Cover Page - shares	3 Months Ended
	Mar. 28, 2020	Apr. 20, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Document Quarterly Report	true
Entity Central Index Key	0000853816
Document Transition Report	false
Current Fiscal Year End Date	--12-28
Document Period End Date	Mar. 28, 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		38,370,508
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