Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Change in Accounting Estimate [Line Items]
Operating income	$ 87,068	$ 60,567	$ 245,647	$ 173,919
Net income attributable to Landstar System, Inc. and subsidiary	$ 66,985	$ 42,443	$ 187,025	$ 112,336
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.63	$ 1.01	$ 4.50	$ 2.68
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.63	$ 1.01	$ 4.50	$ 2.67
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 3,352	$ 1,124	$ 11,596	$ 1,327
Net income attributable to Landstar System, Inc. and subsidiary	$ 2,531	$ 699	$ 8,755	$ 825
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.06	$ 0.02	$ 0.21	$ 0.02
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.06	$ 0.02	$ 0.21	$ 0.02