| Document and Entity Information | Oct. 21, 2020 |
| Cover [Abstract] | |
| Entity Registrant Name | LANDSTAR SYSTEM INC |
| Entity File Number | 021-238 |
| Amendment Flag | false |
| Entity Central Index Key | 0000853816 |
| Document Type | 8-K |
| Document Period End Date | Oct. 21, 2020 |
| Entity Incorporation State 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, State or Province | FL |
| Entity Address, Postal Zip Code | 32224 |
| City Area Code | (904) |
| Local Phone Number | 398-9400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock |
| Trading Symbol | LSTR |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
