STOCK-BASED COMPENSATION - Summary of Compensation Cost for Stock-Based Payment Arrangements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 7,624	$ 8,878	$ 28,576	$ 25,467
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	422	719	1,409	3,073
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	8,716	6,771	24,359	19,374
Performance share units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	1,969	1,388	6,291	3,020
Stock-based compensation benefit included in restructuring charges
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ (3,483)	$ 0	$ (3,483)	$ 0