SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	$ 1,817,210	$ 1,630,862	$ 1,643,896
Other comprehensive income, before reclassifications, net of tax	(6,350)	25,580
Amounts reclassified from accumulated other comprehensive income, net of tax	1,217	4,122
Other comprehensive (loss) income, net of tax	(5,133)	29,702	9,787
Balance as of end of period	1,990,735	1,817,210	1,630,862
Tax on other comprehensive income before reclassifications	1,922	(7,727)
Tax on amounts reclassified from accumulated other comprehensive income	(366)	(1,247)
Tax on net current-period other comprehensive income	1,556	(8,974)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	(4,826)	(34,528)	(44,315)
Other comprehensive (loss) income, net of tax	(5,133)	29,702	9,787
Balance as of end of period	(9,959)	(4,826)	(34,528)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	(8,322)	(9,376)
Other comprehensive income, before reclassifications, net of tax	0	0
Amounts reclassified from accumulated other comprehensive income, net of tax	1,053	1,054
Other comprehensive (loss) income, net of tax	1,053	1,054
Balance as of end of period	(7,269)	(8,322)	(9,376)
Tax on other comprehensive income before reclassifications	0	0
Tax on amounts reclassified from accumulated other comprehensive income	(317)	(317)
Tax on net current-period other comprehensive income	(317)	(317)
Postemployment Benefit Obligation
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	3,496	(25,152)
Other comprehensive income, before reclassifications, net of tax	(6,350)	25,580
Amounts reclassified from accumulated other comprehensive income, net of tax	164	3,068
Other comprehensive (loss) income, net of tax	(6,186)	28,648
Balance as of end of period	(2,690)	3,496	$ (25,152)
Tax on other comprehensive income before reclassifications	1,922	(7,727)
Tax on amounts reclassified from accumulated other comprehensive income	(49)	(930)
Tax on net current-period other comprehensive income	$ 1,873	$ (8,657)