Summary of Significant Accounting Policies (Variable Interest Entities) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Oct. 01, 2019
Variable Interest Entity [Line Items]
Current assets	$ 695,712			$ 637,220
Property, plant and equipment	364,461	$ 333,584		336,396
Goodwill	397,205	392,890	$ 411,346
Other non-current assets	27,509	25,715
Total liabilities	(1,362,395)			$ (1,418,175)
Total revenues	1,429,456	1,444,441	1,339,541
Total operating expenses	(341,962)	(371,249)	(345,702)
Income before interest expense and income taxes	168,448	59,672	66,846
Treated Water Outsourcing [Member]
Variable Interest Entity [Line Items]
Cash	2,088	3,903
Current assets	4,016	6,324
Property, plant and equipment	1,145	2,186
Goodwill	2,206	2,206
Other non-current assets	0	3
Total liabilities	(1,324)	(2,388)
Total revenues	5,944	10,633	15,526
Total operating expenses	(4,519)	(8,732)	(12,996)
Income before interest expense and income taxes	1,425	$ 1,901	$ 2,530
Frontier [Member]
Variable Interest Entity [Line Items]
Cash	1,675
Current assets	4,024
Property, plant and equipment	3,159
Goodwill	1,798
Other non-current assets	9,918
Total liabilities	(3,692)
Total revenues	5,365
Total operating expenses	(8,219)
Income before interest expense and income taxes	$ (2,854)