Summary of Significant Accounting Policies (Impact of ASC Topic 842 Adoption) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Oct. 01, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Prepaid and other current assets	$ 20,051		$ 21,867	$ 21,940
Total current assets	642,722		637,220	637,293
Property, plant, and equipment, net	356,424		336,396	333,584
Operating lease right-of-use assets, net	39,427		42,073	0
Assets	1,777,896		1,782,660	1,737,848
Accrued expenses and other liabilities	124,016		115,435	101,839
Total current liabilities	335,292		335,817	322,221
Obligation under operating leases	31,008		29,308	0
Other non‑current liabilities	93,909		81,906	78,661
Total non-current liabilities	995,274		1,082,358	1,049,805
Total liabilities	1,330,566		1,418,175	1,372,026
Retained deficit	(93,972)		(176,313)	(174,976)
Total Evoqua Water Technologies Corp. equity	445,201		361,422	362,759
Total shareholder's equity	447,330	$ 422,503	364,485	365,822	$ 363,591	$ 355,377	$ 362,016
Total liabilities and shareholders' equity	$ 1,777,896		1,782,660	$ 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			2,812
Operating lease right-of-use assets, net			42,073
Assets			44,812
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			(1,337)
Total Evoqua Water Technologies Corp. equity			(1,337)
Total shareholder's equity			(1,337)
Total liabilities and shareholders' equity			$ 44,812