Description of Business - Acquisitions/Strategic Initiatives/Divestitures (Details) $ in Thousands, € in Millions, £ in Millions, kr in Millions, $ in Millions															3 Months Ended		12 Months Ended
	Nov. 01, 2018 DKK (kr)	Nov. 01, 2018 USD ($)	Aug. 31, 2018 USD ($)	Jul. 20, 2018 USD ($)	May 04, 2018 USD ($)	Mar. 01, 2017 GBP (£)	Mar. 01, 2017 USD ($)	Jul. 01, 2016 NZD ($)	Jul. 01, 2016 USD ($)	Jun. 02, 2016 EUR (€)	Jun. 02, 2016 USD ($)	May 13, 2016 USD ($) country	Feb. 18, 2016 USD ($)	Feb. 01, 2016 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2017	Jan. 31, 2016
Infrastructure Solutions
Business Acquisition [Line Items]
Proceeds from sale of productive assets	kr 10.5	$ 1,600
Tube-supply agreement, term	5 years	5 years
Trade name entitlement agreement, term	5 years	5 years
Environmental Techniques
Business Acquisition [Line Items]
Total consideration recorded						£ 6.5	$ 8,000
CSL and BCS
Business Acquisition [Line Items]
Total consideration recorded								$ 8.9	$ 6,400
Payments to acquire businesses								7.5	5,500
Milestone payments								$ 2.0	$ 1,400
LMJ
Business Acquisition [Line Items]
Total consideration recorded										€ 2.9	$ 3,200
Fyfe Europe
Business Acquisition [Line Items]
Total consideration recorded												$ 3,000
Payments to acquire businesses												$ 2,800
Number of countries in which entity operates | country												72
Underground Solutions
Business Acquisition [Line Items]
Total consideration recorded													$ 88,370
Payments to acquire businesses													85,000
Discounted value of net operating loss tax assets													5,000
Consideration transferred, funds held in escrow													6,300
Foreign earnings repatriated													29,700
Foreign earnings repatriated, tax impact													$ 3,200
Hebna
Business Acquisition [Line Items]
Total consideration recorded					$ 6,000										$ 3,000	$ 3,000
Energy Services
Business Acquisition [Line Items]
Total consideration recorded				$ 3,000
Bayou
Business Acquisition [Line Items]
Ownership percentage by parent			51.00%
Proceeds from divestiture of businesses			$ 46,000														$ 35,000
Consideration received, cash			38,000
Consideration received, performance based, additional earn-out payments			4,000
Gain (loss) on disposition of business, non-cash																	$ 7,000
Bayou Perma-Pipe Canada, Ltd
Business Acquisition [Line Items]
Proceeds from divestiture of businesses														$ 7,600
Consideration received														9,600
Disposal group note receivable														$ 2,000
Loans Payable | Bayou
Business Acquisition [Line Items]
Consideration received, loans receivable			$ 8,000
Debt term			2 years
OMAN | Co-venturer
Business Acquisition [Line Items]
Purchase priced paid, percentage					50.00%
Aegion International | OMAN | Co-venturer
Business Acquisition [Line Items]
Ownership percentage by parent																	51.00%
Aegion South Africa Proprietary Limited
Business Acquisition [Line Items]
Ownership percentage by parent																		60.00%
Ownership percentage by noncontrolling owners																		40.00%
Bayou Perma-Pipe Canada, Ltd
Business Acquisition [Line Items]
Ownership percentage by parent																			51.00%
Other expense | Bayou
Business Acquisition [Line Items]
Gain (loss) on disposition of business, adjustment															$ 8,700