Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Change in Accounting Estimate [Line Items]
Operating income	$ 83,426	$ 61,018	$ 158,579	$ 113,352
Net income attributable to Landstar System, Inc. and subsidiary	$ 62,523	$ 37,510	$ 120,040	$ 69,893
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.51	$ 0.89	$ 2.88	$ 1.67
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.51	$ 0.89	$ 2.87	$ 1.66
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 5,686	$ 307	$ 8,244	$ 203
Net income attributable to Landstar System, Inc. and subsidiary	$ 4,293	$ 189	$ 6,224	$ 125
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.10		$ 0.15
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.10		$ 0.15