SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	$ 1,990,735	$ 1,817,210	$ 1,630,862
Other comprehensive income, before reclassifications, net of tax	3,360	(6,350)
Amounts reclassified from accumulated other comprehensive income, net of tax	78	1,217
Other comprehensive income (loss), net of tax	3,438	(5,133)	29,702
Balance as of end of period	2,200,443	1,990,735	1,817,210
Tax on other comprehensive income before reclassifications	(1,002)	1,922
Tax on amounts reclassified from accumulated other comprehensive income	(21)	(366)
Tax on net current-period other comprehensive income	(1,023)	1,556
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	(9,959)	(4,826)	(34,528)
Other comprehensive income (loss), net of tax	3,438	(5,133)	29,702
Balance as of end of period	(6,521)	(9,959)	(4,826)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	(7,269)	(8,322)
Other comprehensive income, before reclassifications, net of tax	0	0
Amounts reclassified from accumulated other comprehensive income, net of tax	1,054	1,053
Other comprehensive income (loss), net of tax	1,054	1,053
Balance as of end of period	(6,215)	(7,269)	(8,322)
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	(2,690)	3,496
Other comprehensive income, before reclassifications, net of tax	3,360	(6,350)
Amounts reclassified from accumulated other comprehensive income, net of tax	(976)	164
Other comprehensive income (loss), net of tax	2,384	(6,186)
Balance as of end of period	(306)	(2,690)	$ 3,496
Tax on other comprehensive income before reclassifications	(1,002)	1,922
Tax on amounts reclassified from accumulated other comprehensive income	296	(49)
Tax on net current-period other comprehensive income	$ (706)	$ 1,873