General - Additional Information (Details) - USD ($) $ in Millions				3 Months Ended			9 Months Ended	12 Months Ended
	Aug. 31, 2018	Jul. 20, 2018	May 04, 2018	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2016	Dec. 31, 2014
General [Line Items]
Increase in foreign currency adjustment, error adjustment				$ 0.6			$ 1.4
Decrease in equity-based compensation expense, error adjustment							$ 1.1
Hebna
General [Line Items]
Total consideration recorded			$ 6.0	$ 3.0	$ 3.0
Energy Services
General [Line Items]
Total consideration recorded		$ 3.0
Aegion South Africa Proprietary Limited
General [Line Items]
Ownership percentage by noncontrolling owners						40.00%
OMAN | Co-venturer
General [Line Items]
Purchase price paid, percentage			50.00%
Aegion International | Aegion South Africa Proprietary Limited
General [Line Items]
Ownership percentage by parent						60.00%
Aegion International | OMAN | Co-venturer
General [Line Items]
Ownership percentage by parent				51.00%			51.00%
Restatement Adjustment
General [Line Items]
Intercompany accounts and stock compensation, error, net									$ 9.0
Decrease in retained earnings, error adjustment								$ 9.0
Increase in additional paid-in capital, error adjustment								1.1
Increase in accumulated other comprehensive income loss, error adjustment								$ 7.9
Increase in operating expense, error adjustment						$ 0.3	$ 0.3
Bayou
General [Line Items]
Ownership percentage by parent	51.00%
Proceeds from divestiture of businesses	$ 46.0			$ 35.0
Consideration received, cash	38.0
Consideration received, performance based, additional earn-out payments	4.0
Gain (loss) on disposition of business, non-cash				$ 8.7
Loans Payable | Bayou
General [Line Items]
Consideration received, loans receivable	$ 8.0
Debt instrument, term	2 years