Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 30, 2019	Mar. 31, 2018
Change in Accounting Estimate [Line Items]
Operating income	$ 80,896	$ 75,153
Net income attributable to Landstar System, Inc. and subsidiary	$ 63,317	$ 57,517
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.58	$ 1.37
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.58	$ 1.37
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 1,407	$ 2,558
Net income attributable to Landstar System, Inc. and subsidiary	$ 1,067	$ 1,931
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.03	$ 0.05
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.03	$ 0.05