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