New Revenue Standard Requirements Impact of Adoption on Consolidated Income Statement and Balance Sheet (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 30, 2017
Revenues
Revenues	$ 1,047,926	$ 780,908
Costs and expenses
Purchased transportation	810,297	595,523
Commissions to agents	82,125	63,798
Income taxes	16,880	18,868
Net income attributable to Landstar System, Inc. and subsidiary	57,517	32,383
ASSETS
Trade accounts receivable, net	631,828		$ 641,043	$ 631,164
Liabilities
Accounts payable	282,480		293,769	285,132
Other current liabilities	62,206			47,211
Equity
Retained earnings	1,663,140		1,611,931	1,611,158
Transportation Logistics
Revenues
Revenues	1,035,640	769,348
Insurance
Revenues
Revenues	12,286	$ 11,560
Calculated under Revenue Guidance in Effect before Topic 606
Costs and expenses
Purchased transportation	808,561
Commissions to agents	81,974
Income taxes	16,805
Net income attributable to Landstar System, Inc. and subsidiary	57,284
ASSETS
Trade accounts receivable, net	619,754			631,164
Liabilities
Accounts payable	271,956			285,132
Other current liabilities	61,662
Equity
Retained earnings	1,662,134			$ 1,611,158
Calculated under Revenue Guidance in Effect before Topic 606 | Transportation Logistics
Revenues
Revenues	1,033,445
Calculated under Revenue Guidance in Effect before Topic 606 | Insurance
Revenues
Revenues	12,286
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Costs and expenses
Purchased transportation	1,736
Commissions to agents	151
Income taxes	75
Net income attributable to Landstar System, Inc. and subsidiary	233
ASSETS
Trade accounts receivable, net	12,074		9,879
Liabilities
Accounts payable	10,524		8,637
Other current liabilities	544
Equity
Retained earnings	1,006		$ 773
Difference between Revenue Guidance in Effect before and after Topic 606 | Transportation Logistics | Accounting Standards Update 2014-09
Revenues
Revenues	$ 2,195