Fair Value - Partnership's Financial Assets at Fair Value (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Investments segregated under federal regulations:
Total investments segregated under federal regulations	$ 3,006	$ 1,334
Investment securities:
Total investment securities	201	161
Inventory securities:
Total inventory securities	36	69
Mutual Funds [Member]
Investment securities:
Total investment securities	192	136
Inventory securities:
Total inventory securities	7	5
Government and Agency Obligations [Member]
Investment securities:
Total investment securities	4	19
Other [Member]
Inventory securities:
Total inventory securities		2
U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	2,706	1,109
State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	11	40
Equities [Member]
Investment securities:
Total investment securities	4	5
Inventory securities:
Total inventory securities	17	17
Corporate Bonds and Notes [Member]
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	1	2
Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	150	100
Investments segregated under federal regulations:
Total investments segregated under federal regulations	300	225
Inventory securities:
Total inventory securities		3
Level I [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	2,706	1,109
Investment securities:
Total investment securities	200	160
Inventory securities:
Total inventory securities	24	23
Level I [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	192	136
Inventory securities:
Total inventory securities	7	5
Level I [Member] | Government and Agency Obligations [Member]
Investment securities:
Total investment securities	4	19
Level I [Member] | Other [Member]
Inventory securities:
Total inventory securities		1
Level I [Member] | U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	2,706	1,109
Level I [Member] | State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	0	0
Level I [Member] | Equities [Member]
Investment securities:
Total investment securities	4	5
Inventory securities:
Total inventory securities	17	17
Level I [Member] | Corporate Bonds and Notes [Member]
Investment securities:
Total investment securities	0	0
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
Inventory securities:
Total inventory securities		0
Level II [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	300	225
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	12	46
Level II [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	0	0
Inventory securities:
Total inventory securities	0	0
Level II [Member] | Government and Agency Obligations [Member]
Investment securities:
Total investment securities	0	0
Level II [Member] | Other [Member]
Inventory securities:
Total inventory securities		1
Level II [Member] | U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	0	0
Level II [Member] | State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	11	40
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]
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	1	2
Level II [Member] | Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	150	100
Investments segregated under federal regulations:
Total investments segregated under federal regulations	300	225
Inventory securities:
Total inventory securities		3
Level III [Member]
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	0
Level III [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	0	0
Inventory securities:
Total inventory securities	0	0
Level III [Member] | Government and Agency Obligations [Member]
Investment securities:
Total investment securities	0	0
Level III [Member] | Other [Member]
Inventory securities:
Total inventory securities		0
Level III [Member] | U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	0	0
Level III [Member] | State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	0	0
Level III [Member] | Equities [Member]
Investment securities:
Total investment securities	0	0
Inventory securities:
Total inventory securities	0	0
Level III [Member] | Corporate Bonds and Notes [Member]
Investment securities:
Total investment securities	0	0
Inventory securities:
Total inventory securities	0	0
Level III [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
Inventory securities:
Total inventory securities		$ 0