EMPLOYEE RETIREMENT PLANS - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Contribution Plan Disclosure [Line Items]
Benefit costs	$ 1,403	$ 1,345	$ 1,236
North America
Defined Contribution Plan Disclosure [Line Items]
Plan expense	7,658	6,698	$ 5,562
KMG 401(k) Plan | Foreign Plan
Defined Contribution Plan Disclosure [Line Items]
Plan contributions	$ 1,766	$ 1,356