Investments, Equity Method and Joint Ventures (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 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	$ 717,132		$ 717,132				$ 695,712
Property, plant, and equipment, net	371,868		371,868				364,461
Goodwill	404,393		404,393				397,205
Other non‑current assets	37,836		37,836				27,509
Liabilities	(1,354,914)		(1,354,914)				(1,362,395)
Operating Expenses	(90,151)	$ (99,295)	(169,485)	$ (186,763)
Operating profit (loss)	16,178	21,155	32,412	90,847
Open Option Contracts Written, at Fair Value	(1,490)		(1,490)				0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Schedule of Equity Method Investments [Line Items]
Open Option Contracts Written, at Fair Value	(7,739)		(7,739)				(7,739)
Other Current Liabilities | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Schedule of Equity Method Investments [Line Items]
Open Option Contracts Written, at Fair Value	$ 1,490		$ 1,490				0
Frontier
Schedule of Equity Method Investments [Line Items]
Percentage of voting interests acquired	8.00%		8.00%					60.00%
Remaining percentage to be acquired								40.00%
Frontier | Subsequent Event
Schedule of Equity Method Investments [Line Items]
Percentage of voting interests acquired					68.00%
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	$ 3,462		$ 3,462				4,016
Property, plant, and equipment, net	1,024		1,024				1,145
Goodwill	2,206		2,206				2,206
Liabilities	(1,168)		(1,168)				(1,324)
Cash	1,552		1,552				2,088
Revenues	834	1,210	1,668	3,852
Operating Expenses	(732)	(1,086)	(1,469)	(3,044)
Operating profit (loss)	102	124	199	808
Frontier
Schedule of Equity Method Investments [Line Items]
Current assets	5,045		5,045				4,024
Property, plant, and equipment, net	3,094		3,094				3,159
Goodwill	1,798		1,798				1,798
Other non‑current assets	9,092		9,092				9,918
Liabilities	(5,095)		(5,095)				(3,692)
Cash	1,182		1,182				$ 1,675
Revenues	2,078	1,078	2,848	2,723
Operating Expenses	(2,401)	(2,364)	(4,139)	(4,301)
Operating profit (loss)	$ (323)	$ (1,286)	$ (1,291)	$ (1,578)