New Revenue Standard Requirements Impact of Adoption on Consolidated Income Statement and Balance Sheet (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Dec. 31, 2017	Dec. 30, 2017
Revenues
Revenues	$ 1,182,786	$ 870,434	$ 2,230,712	$ 1,651,342
Costs and expenses
Purchased transportation	916,940	667,588	1,727,237	1,263,111
Commissions to agents	94,399	70,282	176,524	134,080
Income taxes	20,095	22,689	36,975	41,557
Net income attributable to Landstar System, Inc. and subsidiary	62,523	37,510	120,040	69,893
ASSETS
Trade accounts receivable, net	688,961		688,961		$ 641,043	$ 631,164
Other current assets	29,553		29,553		13,925	14,394
Liabilities
Accounts payable	311,768		311,768		293,769	285,132
Equity
Retained earnings	1,719,475		1,719,475		1,611,931	1,611,158
Transportation Logistics
Revenues
Revenues	1,169,921	858,807	2,205,561	1,628,155
Insurance
Revenues
Revenues	12,865	$ 11,627	25,151	$ 23,187
Calculated under Revenue Guidance in Effect before Topic 606
Costs and expenses
Purchased transportation			1,723,814
Commissions to agents			176,222
Income taxes			36,834
Net income attributable to Landstar System, Inc. and subsidiary			119,605
ASSETS
Trade accounts receivable, net	674,781		674,781			631,164
Other current assets	30,163		30,163			14,394
Liabilities
Accounts payable	299,406		299,406			285,132
Equity
Retained earnings	1,718,267		1,718,267			$ 1,611,158
Calculated under Revenue Guidance in Effect before Topic 606 | Transportation Logistics
Revenues
Revenues			2,201,260
Calculated under Revenue Guidance in Effect before Topic 606 | Insurance
Revenues
Revenues			25,151
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Costs and expenses
Purchased transportation			3,423
Commissions to agents			302
Income taxes			141
Net income attributable to Landstar System, Inc. and subsidiary			435
ASSETS
Trade accounts receivable, net	14,180		14,180		9,879
Other current assets	(610)		(610)		(469)
Liabilities
Accounts payable	12,362		12,362		8,637
Equity
Retained earnings	$ 1,208		1,208		$ 773
Difference between Revenue Guidance in Effect before and after Topic 606 | Transportation Logistics | Accounting Standards Update 2014-09
Revenues
Revenues			$ 4,301