EMPLOYEE BENEFIT PLANS - PENSION AND OTHER POSTEMPLOYMENT BENEFIT PLANS, NARRATIVE (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2017 USD ($) plan	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of noncontributory defined benefit retirement plans (in plans) | plan	2
Required number of years of service (more than)	1 year
Years of service and average compensation, basis period for plan benefits	60 months
Long-term real rate of return on assets (as a percent)	5.00%
OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions	$ 6,049	$ 3,235	$ 5,700
OPEB | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated future employer contributions over the next five years	4,000
OPEB | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated future employer contributions over the next five years	7,000
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer discretionary contributions	0	30,000	$ 0
Employer contributions	$ 74	$ 30,071