EMPLOYEE RETIREMENT PLANS - Purchase Obligations in Foreign Jurisdictions (Details) - Postretirement obligations - Benefit plan KMG employees - Foreign Plan - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 11,627	$ 11,121
Accumulated benefit obligation	8,680	8,314
Pension cost included in Accumulated other comprehensive income (loss)	$ (764)	$ (1,811)
Weighted average discount rate	1.32%	0.73%
Weighted average rate of increases in future compensation levels	3.01%	2.89%