Quarterly Financial Data (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
Schedule Of Quarterly Financial Data [Line Items]
Revenue	$ 994,879		$ 1,011,658		$ 1,045,040		$ 1,033,000		$ 1,182,351		$ 1,202,081		$ 1,182,786		$ 1,047,926		$ 4,084,577	$ 4,615,144	$ 3,646,364
Operating income	66,519		70,638		80,851		80,896		86,088		87,068		83,426		75,153		298,904	331,735	243,968
Income before income taxes	65,656		69,874		80,142		80,091		85,189		86,252		82,587		74,353		295,763	328,381	240,802
Income taxes	15,608		16,619		19,042		16,791		16,889		19,304		20,095		16,880		68,060	73,168	63,806
Net income	50,048		53,255		61,100		63,300		68,300		66,948		62,492		57,473		227,703	255,213	176,996
Less: Net income/loss attributable to noncontrolling interest							(17)		44		(37)		(31)		(44)		(17)	(68)	(92)
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 common 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
Dividends per common share	$ 2.185		$ 0.185		$ 0.165		$ 0.165		$ 0.165		$ 0.165		$ 0.150		$ 0.150		$ 2.70	$ 0.63	$ 1.88

[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.