Acquisitions Opening Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Apr. 01, 2021	Dec. 17, 2020	Sep. 30, 2020
Business Acquisition [Line Items]
Goodwill	$ 409,756			$ 397,205
Water consulting specialists, inc
Business Acquisition [Line Items]
Current assets	1,813
Property, plant and equipment	221
Goodwill	4,361
Intangible assets	7,336
Other non-current assets	86
Total assets acquired	13,817
Total liabilities assumed	(1,792)
Net assets acquired	12,025	$ 12,025
Ultrapure
Business Acquisition [Line Items]
Current assets	2,366
Property, plant and equipment	963
Goodwill	2,836
Intangible assets	3,751
Other non-current assets	21
Total assets acquired	9,937
Total liabilities assumed	(904)
Net assets acquired	$ 9,033		$ 8,743