Share Based Compensation (Narrative) (Details) - USD ($) $ / shares in Units, shares in Thousands			12 Months Ended
	Nov. 02, 2019	Dec. 21, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Feb. 18, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, par value (in USD per share)			$ 0.01	$ 0.01
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized		11,297
Share-based Payment Arrangement, Expensed and Capitalized, Amount			$ 10,535,000
Proceeds from issuance of common stock			18,927,000	$ 363,000	$ 137,605,000
Tax cuts and jobs act of 2017, change in tax rate, deferred tax liability, income tax (expense) benefit				$ 3,641,000
Intrinsic value, exercised			$ 27,973,000
Weighted average grant date fair value, granted (in dollars per share)			$ 6.06	$ 3.87	$ 7.91
Fair value of options vested			$ 3,906,000	$ 4,064,000	$ 2,623,000
Stock-settled RSU's (shares)			1,197
Share-based compensation			$ 10,509,000	$ 19,903,000	$ 15,742,000
Compensation cost not yet recognized, period for recognition			$ 10,445,000
Share-based Payment Arrangement, Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (shares)			11,083
Number of shares available for grant (shares)			2,149
Compensation cost not yet recognized, period for recognition			$ 8,815,000
Period for recognition for unrecognized compensation expense			2 years 3 months 18 days
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	1,158		1,555	24
Stock-settled RSU's (shares)			750	2,002	1,213
Aggregate value			$ 25,000
Period for recognition for unrecognized compensation expense			2 years 7 months 6 days
Employee Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (shares)			5,100			5,000
Number of shares available for grant (shares)			6,025
Employee Stock Ownership Plan (ESOP), Plan [Domain]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, par value (in USD per share)		$ 0.01
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized		5,000
Employee Stock Ownership Plan (ESOP), Compensation Expense			$ 392,000	$ 400,000
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period			58	46
Evoqua Water Technologies Corp. Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock awards vesting percentage			25.00%
Expiration period			10 years
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Issued During Period, Shares, New Issues	739		3,478	108	8,333
Shares withheld related to net share settlement (including tax withholdings) (shares)				884	1,324
Treasury Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Issued During Period, Shares, New Issues			531
Shares withheld related to net share settlement (including tax withholdings) (shares)	419			577	659