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PENSION AND OTHER POSTRETIREMENT PLANS - Net Periodic Pension and Other Postretirement Benefit Costs (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2014
Pension      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 7,446 $ 6,764 $ 6,150
Interest cost 9,725 8,740 8,927
Expected return on plan assets (9,625) (10,151) (9,666)
Prior service cost 183 180 206
Net actuarial loss (gain) 7,468 6,203 3,927
Net benefit cost 14,831 11,376 9,132
Other Postretirement      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 402 454 436
Interest cost 845 885 919
Expected return on plan assets 0 0 0
Prior service cost 195 195 195
Net actuarial loss (gain) 0 0 (87)
Net benefit cost $ 1,052 $ 1,144 $ 1,073