OTHER COMPREHENSIVE INCOME (LOSS) - Summary of other comprehensive income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-tax	$ 31,710	$ (9,344)	$ (39,454)
Tax	(5,595)	3,168	7,650
Net of tax	26,115	(6,176)	(31,804)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-tax	6,433	5,601	(8,460)
Tax	0	0	0
Net of tax	6,433	5,601	(8,460)
Pension and other defined benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-tax	22,583	(14,955)	(30,581)
Tax	(4,787)	3,171	7,526
Net of tax	17,796	(11,784)	(23,055)
Cash flow hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Pre-tax	2,694	10	(413)
Tax	(808)	(3)	124
Net of tax	$ 1,886	$ 7	$ (289)