Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 28, 2019	Sep. 29, 2018	Sep. 28, 2019	Sep. 29, 2018
Change in Accounting Estimate [Line Items]
Operating income	$ 70,638	$ 87,068	$ 232,385	$ 245,647
Net income attributable to Landstar System, Inc. and subsidiary	$ 53,255	$ 66,985	$ 177,672	$ 187,025
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.35	$ 1.63	$ 4.45	$ 4.50
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.35	$ 1.63	$ 4.45	$ 4.50
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 6,597	$ 3,352	$ 9,497	$ 11,596
Net income attributable to Landstar System, Inc. and subsidiary	$ 5,001	$ 2,531	$ 7,199	$ 8,755
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.13	$ 0.06	$ 0.18	$ 0.21
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.13	$ 0.06	$ 0.18	$ 0.21