Fair Value - Partnership's Financial Assets at Fair Value (Detail) - USD ($) $ in Millions	Sep. 25, 2020	Dec. 31, 2019
Investments segregated under federal regulations:
Total investments segregated under federal regulations	$ 8,797
Investment securities:
Total investment securities	911	$ 332
Inventory securities:
Total inventory securities	68	50
Certificates of Deposit [Member]
Cash equivalents:
Total cash equivalents	226	326
Investments segregated under federal regulations:
Total investments segregated under federal regulations	100
Investment securities:
Total investment securities	1
Inventory securities:
Total inventory securities	2	9
U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	8,697	3,394
Mutual Funds [Member]
Investment securities:
Total investment securities	329	328
Inventory securities:
Total inventory securities	29	2
State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	27	18
Government and Agency Obligations [Member]
Investment securities:
Total investment securities	578	3
Equities [Member]
Investment securities:
Total investment securities	3	1
Inventory securities:
Total inventory securities	8	18
Corporate Bonds and Notes [Member]
Inventory securities:
Total inventory securities	2	3
Level I [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	8,697
Investment securities:
Total investment securities	910	332
Inventory securities:
Total inventory securities	37	20
Level I [Member] | U.S. Treasuries [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	8,697	3,394
Level I [Member] | Mutual Funds [Member]
Investment securities:
Total investment securities	329	328
Inventory securities:
Total inventory securities	29	2
Level I [Member] | Government and Agency Obligations [Member]
Investment securities:
Total investment securities	578	3
Level I [Member] | Equities [Member]
Investment securities:
Total investment securities	3	1
Inventory securities:
Total inventory securities	8	18
Level II [Member]
Investments segregated under federal regulations:
Total investments segregated under federal regulations	100
Investment securities:
Total investment securities	1	0
Inventory securities:
Total inventory securities	31	30
Level II [Member] | Certificates of Deposit [Member]
Cash equivalents:
Total cash equivalents	226	326
Investments segregated under federal regulations:
Total investments segregated under federal regulations	100
Investment securities:
Total investment securities	1
Inventory securities:
Total inventory securities	2	9
Level II [Member] | State and Municipal Obligations [Member]
Inventory securities:
Total inventory securities	27	18
Level II [Member] | Corporate Bonds and Notes [Member]
Inventory securities:
Total inventory securities	$ 2	$ 3