SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	$ 1,630,862	$ 1,643,896	$ 1,381,833
Other comprehensive income, before reclassifications, net of tax	25,580	5,494
Amounts reclassified from accumulated other comprehensive income, net of tax	4,122	4,293
Net current-period other comprehensive income, net of tax	29,702	9,787	(12,528)
Balance as of end of period	1,817,210	1,630,862	1,643,896
Tax on other comprehensive income before reclassifications	(7,727)	(1,618)
Tax on amounts reclassified from accumulated other comprehensive income	(1,247)	(1,307)
Tax on net current-period other comprehensive income	(8,974)	(2,925)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	(9,376)	(10,397)
Other comprehensive income, before reclassifications, net of tax	0	0
Amounts reclassified from accumulated other comprehensive income, net of tax	1,054	1,021
Net current-period other comprehensive income, net of tax	1,054	1,021
Balance as of end of period	(8,322)	(9,376)	(10,397)
Tax on other comprehensive income before reclassifications	0	0
Tax on amounts reclassified from accumulated other comprehensive income	(317)	(350)
Tax on net current-period other comprehensive income	(317)	(350)
Postemployment Benefit Obligation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	(25,152)	(33,918)
Other comprehensive income, before reclassifications, net of tax	25,580	5,494
Amounts reclassified from accumulated other comprehensive income, net of tax	3,068	3,272
Net current-period other comprehensive income, net of tax	28,648	8,766
Balance as of end of period	3,496	(25,152)	(33,918)
Tax on other comprehensive income before reclassifications	(7,727)	(1,618)
Tax on amounts reclassified from accumulated other comprehensive income	(930)	(957)
Tax on net current-period other comprehensive income	(8,657)	(2,575)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	(34,528)	(44,315)	(31,787)
Net current-period other comprehensive income, net of tax	29,702	9,787	(12,528)
Balance as of end of period	$ (4,826)	$ (34,528)	$ (44,315)