Note 5 - Assets and Liabilities Held for Sale - Assets and Liabilities Held for Sale (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended			12 Months Ended
	Sep. 30, 2019		Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019		Sep. 30, 2018	Dec. 31, 2018
Disposal Group, Including Discontinued Operation, Contract Assets, Current	$ 9,400				$ 9,400			$ 1,800
Total current assets	28,819				28,819			7,792
Impairment of assets held for sale	0		$ 11,900	$ 0	11,946		$ 0
Disposal Group, Including Discontinued Operation, Contract Liabilities, Current	400				400			100
Total current liabilities	20,179				20,179			5,260
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Receivables, net	7,446	[1]			7,446	[1]		1,309	[2]
Retainage	481	[1]			481	[1]		15	[2]
Disposal Group, Including Discontinued Operation, Contract Assets, Current	9,431	[1]			9,431	[1]		1,777	[2]
Inventories	3,003	[1]			3,003	[1]		2,123	[2]
Prepaid expenses and other current assets	2,857	[1]			2,857	[1]		300	[2]
Total current assets	23,218	[1]			23,218	[1]		5,524	[2]
Property, plant & equipment, less accumulated depreciation	7,757	[1]			7,757	[1]		2,268	[2]
Goodwill	4,224	[1]			4,224	[1]		0	[2]
Intangible assets, less accumulated amortization	1,425	[1]			1,425	[1]		0	[2]
Operating lease assets	2,213	[1]			2,213	[1]		0	[2]
Deferred income tax assets	707	[1]			707	[1]		0	[2]
Other assets	87	[1]			87	[1]		0	[2]
Impairment of assets held for sale					(10,812)	[1]		0	[2]
Disposal Group, Including Discontinued Operation, Assets, Total	28,819	[1]			28,819	[1]		7,792	[2]
Accounts payable	6,290	[1]			6,290	[1]		1,331	[2]
Accrued expenses	12,400	[1]			12,400	[1]		3,891	[2]
Disposal Group, Including Discontinued Operation, Contract Liabilities, Current	404	[1]			404	[1]		38	[2]
Current maturities of long-term debt	0	[1]			0	[1]		0	[2]
Total current liabilities	19,094	[1]			19,094	[1]		5,260	[2]
Operating lease liabilities	1,058	[1]			1,058	[1]		0	[2]
Other non-current liabilities	27	[1]			27	[1]		0	[2]
Disposal Group, Including Discontinued Operation, Liabilities, Total	$ 20,179	[1]			$ 20,179	[1]		$ 5,260	[2]

[1] Includes Insituform Australia, Insituform Netherlands, Insituform Spain, Environmental Techniques and United Mexico.

[2] Includes Insituform Australia.