SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Oct. 01, 2018
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period		$ 1,569,162	$ 1,467,374	$ 1,418,978	$ 1,236,643
Balance as of end of period		1,572,197	1,449,962	1,572,197	1,449,962
Investments in Equity Securities
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period		0	0	3,446	11,044
Other comprehensive (loss), before reclassifications, net of tax			(2,364)		(25,055)
Amounts reclassified from accumulated other comprehensive income, net of tax		0	0	0	11,647
Net current-period other comprehensive income, net of tax			(2,364)		(13,408)
Reclassification to retained earnings	[1]					$ (3,446)
Balance as of end of period		0	(2,364)	0	(2,364)
Tax on other comprehensive income (loss) before reclassifications			854		9,071
Tax on amounts reclassified from accumulated other comprehensive income		0	0	0	(858)
Tax on net current-period other comprehensive income (loss)			854		8,213
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period		(15,517)	(13,788)	(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	844	784
Net current-period other comprehensive income, net of tax			272		784
Reclassification to retained earnings						0
Balance as of end of period		(15,212)	(13,516)	(15,212)	(13,516)
Tax on other comprehensive income (loss) before reclassifications			0		0
Tax on amounts reclassified from accumulated other comprehensive income		(118)	(104)	(333)	(344)
Tax on net current-period other comprehensive income (loss)			(104)		(344)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period		(15,517)	(13,788)	(12,610)	(3,256)
Other comprehensive (loss), before reclassifications, net of tax			(2,364)		(25,055)
Amounts reclassified from accumulated other comprehensive income, net of tax		305	272	844	12,431
Net current-period other comprehensive income, net of tax			(2,092)		(12,624)
Reclassification to retained earnings						$ (3,446)
Balance as of end of period		(15,212)	(15,880)	(15,212)	(15,880)
Tax on other comprehensive income (loss) before reclassifications			854		9,071
Tax on amounts reclassified from accumulated other comprehensive income		$ (118)	(104)	$ (333)	(1,202)
Tax on net current-period other comprehensive income (loss)			$ 750		$ 7,869

[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.