EMPLOYEE BENEFIT PLANS - WEIGHTED AVERAGE ASSUMPTIONS (Details)		12 Months Ended
		Sep. 30, 2018		Sep. 30, 2017		Sep. 30, 2016
Represented
Obligations:
Compensation increase				3.25%
Nonrepresented
Obligations:
Compensation increase				3.50%
Pension
Benefit costs:
Discount rate						4.50%
Expected asset return		7.50%		7.75%		8.75%
Obligations:
Discount rate	[1]			4.03%
Pension | Represented
Benefit costs:
Discount rate		4.04%		3.96%
Compensation increase		3.25%	[1]	3.25%	[1]	3.25%
Obligations:
Discount rate		4.36%				3.96%
Compensation increase	[1]	3.25%		3.25%		3.25%
Pension | Nonrepresented
Benefit costs:
Discount rate		4.03%		3.94%
Compensation increase		3.50%	[1]	3.50%	[1]	3.50%
Obligations:
Discount rate		4.35%				3.94%
Compensation increase	[1]	3.50%		3.50%		3.50%
OPEB
Benefit costs:
Expected asset return		7.50%		7.75%		8.75%
Compensation increase						3.50%
Obligations:
Compensation increase						3.50%
OPEB | Represented
Benefit costs:
Discount rate		4.12%	[1]	4.08%	[1]	4.60%
Compensation increase		3.25%	[1]	3.25%
Obligations:
Discount rate	[1]	4.38%		4.12%		4.08%
Compensation increase	[1]	3.25%
OPEB | Nonrepresented
Benefit costs:
Discount rate		4.08%	[1]	4.01%	[1]	4.55%
Compensation increase		3.50%	[1]	3.50%
Obligations:
Discount rate	[1]	4.37%		4.08%		4.01%
Compensation increase	[1]	3.50%

[1] Percentages for represented and nonrepresented plans, respectively.