SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Oct. 01, 2018
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period		$ 1,497,050	$ 1,347,770	$ 1,418,978	$ 1,236,643
Balance as of end of period		1,569,162	1,467,374	1,569,162	1,467,374
Investments in Equity Securities
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period		0	5,600	3,446	11,044
Other comprehensive (loss), before reclassifications, net of tax			(20,401)		(22,691)
Amounts reclassified from accumulated other comprehensive income, net of tax		0	14,801	0	11,647
Net current-period other comprehensive income, net of tax			(5,600)		(11,044)
Reclassification to retained earnings	[1]					$ (3,446)
Balance as of end of period		0	0	0	0
Tax on other comprehensive income (loss) before reclassifications			(7,366)		(8,217)
Tax on amounts reclassified from accumulated other comprehensive income		0	3,036	0	858
Tax on net current-period other comprehensive income (loss)			(4,330)		(7,359)
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period		(15,822)	(14,060)	(16,056)	(14,300)
Other comprehensive (loss), before reclassifications, net of tax			0		0
Amounts reclassified from accumulated other comprehensive income, net of tax	[2]	305	272	539	512
Net current-period other comprehensive income, net of tax			272		512
Reclassification to retained earnings						0
Balance as of end of period		(15,517)	(13,788)	(15,517)	(13,788)
Tax on other comprehensive income (loss) before reclassifications			0		0
Tax on amounts reclassified from accumulated other comprehensive income		(119)	104	(215)	240
Tax on net current-period other comprehensive income (loss)			104		240
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period		(15,822)	(8,460)	(12,610)	(3,256)
Other comprehensive (loss), before reclassifications, net of tax			(20,401)		(22,691)
Amounts reclassified from accumulated other comprehensive income, net of tax		305	15,073	539	12,159
Net current-period other comprehensive income, net of tax			(5,328)		(10,532)
Reclassification to retained earnings						$ (3,446)
Balance as of end of period		(15,517)	(13,788)	(15,517)	(13,788)
Tax on other comprehensive income (loss) before reclassifications			(7,366)		(8,217)
Tax on amounts reclassified from accumulated other comprehensive income		$ (119)	3,140	$ (215)	1,098
Tax on net current-period other comprehensive income (loss)			$ (4,226)		$ (7,119)

[1] Due to the adoption of ASU No. 2016-01, an amendment to ASC 825, Financial Instruments. See Note 2. Summary of Significant Accounting Policies - Recently Adopted Updates to the Accounting Standards Codification section for more details.

[2] Included in the computation of net periodic pension cost, a component of operations and maintenance expense on the Unaudited Condensed Consolidated Statements of Operations.