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, 2017
Sep. 30, 2016
Sep. 30, 2015
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 8,553 $ 7,446 $ 6,764
Interest cost 7,362 9,725 8,740
Expected return on plan assets (9,249) (9,625) (10,151)
Prior service cost (181) (183) (180)
Net actuarial loss (gain) 10,034 7,468 6,203
Net benefit cost 16,519 14,831 11,376
Other Postretirement      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 392 402 454
Interest cost 626 845 885
Expected return on plan assets 0 0 0
Prior service cost (195) (195) (195)
Net actuarial loss (gain) 0 0 0
Net benefit cost $ 823 $ 1,052 $ 1,144