Note 14 - Subsequent Events (Details Textual) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Oct. 11, 2019	Oct. 10, 2019
Impairment of Long-Lived Assets to be Disposed of	$ 0	$ 11,900	$ 0	$ 11,946	$ 0
United Mexico [Member] | Subsequent Event [Member] | Miller Pipeline de Mexico, S.A. de C.V. [Member]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners							45.00%
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | United Mexico [Member] | Miller Pipeline de Mexico, S.A. de C.V. [Member] | Subsequent Event [Member]
Noncontrolling Interest, Ownership Percentage Sold by Parent						55.00%
Infrastructure Solutions [Member]
Impairment of Long-Lived Assets to be Disposed of				9,000
Corrosion Protection [Member]
Impairment of Long-Lived Assets to be Disposed of				$ 2,900
Corrosion Protection [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | United Mexico [Member]
Impairment of Long-Lived Assets to be Disposed of		1,800
CIPP Contracting Operations of Insituform Netherlands [Member] | Infrastructure Solutions [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Impairment of Long-Lived Assets to be Disposed of		$ 3,900