Annual report pursuant to Section 13 and 15(d)

PENSION AND OTHER POSTRETIREMENT PLANS - Net Periodic Pension and Other Postretirement Benefit Cost (Details)

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PENSION AND OTHER POSTRETIREMENT PLANS - Net Periodic Pension and Other Postretirement Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2017
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 7,998 $ 8,159 $ 8,553
Interest cost 9,202 8,210 7,362
Expected return on plan assets (10,304) (10,136) (9,249)
Prior service cost (186) (138) (181)
Net actuarial loss 4,245 7,018 10,034
Net benefit cost 10,955 13,113 16,519
Other Postretirement      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 244 335 392
Interest cost 718 631 626
Expected return on plan assets 0 0 0
Prior service cost (195) (195) (195)
Net actuarial loss (59) 0 0
Net benefit cost $ 708 $ 771 $ 823