SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCOUNTING STANDARDS UPDATE - COMPENSATION (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	$ 62,959	$ 56,797	$ 123,061	$ 110,957
Total operating expenses	789,254	840,299	1,512,278	1,469,408
Operating income	77,001	178,744	165,744	254,940
Other income (expense), net	2,758	$ 1,028	3,627	$ 7,004
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	56,797		110,957
Total operating expenses	840,299		1,469,408
Operating income	178,744		254,940
Other income (expense), net	1,028		7,004
Accounting Standards Update 2017-07 | As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	57,749		112,860
Total operating expenses	841,251		1,471,311
Operating income	177,792		253,037
Other income (expense), net	1,980		8,907
Accounting Standards Update 2017-07 | Effect of Change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operation and maintenance	(952)		(1,903)
Total operating expenses	(952)		(1,903)
Operating income	952		1,903
Other income (expense), net	$ (952)		$ (1,903)