Investments, Equity Method and Joint Ventures (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2020	Oct. 01, 2019
Schedule of Equity Method Investments [Line Items]
Current assets	$ 679,540		$ 695,712
Property, plant, and equipment, net	369,915		364,461
Goodwill	408,593		397,205
Other non‑current assets	33,166		27,509
Liabilities	(1,341,705)		(1,362,395)
Operating Expenses	(79,334)	$ (87,468)
Operating profit (loss)	16,234	69,692
Frontier [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of voting interests acquired				60.00%
Remaining percentage to be acquired				40.00%
Treated Water Outsourcing [Member]
Schedule of Equity Method Investments [Line Items]
Current assets	3,560		4,016
Property, plant, and equipment, net	1,084		1,145
Goodwill	2,206		2,206
Liabilities	(1,219)		(1,324)
Cash	1,743		2,088
Revenues	834	2,642
Operating Expenses	(737)	(1,958)
Operating profit (loss)	97	684
Frontier [Member]
Schedule of Equity Method Investments [Line Items]
Current assets	3,528		4,024
Property, plant, and equipment, net	3,210		3,159
Goodwill	1,798		1,798
Other non‑current assets	9,505		9,918
Liabilities	(3,798)		(3,692)
Cash	2,143		$ 1,675
Revenues	770	1,645
Operating Expenses	(1,738)	(1,937)
Operating profit (loss)	$ (968)	$ (292)