SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE LOSS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	$ 1,698,214	$ 1,608,092	$ 1,643,896	$ 1,345,615
Balance as of end of period	1,821,833	1,654,737	1,821,833	1,654,737
Cash Flow Hedges
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(10,166)	0	(10,397)	0
Other comprehensive (loss) income, before reclassifications, net of tax		(9,711)		(9,711)
Amounts reclassified from accumulated other comprehensive income, net of tax	263	0	494	0
Net current-period, other comprehensive income, net of tax		(9,711)		(9,711)
Balance as of end of period	(9,903)	(9,711)	(9,903)	(9,711)
Tax on amounts reclassified from accumulated other comprehensive income	(79)	0	(191)	0
Other comprehensive income (loss), tax		2,781		2,781
Other comprehensive income (loss) before reclassifications, tax		2,781		2,781
Postemployment Benefit Obligation
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(33,106)	(31,028)	(33,918)	(31,787)
Other comprehensive (loss) income, before reclassifications, net of tax		5,378		5,378
Amounts reclassified from accumulated other comprehensive income, net of tax	813	722	1,625	1,481
Net current-period, other comprehensive income, net of tax		6,100		6,859
Balance as of end of period	(32,293)	(24,928)	(32,293)	(24,928)
Tax on amounts reclassified from accumulated other comprehensive income	(244)	(72)	(489)	(289)
Other comprehensive income (loss), tax		(1,753)		(1,970)
Other comprehensive income (loss) before reclassifications, tax		(1,681)		(1,681)
Total
Increase (Decrease) in Accumulated Other Comprehensive Income [Roll Forward]
Balance as of beginning of period	(43,272)	(31,028)	(44,315)	(31,787)
Other comprehensive (loss) income, before reclassifications, net of tax		(4,333)		(4,333)
Amounts reclassified from accumulated other comprehensive income, net of tax	1,076	722	2,119	1,481
Net current-period, other comprehensive income, net of tax		(3,611)		(2,852)
Balance as of end of period	(42,196)	(34,639)	(42,196)	(34,639)
Tax on amounts reclassified from accumulated other comprehensive income	$ (323)	(72)	$ (680)	(289)
Other comprehensive income (loss), tax		1,028		811
Other comprehensive income (loss) before reclassifications, tax		$ 1,100		$ 1,100