Note 7 - Leases (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019		Jan. 01, 2019	Dec. 31, 2018	[2]
Lessee, Operating Lease, Renewal Term	5 years
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 19,800
Operating Leases, Future Minimum Payments, Due in Two Years	15,100
Operating Leases, Future Minimum Payments, Due in Three Years	11,500
Operating Leases, Future Minimum Payments, Due in Four Years	8,100
Operating Leases, Future Minimum Payments, Due in Five Years	5,400
Operating Leases, Future Minimum Payments, Due Thereafter	7,200
Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Operating Lease, Right-of-Use Asset	326	[1]		$ 0
Disposal Group Including Discontinued Operation, Operating Lease Liability Current	200
Disposal Group, Including Discontinued Operation, Operating Lease, Liability, Noncurrent	$ 174	[1]		$ 0
Minimum [Member]
Lessee, Operating Lease, Remaining Lease Term	1 year
Maximum [Member]
Lessee, Operating Lease, Remaining Lease Term	20 years
Accounting Standards Update 2016-02 [Member]
Operating Lease, Liability, Total			$ 70,500

[1] Includes Insituform Australia, Insituform Spain, Environmental Techniques and land held at Corporate.

[2] Includes Insituform Australia.