ACCUMULATED OTHER COMPREHENSIVE LOSS - Schedule of Change in Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,539,900	$ 1,468,119	$ 1,249,787
Other comprehensive income before reclassifications	950	14,646
Amounts reclassified from accumulated other comprehensive loss	156	200
Total other comprehensive income	1,106	14,846	16,206
Balance at end of period	1,398,341	1,539,900	1,468,119
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(19,094)	(33,940)	(50,146)
Total other comprehensive income	1,106	14,846	16,206
Balance at end of period	(17,988)	(19,094)	(33,940)
Defined benefit pension items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(10,408)	(10,801)
Other comprehensive income before reclassifications	(162)	193
Amounts reclassified from accumulated other comprehensive loss	156	200	501
Total other comprehensive income	(6)	393
Balance at end of period	(10,414)	(10,408)	(10,801)
Currency translation adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(8,686)	(23,139)
Other comprehensive income before reclassifications	1,112	14,453
Amounts reclassified from accumulated other comprehensive loss	0	0
Total other comprehensive income	1,112	14,453
Balance at end of period	$ (7,574)	$ (8,686)	$ (23,139)