Fair Value - Partnership's Financial Assets at Fair Value (Detail) - USD ($) $ in Millions	Jun. 26, 2020	Dec. 31, 2019
Cash equivalents:
Total cash equivalents	$ 401
Investments segregated under federal regulations:
Total investments segregated under federal regulations	8,096
Investment securities:
Total investment securities	395	$ 332
Inventory securities:
Total inventory securities	59	50
Certificates of Deposit [Member]
Cash equivalents:
Total cash equivalents	201	326
Investments segregated under federal regulations:
Total investments segregated under federal regulations	100
Investment securities:
Total investment securities	2
Inventory securities:
Total inventory securities	1	9
U.S. Treasuries [Member]
Cash equivalents:
Total cash equivalents	200
Investments segregated under federal regulations:
Total investments segregated under federal regulations	7,996	3,394
Mutual Funds [Member]
Investment securities:
Total investment securities	300	328
Inventory securities:
Total inventory securities	9	2
State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	18	18
Government and Agency Obligations [Member]
Investment securities:
Total investment securities	90	3
Equities [Member]
Investment securities:
Total investment securities	3	1
Inventory securities:
Total inventory securities	26	18
Corporate Bonds and Notes [Member]
Inventory securities:
Total inventory securities	5	3
Level I [Member]
Cash equivalents:
Total cash equivalents	200
Investments segregated under federal regulations:
Total investments segregated under federal regulations	7,996
Investment securities:
Total investment securities	393	332
Inventory securities:
Total inventory securities	35	20
Level I [Member] | U.S. Treasuries [Member]
Cash equivalents:
Total cash equivalents	200
Investments segregated under federal regulations:
Total investments segregated under federal regulations	7,996	3,394
Level I [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	300	328
Inventory securities:
Total inventory securities	9	2
Level I [Member] | Government and Agency Obligations [Member]
Investment securities:
Total investment securities	90	3
Level I [Member] | Equities [Member]
Investment securities:
Total investment securities	3	1
Inventory securities:
Total inventory securities	26	18
Level II [Member]
Cash equivalents:
Total cash equivalents	201
Investments segregated under federal regulations:
Total investments segregated under federal regulations	100
Investment securities:
Total investment securities	2	0
Inventory securities:
Total inventory securities	24	30
Level II [Member] | Certificates of Deposit [Member]
Cash equivalents:
Total cash equivalents	201	326
Investments segregated under federal regulations:
Total investments segregated under federal regulations	100
Investment securities:
Total investment securities	2
Inventory securities:
Total inventory securities	1	9
Level II [Member] | State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	18	18
Level II [Member] | Corporate Bonds and Notes [Member]
Inventory securities:
Total inventory securities	$ 5	$ 3