Effect of Increase in Cost of Insurance and Claims (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
Change in Accounting Estimate [Line Items]
Operating income	$ 70,049		$ 60,567		$ 61,018		$ 52,334		$ 63,765		$ 58,461		$ 53,093		$ 47,932		$ 243,968	$ 223,251	$ 241,706
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 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
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income																	$ 4,144	$ 1,079	$ 4,852
Net income attributable to Landstar System, Inc. and subsidiary																	$ 2,578	$ 667	$ 2,999
Earnings per share attributable to Landstar System, Inc. and subsidiary																	$ 0.06	$ 0.02	$ 0.07
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary																	$ 0.06	$ 0.02	$ 0.07

[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.