Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 29, 2019	Jun. 30, 2018	Jun. 29, 2019	Jun. 30, 2018
Change in Accounting Estimate [Line Items]
Operating income	$ 80,851	$ 83,426	$ 161,747	$ 158,579
Net income attributable to Landstar System, Inc. and subsidiary	$ 61,100	$ 62,523	$ 124,417	$ 120,040
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.53	$ 1.51	$ 3.11	$ 2.88
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.53	$ 1.51	$ 3.11	$ 2.87
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 1,493	$ 5,686	$ 2,900	$ 8,244
Net income attributable to Landstar System, Inc. and subsidiary	$ 1,131	$ 4,293	$ 2,198	$ 6,224
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.03	$ 0.10	$ 0.05	$ 0.15
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.03	$ 0.10	$ 0.05	$ 0.15