SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCOUNTING STANDARDS UPDATE - COMPENSATION (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	$ 62,559	$ 68,496	$ 185,620	$ 179,453
Total operating expenses	438,961	580,150	1,951,239	2,049,558
Operating income	(4,019)	(36,715)	161,725	218,225
Other income (expense), net	1,829	$ 1,731	5,456	$ 8,735
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	68,496		179,453
Total operating expenses	580,150		2,049,558
Operating income	(36,715)		218,225
Other income (expense), net	1,731		8,735
Accounting Standards Update 2017-07 | As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	69,447		182,307
Total operating expenses	581,101		2,052,412
Operating income	(37,666)		215,371
Other income (expense), net	2,682		11,589
Accounting Standards Update 2017-07 | Effect of Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	(951)		(2,854)
Total operating expenses	(951)		(2,854)
Operating income	951		2,854
Other income (expense), net	$ (951)		$ (2,854)