Note 12 - Segment Reporting (Details Textual) $ in Thousands		3 Months Ended					9 Months Ended
		Sep. 30, 2020 USD ($)		Jun. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)		Sep. 30, 2020 USD ($)		Sep. 30, 2019 USD ($)
Number of Reportable Segments							3
Impairment of Long-Lived Assets to be Disposed of							$ 0		$ 11,946
Goodwill, Impairment Loss		$ 39,430			$ 0		40,688	[1],[2]	0
Impairment of Intangible Assets, Finite-lived		0			0		957		0
Infrastructure Solutions [Member]
Impairment of Long-Lived Assets to be Disposed of							700		9,000
Goodwill, Impairment Loss	[1],[2]						(0)
Corrosion Protection [Member]
Impairment of Long-Lived Assets to be Disposed of									2,900
Goodwill, Impairment Loss	[1],[2]						(0)
Energy Services [Member]
Goodwill, Impairment Loss		39,400		$ 1,300			40,688	[1],[2]
Impairment of Intangible Assets, Finite-lived							1,000
Other Nonoperating Income (Expense) [Member] | Insituform Australia and Insituform Spain [Member]
Gain (Loss) on Disposition of Business							100
Restructuring 2017 [Member]
Restructuring Charges, Total		4,243	[3]		8,607	[4]	12,699	[5]	17,989	[6]
Restructuring 2017 [Member] | Operating Income (Loss) [Member] | Infrastructure Solutions [Member]
Restructuring Charges, Total		200			1,200		400		4,700
Restructuring 2017 [Member] | Operating Income (Loss) [Member] | Infrastructure Solutions [Member] | Divestiture of International Operations [Member]
Restructuring Charges, Total					500		200		1,000
Restructuring 2017 [Member] | Operating Income (Loss) [Member] | Corrosion Protection [Member]
Restructuring Charges, Total		4,500			800		7,600		4,300
Restructuring 2017 [Member] | Operating Income (Loss) [Member] | Energy Services [Member]
Restructuring Charges, Total		900			100		2,200		200
Restructuring 2017 [Member] | Operating Income (Loss) [Member] | Corporate Segment [Member]
Restructuring Charges, Total		500			1,100		2,000		2,400
Restructuring 2017 [Member] | Operating Income (Loss) [Member] | Corporate Segment [Member] | Divestiture of International Operations [Member]
Restructuring Charges, Total		700			1,300		2,000		1,600
Restructuring 2017 [Member] | Other Nonoperating Income (Expense) [Member]
Restructuring Charges, Total		1,500			$ 5,300		$ 1,800		$ 6,500
Restructuring 2017 [Member] | Other Nonoperating Income (Expense) [Member] | Divestiture of International Operations [Member]
Restructuring Charges, Total		$ 600

[1] During the second quarter of 2020, the Company recorded a $1.3 million goodwill impairment related to restructuring activities within Energy Services (see Note 4).

[2] During the third quarter of 2020, the Company recorded a $39.4 million goodwill impairment within Energy Services (see Note 2).

[3] Total pre-tax restructuring charges for the quarter ended September 30, 2020 include cash charges of $3.7 million and non-cash charges of $0.5 million. Cash charges consist of charges incurred during the quarter that will be settled in cash, either during the current period or future periods.

[4] Total pre-tax restructuring charges for the quarter ended September 30, 2019 include cash charges of $3.1 million and non-cash charges of $5.5 million. Cash charges consist of charges incurred during the quarter that will be settled in cash, either during the current period or future periods.

[5] Total pre-tax restructuring charges for the nine months ended September 30, 2020 include cash charges of $9.5 million and non-cash charges of $3.2 million. Cash charges consist of charges incurred during the quarter that will be settled in cash, either during the current period or future periods.

[6] Total pre-tax restructuring charges for the nine months ended September 30, 2019 include cash charges of $11.6 million and non-cash charges of $6.4 million. Cash charges consist of charges incurred during the quarter that will be settled in cash, either during the current period or future periods.