Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 26, 2020	Dec. 28, 2019	Dec. 29, 2018
Change in Accounting Estimate [Line Items]
Operating income	$ 252,950	$ 298,904	$ 331,735
Net income attributable to Landstar System, Inc. and subsidiary	$ 192,106	$ 227,720	$ 255,281
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 4.98	$ 5.72	$ 6.19
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 4.98	$ 5.72	$ 6.18
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 9,196	$ 16,679	$ 13,960
Net income attributable to Landstar System, Inc. and subsidiary	$ 6,989	$ 12,683	$ 10,582
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.18	$ 0.32	$ 0.26
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.18	$ 0.32	$ 0.26