Summary of Significant Accounting Policies - Treated Water Outsourcing (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Subsidiary or Equity Method Investee [Line Items]
Current assets (including cash of $1,907 and $3,033)	$ 530,872		$ 530,872		$ 512,240
Cash	3,033		3,033		1,907
Property, plant and equipment	285,687		285,687		280,043
Goodwill	329,898		329,898		321,913
Other non-current assets	22,747		22,747		22,399
Total liabilities	(1,138,783)		(1,138,783)		(1,256,734)
Total revenues	333,690	$ 299,901	630,740	$ 579,773
Income from operations	46,288	30,854	75,998	43,959
Treated Water Outsourcing [Member]
Subsidiary or Equity Method Investee [Line Items]
Current assets (including cash of $1,907 and $3,033)	8,043		8,043		12,006
Property, plant and equipment	5,704		5,704		6,107
Goodwill	2,206		2,206		2,206
Other non-current assets	0		0		2,735
Total liabilities	(6,509)		(6,509)		$ (12,781)
Total revenues	3,577	4,004	9,566	8,075
Total operating expenses	(2,622)	(3,485)	(7,196)	(5,768)
Income from operations	$ 955	$ 519	$ 2,370	$ 2,307