v3.19.3
Cover Page - shares
9 Months Ended
Sep. 28, 2019
Oct. 21, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Document Quarterly Report true  
Entity Central Index Key 0000853816  
Document Transition Report false  
Current Fiscal Year End Date --12-28  
Document Period End Date Sep. 28, 2019  
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   39,462,908
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