Fair Value - Partnership's Financial Assets at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments segregated under federal regulations:
Total investments segregated under federal regulations		$ 4,149
Investment securities:
Total investment securities	$ 258	224
Inventory securities:
Total inventory securities	50	43
Equities [Member]
Investment securities:
Total investment securities	2	5
Inventory securities:
Total inventory securities	16	17
Corporate Bonds and Notes [Member]
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	2	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	2,399	3,999
Mutual Funds [Member]
Investment securities:
Total investment securities	252	209
Inventory securities:
Total inventory securities	4	7
State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	24	16
Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	275	150
Investments segregated under federal regulations:
Total investments segregated under federal regulations		150
Inventory securities:
Total inventory securities	4
Level I [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations		3,999
Investment securities:
Total investment securities	257	223
Inventory securities:
Total inventory securities	20	24
Level I [Member] | Equities [Member]
Investment securities:
Total investment securities	2	5
Inventory securities:
Total inventory securities	16	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	2,399	3,999
Level I [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	252	209
Inventory securities:
Total inventory securities	4	7
Level II [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations		150
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	30	19
Level II [Member] | Corporate Bonds and Notes [Member]
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	2	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	24	16
Level II [Member] | Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	275	150
Investments segregated under federal regulations:
Total investments segregated under federal regulations		$ 150
Inventory securities:
Total inventory securities	$ 4