Fair Value - Partnership's Financial Assets at Fair Value (Detail) - USD ($) $ in Millions	Sep. 28, 2018	Dec. 31, 2017
Investments segregated under federal regulations:
Total investments segregated under federal regulations	$ 1,695
Investment securities:
Total investment securities	270	$ 258
Inventory securities:
Total inventory securities	83	50
Equities [Member]
Investment securities:
Total investment securities	2	2
Inventory securities:
Total inventory securities	16	16
Corporate Bonds and Notes [Member]
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	6	2
Government and Agency Obligations [Member]
Investment securities:
Total investment securities	3	3
U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	1,495	2,399
Mutual Funds [Member]
Investment securities:
Total investment securities	264	252
Inventory securities:
Total inventory securities	28	4
State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	30	24
Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	277	275
Investments segregated under federal regulations:
Total investments segregated under federal regulations	200
Inventory securities:
Total inventory securities	3	4
Level I [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	1,495
Investment securities:
Total investment securities	269	257
Inventory securities:
Total inventory securities	44	20
Level I [Member] | Equities [Member]
Investment securities:
Total investment securities	2	2
Inventory securities:
Total inventory securities	16	16
Level I [Member] | Government and Agency Obligations [Member]
Investment securities:
Total investment securities	3	3
Level I [Member] | U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	1,495	2,399
Level I [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	264	252
Inventory securities:
Total inventory securities	28	4
Level II [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	200
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	39	30
Level II [Member] | Corporate Bonds and Notes [Member]
Investment securities:
Total investment securities	1	1
Inventory securities:
Total inventory securities	6	2
Level II [Member] | State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	30	24
Level II [Member] | Certificates of Deposit [Member]
Cash equivalents:
Cash and cash equivalents	277	275
Investments segregated under federal regulations:
Total investments segregated under federal regulations	200
Inventory securities:
Total inventory securities	$ 3	$ 4