EMPLOYEE BENEFIT PLANS - WEIGHTED AVERAGE ASSUMPTIONS (Details)	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Pension
Benefit costs:
Discount rate		4.50%	4.55%
Expected asset return	7.75%	8.75%	8.75%
Compensation increase			3.25%
Obligations:
Discount rate	4.03%		4.50%
Pension | Represented
Benefit costs:
Discount rate	3.96%
Compensation increase	3.25%	3.25%
Obligations:
Discount rate		3.96%
Compensation increase	3.25%	3.25%	3.25%
Pension | Nonrepresented
Benefit costs:
Discount rate	3.94%
Compensation increase	3.50%	3.50%
Obligations:
Discount rate		3.94%
Compensation increase	3.50%	3.50%	3.50%
OPEB
Benefit costs:
Discount rate			4.55%
Expected asset return	7.75%	8.75%	8.75%
Compensation increase		3.50%	3.50%
Obligations:
Compensation increase		3.50%	3.50%
OPEB | Represented
Benefit costs:
Discount rate	4.08%	4.60%
Compensation increase	3.25%
Obligations:
Discount rate	4.12%	4.08%	4.60%
Compensation increase	3.25%
OPEB | Nonrepresented
Benefit costs:
Discount rate	4.01%	4.55%
Compensation increase	3.50%
Obligations:
Discount rate	4.08%	4.01%	4.55%
Compensation increase	3.50%