Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 25, 2021	Dec. 26, 2020	Dec. 28, 2019
Change in Accounting Estimate [Line Items]
Operating income	$ 505,668	$ 252,950	$ 298,904
Net income attributable to Landstar System, Inc. and subsidiary	$ 381,524	$ 192,106	$ 227,720
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 9.98	$ 4.98	$ 5.72
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 9,708	$ 9,196	$ 16,679
Net income attributable to Landstar System, Inc. and subsidiary	$ 7,359	$ 6,989	$ 12,683
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.19	$ 0.18	$ 0.32