Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 30, 2017		Sep. 30, 2017		Jul. 01, 2017		Apr. 01, 2017		Dec. 31, 2016		Sep. 24, 2016		Jun. 25, 2016		Mar. 26, 2016		Dec. 30, 2017	Dec. 31, 2016	Dec. 26, 2015
Schedule Of Quarterly Financial Data [Line Items]
Revenue	$ 1,051,592		$ 943,430		$ 870,434		$ 780,908		$ 892,829		$ 787,938		$ 775,223		$ 711,644		$ 3,646,364	$ 3,167,634	$ 3,321,091
Operating income	70,049		60,567		61,018		52,334		63,765		58,461		53,093		47,932		243,968	223,251	241,706
Income before income taxes	69,442		59,910		60,199		51,251		62,696		57,513		52,205		47,043		240,802	219,457	238,757
Income taxes	4,759		17,490		22,689		18,868		23,122		21,235		19,891		17,859		63,806	82,107	91,068
Net income	64,683		42,420		37,510		32,383		$ 39,574		$ 36,278		$ 32,314		$ 29,184		176,996	137,350	147,689
Less: Net loss attributable to noncontrolling interest	(69)		(23)														92
Net income attributable to Landstar System, Inc. and subsidiary	$ 64,752		$ 42,443		$ 37,510		$ 32,383										$ 177,088	$ 137,350	$ 147,689
Earnings per common share attributable to Landstar System, Inc. and subsidiary	$ 1.54	[1]	$ 1.01	[1]	$ 0.89	[1]	$ 0.77	[1]	$ 0.95	[1]	$ 0.86	[1]	$ 0.77	[1]	$ 0.69	[1]	$ 4.22	$ 3.26	$ 3.38
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	1.54	[1]	1.01	[1]	0.89	[1]	0.77	[1]	0.94	[1]	0.86	[1]	0.76	[1]	0.69	[1]	4.21	3.25	3.37
Dividends per common share	$ 1.60		$ 0.10		$ 0.09		$ 0.09		$ 0.09		$ 0.09		$ 0.08		$ 0.08		$ 1.88	$ 0.34	$ 0.30

[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.