Investments, Equity Method and Joint Ventures (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Apr. 08, 2021	Feb. 28, 2021	Sep. 30, 2020	Oct. 01, 2019
Schedule of Equity Method Investments [Line Items]
Obligation to absorb risk of loss, percentage of partner's equity	100.00%		100.00%
Current assets	$ 664,060		$ 664,060				$ 695,712
Property, plant, and equipment, net	372,399		372,399				364,461
Goodwill	409,756		409,756				397,205
Other non‑current assets	50,252		50,252				27,509
Liabilities	(1,297,979)		(1,297,979)				(1,362,395)
Operating Expenses	(90,121)	$ (77,504)	(259,606)	$ (264,267)
Operating profit (loss)	28,310	33,066	60,722	123,913
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Schedule of Equity Method Investments [Line Items]
Purchase of outstanding equity	$ 6,249		$ 6,249				7,739
Frontier
Schedule of Equity Method Investments [Line Items]
Percentage of voting interests acquired	68.00%		68.00%		8.00%			60.00%
Remaining percentage to be acquired								40.00%
Purchase of outstanding equity					$ 1,490
Frontier | Maximum
Schedule of Equity Method Investments [Line Items]
Remaining percentage to be acquired						10.00%
Treated Water Outsourcing [Member]
Schedule of Equity Method Investments [Line Items]
Partnership interest			50.00%
Current assets	$ 3,529		$ 3,529				4,016
Property, plant, and equipment, net	963		963				1,145
Goodwill	2,206		2,206				2,206
Liabilities	(1,287)		(1,287)				(1,324)
Cash	1,580		1,580				2,088
Revenues	843	1,328	2,511	5,180
Operating Expenses	(748)	(803)	(2,217)	(3,847)
Operating profit (loss)	95	525	294	1,333
Frontier
Schedule of Equity Method Investments [Line Items]
Current assets	10,034		10,034				4,024
Property, plant, and equipment, net	2,974		2,974				3,159
Goodwill	1,798		1,798				1,798
Other non‑current assets	8,678		8,678				9,918
Liabilities	(9,039)		(9,039)				(3,692)
Cash	2,947		2,947				$ 1,675
Revenues	4,945	1,411	7,793	4,134
Operating Expenses	(4,448)	(2,003)	(8,587)	(6,304)
Operating profit (loss)	$ 497	$ (592)	$ (794)	$ (2,170)