Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Change in Accounting Estimate [Line Items]
Operating income	$ 82,352	$ 70,638	$ 168,538	$ 232,385
Net income attributable to Landstar System, Inc. and subsidiary	$ 61,886	$ 53,255	$ 127,035	$ 177,672
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.61	$ 1.35	$ 3.28	$ 4.45
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.61	$ 1.35	$ 3.28	$ 4.45
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 1,123	$ 6,597	$ 6,162	$ 9,497
Net income attributable to Landstar System, Inc. and subsidiary	$ 851	$ 5,001	$ 4,671	$ 7,199
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.02	$ 0.13	$ 0.12	$ 0.18
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.02	$ 0.13	$ 0.12	$ 0.18