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Note 1 - Description of Business (Details Textual) - USD ($)		1 Months Ended		3 Months Ended	12 Months Ended
	Aug. 31, 2018	Aug. 31, 2018	May 31, 2018	Jun. 30, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 30, 2019
Proceeds from Divestiture of Businesses					$ 3,602,000	$ 0	$ 37,942,000
Goodwill, Ending Balance					$ 210,665,000	$ 208,858,000	212,656,000
Discontinued Operations, Disposed of by Sale [Member] \| Bayou Companies, LLC [Member]
Joint Venture Ownership Percentage Sold	51.00%
Disposal Group, Including Discontinued Operation, Consideration	$ 46,000,000	$ 46,000,000
Proceeds from Divestiture of Businesses		38,000,000
Noncash or Part Noncash Divestiture, Amount of Consideration Received		$ 8,000,000
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal							$ 7,000,000.0
Discontinued Operations, Disposed of by Sale [Member] \| Bayou Companies, LLC [Member] \| Notes Receivable [Member]
Notes Receivable, Term (Year)		2 years
United Mexico [Member] \| Miller Pipeline de Mexico, S.A. de C.V. [Member]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners								45.00%
United Mexico [Member] \| Miller Pipeline de Mexico, S.A. de C.V. [Member] \| Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Noncontrolling Interest, Ownership Percentage Sold by Parent								55.00%
Aegion International [Member] \| OMAN
Business Acquisition, Percentage of Voting Interests Acquired			50.00%
Noncontrolling Interest, Ownership Percentage by Parent			51.00%
New Mountain Capital, L.L.C. [Member] \| Forecast [Member]
Business Combination, Consideration Transferred, Total				$ 963,000,000
Business Acquisition, Share Price (in dollars per share)				$ 26.00
Business Combination, Termination Fee				$ 30,000,000
Hebna [Member]
Business Combination, Consideration Transferred, Total			$ 6,000,000.0
Goodwill, Ending Balance			2,700,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill, Total			2,400,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment, Total			$ 900,000