Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 24, 2016	Sep. 30, 2017	Sep. 24, 2016
Change in Accounting Estimate [Line Items]
Operating income	$ 60,567	$ 58,461	$ 173,919	$ 159,486
Net income attributable to Landstar System, Inc. and subsidiary	$ 42,443	$ 36,278	$ 112,336	$ 97,776
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.01	$ 0.86	$ 2.68	$ 2.32
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.01	$ 0.86	$ 2.67	$ 2.31
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 1,124	$ (2,118)	$ 1,327	$ 698
Net income attributable to Landstar System, Inc. and subsidiary	$ 699	$ (1,309)	$ 825	$ 431
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.02	$ (0.03)	$ 0.02	$ 0.01
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.02	$ (0.03)	$ 0.02	$ 0.01