Summary of Significant Accounting Policies (Treated Water Outsourcing) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2018
Variable Interest Entity [Line Items]
Assets, Current	$ 551,617		$ 551,617		$ 565,560
Revenues	348,628	$ 333,690	671,630	$ 630,740
Operating Expenses	(87,607)	(83,093)	(182,736)	(161,052)
Income from operations	11,468	25,808	5,109	35,635
Property, plant, and equipment, net	333,023		333,023		320,023
Goodwill	407,572		407,572		411,346
Other non‑current assets	25,418		25,418		23,842
Liabilities	(1,304,797)		(1,304,797)		(1,301,601)
Treated Water Outsourcing
Variable Interest Entity [Line Items]
Cash	4,557		4,557		3,304
Assets, Current	5,300		5,300		5,486
Revenues	3,222	3,577	6,378	9,566
Operating Expenses	(2,806)	(2,622)	(5,036)	(7,196)
Income from operations	416	$ 955	1,342	$ 2,370
Property, plant, and equipment, net	4,648		4,648		4,441
Goodwill	2,206		2,206		2,206
Other non‑current assets	3		3		3
Liabilities	$ (3,567)		$ (3,567)		$ (3,608)