Cover Page - USD ($)	12 Months Ended
	Dec. 26, 2020	Jan. 22, 2021	Jun. 27, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Document Annual Report	true
Entity Central Index Key	0000853816
Document Transition Report	false
Current Fiscal Year End Date	--12-26
Document Period End Date	Dec. 26, 2020
Entity Registrant Name	Landstar System, Inc.
Entity Filer Category	Large Accelerated Filer
Trading Symbol	LSTR
Title of 12(b) Security	Common Stock
Security Exchange Name	NASDAQ
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		38,386,768
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity File Number	0-21238
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	06-1313069
Entity Address, Address Line One	13410 Sutton Park Drive South
Entity Address, City or Town	Jacksonville
Entity Address, Postal Zip Code	32224
City Area Code	904
Local Phone Number	398-9400
Entity Address, State or Province	FL
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Public Float			$ 4,086,370,000
ICFR Auditor Attestation Flag	true