Summary of Significant Accounting Policies - Treated Water Outsourcing (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2017
Subsidiary or Equity Method Investee [Line Items]
Current assets (including cash of $1,907 and $2,297)	$ 536,835		$ 536,835		$ 512,240
Cash	2,297		2,297		1,907
Property, plant and equipment	289,178		289,178		280,043
Goodwill	324,272		324,272		321,913
Other non-current assets	22,611		22,611		22,399
Total liabilities	(1,136,005)		(1,136,005)		(1,256,734)
Total revenues	342,475	$ 311,142	973,215	$ 890,916
Total operating expenses	(94,531)	(73,674)	(255,582)	(244,947)
Income from operations	36,400	44,716	112,398	88,675
Treated Water Outsourcing [Member]
Subsidiary or Equity Method Investee [Line Items]
Current assets (including cash of $1,907 and $2,297)	8,188		8,188		12,006
Property, plant and equipment	4,726		4,726		6,107
Goodwill	2,206		2,206		2,206
Other non-current assets	2,735		2,735		2,735
Total liabilities	(9,500)		(9,500)		$ (12,781)
Total revenues	3,020	3,570	12,586	11,645
Total operating expenses	(2,535)	(3,089)	(9,732)	(8,857)
Income from operations	$ 485	$ 481	$ 2,854	$ 2,788