Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 28, 2019		Sep. 28, 2019		Jun. 29, 2019		Mar. 30, 2019		Dec. 29, 2018		Sep. 29, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Change in Accounting Estimate [Line Items]
Operating income	$ 66,519		$ 70,638		$ 80,851		$ 80,896		$ 86,088		$ 87,068		$ 83,426		$ 75,153		$ 298,904	$ 331,735	$ 243,968
Net income attributable to Landstar System, Inc. and subsidiary	$ 50,048		$ 53,255		$ 61,100		$ 63,317		$ 68,256		$ 66,985		$ 62,523		$ 57,517		$ 227,720	$ 255,281	$ 177,088
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.27	[1]	$ 1.35	[1]	$ 1.53	[1]	$ 1.58	[1]	$ 1.69	[1]	$ 1.63	[1]	$ 1.51	[1]	$ 1.37	[1]	$ 5.72	$ 6.19	$ 4.22
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.27	[1]	$ 1.35	[1]	$ 1.53	[1]	$ 1.58	[1]	$ 1.68	[1]	$ 1.63	[1]	$ 1.51	[1]	$ 1.37	[1]	$ 5.72	$ 6.18	$ 4.21
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income																	$ 16,679	$ 13,960	$ 4,144
Net income attributable to Landstar System, Inc. and subsidiary																	$ 12,683	$ 10,582	$ 2,578
Earnings per share attributable to Landstar System, Inc. and subsidiary																	$ 0.32	$ 0.26	$ 0.06
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary																	$ 0.32	$ 0.26	$ 0.06

[1] Due to the changes in the number of average common shares and common stock equivalents outstanding during the year, the sum of earnings per share amounts for each quarter do not necessarily sum in the aggregate to the earnings per share amounts for the full year.