Summary of Significant Accounting Policies (Impact of ASC Topic 842 Adoption) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Oct. 01, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prepaid and other current assets	$ 22,586	$ 21,867	$ 21,940
Total current assets	682,218	637,220	637,293
Property, plant, and equipment, net	339,135	335,710	333,584
Operating lease right-of-use assets, net	42,532	42,073	0
Assets	1,810,302	1,781,974	1,737,848
Accrued expenses and other liabilities	101,656	115,435	101,839
Total current liabilities	405,516	335,817	322,221
Obligation under operating leases	32,871	29,308	0
Other non‑current liabilities	90,346	81,906	78,661
Total non-current liabilities	989,656	1,082,358	1,049,805
Total liabilities	1,395,172	1,418,175	1,372,026
Retained deficit	(123,854)	(176,999)	(174,976)
Total Evoqua Water Technologies Corp. equity	412,956	360,736	362,759
Total shareholder's equity	415,130	363,799	365,822	$ 347,404	$ 362,016
Total liabilities and shareholders' equity	$ 1,810,302	1,781,974	$ 1,737,848
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prepaid and other current assets		(73)
Total current assets		(73)
Property, plant, and equipment, net		2,126
Operating lease right-of-use assets, net		42,073
Assets		44,126
Accrued expenses and other liabilities		13,596
Total current liabilities		13,596
Obligation under operating leases		29,308
Other non‑current liabilities		3,245
Total non-current liabilities		32,553
Total liabilities		46,149
Retained deficit		(2,023)
Total Evoqua Water Technologies Corp. equity		(2,023)
Total shareholder's equity		(2,023)
Total liabilities and shareholders' equity		$ 44,126