SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Oct. 01, 2018
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	$ 1,889,107	$ 1,569,162	$ 1,551,717	$ 1,418,978
Balance as of end of period	1,838,927	1,572,197	1,838,927	1,572,197
Investments in Equity Securities
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	0	0	0	3,446
Other comprehensive (loss) income, before reclassifications, net of tax	0		0
Amounts reclassified from accumulated other comprehensive income, net of tax	0	0	0	0
Net current-period, other comprehensive income, net of tax	0		0
Reclassification to retained earnings					$ (3,446)
Balance as of end of period	0	0	0	0
Other comprehensive income (loss) before reclassifications, tax	0		0
Tax on amounts reclassified from accumulated other comprehensive income	0	0	0	0
Other comprehensive income (loss), tax	0		0
Cash Flow Hedges
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(9,711)	0	0	0
Other comprehensive (loss) income, before reclassifications, net of tax	(626)		(10,337)
Amounts reclassified from accumulated other comprehensive income, net of tax	0	0	0	0
Net current-period, other comprehensive income, net of tax	(626)		(10,337)
Reclassification to retained earnings
Balance as of end of period	(10,337)	0	(10,337)	0
Other comprehensive income (loss) before reclassifications, tax	180		2,961
Tax on amounts reclassified from accumulated other comprehensive income	0	0	0	0
Other comprehensive income (loss), tax	180		2,961
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(24,928)	(15,517)	(31,787)	(16,056)
Other comprehensive (loss) income, before reclassifications, net of tax	0		5,378
Amounts reclassified from accumulated other comprehensive income, net of tax	663	305	2,144	844
Net current-period, other comprehensive income, net of tax	663		7,522
Reclassification to retained earnings					0
Balance as of end of period	(24,265)	(15,212)	(24,265)	(15,212)
Other comprehensive income (loss) before reclassifications, tax	0		(1,681)
Tax on amounts reclassified from accumulated other comprehensive income	(191)	(118)	(480)	(333)
Other comprehensive income (loss), tax	(191)		(2,161)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(34,639)	(15,517)	(31,787)	(12,610)
Other comprehensive (loss) income, before reclassifications, net of tax	(626)		(4,959)
Amounts reclassified from accumulated other comprehensive income, net of tax	663	305	2,144	844
Net current-period, other comprehensive income, net of tax	37		(2,815)
Reclassification to retained earnings					$ (3,446)
Balance as of end of period	(34,602)	(15,212)	(34,602)	(15,212)
Other comprehensive income (loss) before reclassifications, tax	180		1,280
Tax on amounts reclassified from accumulated other comprehensive income	(191)	$ (118)	(480)	$ (333)
Other comprehensive income (loss), tax	$ (11)		$ 800