SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCOUNTING STANDARDS UPDATE - COMPENSATION (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	$ 60,102	$ 54,160
Total operating expenses	723,024	629,109
Operating income	88,743	76,196
Other income (expense), net	869	$ 5,976
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	54,160
Total operating expenses	629,109
Operating income	76,196
Other income (expense), net	5,976
Accounting Standards Update 2017-07 | As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	55,111
Total operating expenses	630,060
Operating income	75,245
Other income (expense), net	6,927
Accounting Standards Update 2017-07 | Effect of Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	(951)
Total operating expenses	(951)
Operating income	951
Other income (expense), net	$ (951)