Variable Interest Entities (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Apr. 08, 2021	Feb. 28, 2021	Oct. 01, 2019
Variable Interest Entity [Line Items]
Variable Interest Entity, Obligation To Absorb Risk, Percentage Of Equity	100.00%
Current assets	$ 678,458	$ 695,712
Property, plant, and equipment, net	374,988	364,461
Goodwill	407,376	397,205	$ 392,890
Other non‑current assets	64,188	27,509
Liabilities	(1,286,672)	(1,362,395)
Revenue	1,464,429	1,429,456	1,444,441
Operating Expenses	(363,010)	(341,962)	(371,249)
Earnings before interest, taxes, depreciation and amortization (EBITDA)	$ 99,317	168,448	59,672
Frontier
Variable Interest Entity [Line Items]
Business Acquisition, Percentage Of Voting Interest Remaining						40.00%
Business acquisition, percentage of voting interests acquired	68.00%			8.00%		60.00%
Open Option Contracts Written, at Fair Value				$ 1,490
Goodwill		1,798
Maximum | Frontier
Variable Interest Entity [Line Items]
Business Acquisition, Percentage Of Voting Interest Remaining					10.00%
Frontier
Variable Interest Entity [Line Items]
Cash	$ 2,095	1,675
Current assets	12,495	4,024
Property, plant, and equipment, net	2,113	3,159
Goodwill	1,798	1,798
Other non‑current assets	8,265	9,918
Liabilities	(9,425)	(3,692)
Revenue	14,340	5,365
Operating Expenses	(14,362)	(8,219)
Earnings before interest, taxes, depreciation and amortization (EBITDA)	$ (22)	(2,854)
Treated Water Outsourcing [Member]
Variable Interest Entity [Line Items]
Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage	50.00%
Cash	$ 1,380	2,088
Current assets	3,202	4,016
Property, plant, and equipment, net	903	1,145
Goodwill	2,206	2,206
Liabilities	(1,009)	(1,324)
Revenue	3,315	5,944	10,633
Operating Expenses	(2,922)	(4,519)	(8,732)
Earnings before interest, taxes, depreciation and amortization (EBITDA)	$ 393	$ 1,425	$ 1,901