Note 6 - Valuation Accounts - Valuation and Allowance Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Trade Receivables [Member]
Balance	$ 549	$ 506	$ 639
Net Charged to Costs and Expenses	625	1,415	992
Net Write-Offs	(558)	(1,372)	(1,125)
Balance End of Year	616	549	506
Allowance for Warranty Receivables From Manufacturers [Member]
Balance	114	105	140
Net Charged to Costs and Expenses	713	833	1,677
Net Write-Offs	(617)	(824)	(1,712)
Balance End of Year	210	114	105
Parts Inventory [Member]
Balance	4,885	7,291	5,067
Net Charged to Costs and Expenses	1,414	3,400	4,215
Net Write-Offs	(69)	(5,806)	(1,991)
Balance End of Year	6,230	4,885	7,291
Commercial Vehicle Inventory [Member]
Balance	5,102	6,541	10,644
Net Charged to Costs and Expenses	5,997	9,623	8,417
Net Write-Offs	(5,146)	(11,062)	(12,520)
Balance End of Year	$ 5,953	$ 5,102	$ 6,541