Summary of Significant Accounting Policies (Variable Interest Entities) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Oct. 01, 2019	Sep. 30, 2019
Variable Interest Entity [Line Items]
Document Period End Date			Jun. 30, 2020
Total current assets	$ 642,722		$ 642,722		$ 637,220	$ 637,293
Operating Expenses	(77,504)	$ (84,765)	(264,267)	$ (267,501)
Income from operations	33,066	27,091	123,913	32,200
Property, plant, and equipment, net	356,424		356,424		336,396	333,584
Goodwill	393,427		393,427			392,890
Other non‑current assets	25,427		25,427			25,715
Liabilities	(1,330,566)		(1,330,566)		$ (1,418,175)	(1,372,026)
Treated Water Outsourcing [Member]
Variable Interest Entity [Line Items]
Cash	2,505		2,505			5,921
Total current assets	4,548		4,548			6,324
Revenues	1,328	2,573	5,180	8,951
Operating Expenses	(803)	(2,231)	(3,847)	(7,267)
Income from operations	525	$ 342	1,333	$ 1,684
Property, plant, and equipment, net	1,109		1,109			2,186
Goodwill	2,206		2,206			2,206
Other non‑current assets	3		3			3
Liabilities	(1,402)		(1,402)			$ (2,388)
Frontier [Member]
Variable Interest Entity [Line Items]
Cash	4,106		4,106
Revenues	1,411		4,134
Operating Expenses	(1,498)		(5,047)
Income from operations	(87)		(913)
Property, plant, and equipment, net	3,251		3,251
Goodwill	2,582		2,582
Other non‑current assets	0		0
Liabilities	$ (3,457)		$ (3,457)