,143 | $ 4,149 |
Investment securities: | | |
Total investment securities | 220 | 224 |
Inventory securities: | | |
Total inventory securities | 84 | 43 |
Equities [Member] | | |
Investment securities: | | |
Total investment securities | 4 | 5 |
Inventory securities: | | |
Total inventory securities | 23 | 17 |
Corporate Bonds and Notes [Member] | | |
Investment securities: | | |
Total investment securities | 1 | 1 |
Inventory securities: | | |
Total inventory securities | 5 | 2 |
Government and Agency Obligations [Member] | | |
Investment securities: | | |
Total investment securities | 3 | 9 |
Other [Member] | | |
Inventory securities: | | |
Total inventory securities | | 1 |
U.S. Treasuries [Member] | | |
Investments segregated under federal regulations: | | |
Total investments segregated under federal regulations | 3,993 | 3,999 |
Mutual Funds [Member] | | |
Investment securities: | | |
Total investment securities | 212 | 209 |
Inventory securities: | | |
Total inventory securities | 19 | 7 |
State and Municipal Obligations [Member] | | |
Inventory securities: | | |
Total inventory securities | 33 | 16 |
Certificates of Deposit [Member] | | |
Cash equivalents: | | |
Cash and cash equivalents | 250 | 150 |
Investments segregated under federal regulations: | | |
Total investments segregated under federal regulations | 150 | 150 |
Inventory securities: | | |
Total inventory securities | 4 | |
Level I [Member] | | |
Investments segregated under federal regulations: | | |
Total investments segregated under federal regulations | 3,993 | 3,999 |
Investment securities: | | |
Total investment securities | 219 | 223 |
Inventory securities: | | |
Total inventory securities | 42 | 24 |
Level I [Member] | Equities [Member] | | |
Investment securities: | | |
Total investment securities | 4 | 5 |
Inventory securities: | | |
Total inventory securities | 23 | 17 |
Level I [Member] | Government and Agency Obligations [Member] | | |
Investment securities: | | |
Total investment securities | 3 | 9 |
Level I [Member] | U.S. Treasuries [Member] | | |
Investments segregated under federal regulations: | | |
Total investments segregated under federal regulations | 3,993 | 3,999 |
Level I [Member] | Mutual Funds [Member] | | |
Investment securities: | | |
Total investment securities | 212 | 209 |
Inventory securities: | | |
Total inventory securities | 19 | 7 |
Level II [Member] | | |
Investments segregated under federal regulations: | | |
Total investments segregated under federal regulations | 150 | 150 |
Investment securities: | | |
Total investment securities | 1 | 1 |
Inventory securities: | | |
Total inventory securities | 42 | 19 |
Level II [Member] | Corporate Bonds and Notes [Member] | | |
Investment securities: | | |
Total investment securities | 1 | 1 |
Inventory securities: | | |
Total inventory securities | 5 | 2 |
Level II [Member] | Other [Member] | | |
Inventory securities: | | |
Total inventory securities | | 1 |
Level II [Member] | State and Municipal Obligations [Member] | | |
Inventory securities: | | |
Total inventory securities | 33 | 16 |
Level II [Member] | Certificates of Deposit [Member] | | |
Cash equivalents: | | |
Cash and cash equivalents | 250 | 150 |
Investments segregated under federal regulations: | | |
Total investments segregated under federal regulations | 150 | $ 150 |
Inventory securities: | | |
Total inventory securities | $ 4 | |