SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	$ 2,200,443	$ 1,990,735	$ 1,817,210
Other comprehensive income, before reclassifications, net of tax	(5,213)	3,360
Amounts reclassified from accumulated other comprehensive income, net of tax	271	78
Other comprehensive (loss) income, net of tax	(4,942)	3,438	(5,133)
Balance as of end of period	2,391,666	2,200,443	1,990,735
Tax on other comprehensive income before reclassifications	1,571	(1,002)
Tax on amounts reclassified from accumulated other comprehensive income	(85)	(21)
Tax on net current-period other comprehensive income	1,486	(1,023)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	(6,521)	(9,959)	(4,826)
Other comprehensive (loss) income, net of tax	(4,942)	3,438	(5,133)
Balance as of end of period	(11,463)	(6,521)	(9,959)
Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance as of beginning of period	(6,215)	(7,269)
Other comprehensive income, before reclassifications, net of tax	0	0
Amounts reclassified from accumulated other comprehensive income, net of tax	1,052	1,054
Other comprehensive (loss) income, net of tax	1,052	1,054
Balance as of end of period	(5,163)	(6,215)	(7,269)
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	(306)	(2,690)
Other comprehensive income, before reclassifications, net of tax	(5,213)	3,360
Amounts reclassified from accumulated other comprehensive income, net of tax	(781)	(976)
Other comprehensive (loss) income, net of tax	(5,994)	2,384
Balance as of end of period	(6,300)	(306)	$ (2,690)
Tax on other comprehensive income before reclassifications	1,571	(1,002)
Tax on amounts reclassified from accumulated other comprehensive income	232	296
Tax on net current-period other comprehensive income	$ 1,803	$ (706)