Effect of Increase in Cost of Insurance and Claims (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Apr. 01, 2017
Change in Accounting Estimate [Line Items]
Operating income	$ 75,153	$ 52,334
Net income attributable to Landstar System, Inc. and subsidiary	$ 57,517	$ 32,383
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.37	$ 0.77
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 1.37	$ 0.77
Development of Prior Year Self Insured Claims Estimates
Change in Accounting Estimate [Line Items]
Operating income	$ 2,558	$ (104)
Net income attributable to Landstar System, Inc. and subsidiary	$ 1,931	$ (64)
Earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.05
Diluted earnings per share attributable to Landstar System, Inc. and subsidiary	$ 0.05