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EMPLOYEE BENEFIT PLANS - COMPONENTS OF NET PERIODIC COST (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Pension        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost $ 2,182 $ 2,056 $ 6,547 $ 6,167
Interest cost 2,278 2,647 6,834 7,940
Expected return on plan assets (5,037) (5,145) (15,112) (15,434)
Recognized actuarial loss 2,862 2,606 8,585 7,818
Prior service cost (credit) amortization 25 25 76 76
Net periodic benefit cost 2,310 2,189 6,930 6,567
OPEB        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service cost 1,211 1,213 3,633 3,640
Interest cost 1,518 1,757 4,553 5,270
Expected return on plan assets (1,625) (1,628) (4,873) (4,883)
Recognized actuarial loss 1,978 1,861 5,933 5,582
Prior service cost (credit) amortization (44) (49) (134) (148)
Net periodic benefit cost $ 3,038 $ 3,154 $ 9,112 $ 9,461