EMPLOYEE BENEFIT PLANS - PENSION AND OTHER POSTEMPLOYMENT BENEFIT PLANS, NARRATIVE (Details)		3 Months Ended	12 Months Ended
		Dec. 31, 2015 USD ($)	Sep. 30, 2018 USD ($) plan	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
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%
Other assets, fair value			$ 1,800,000
OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions			6,222,000	$ 6,049,000	$ 3,200,000
OPEB | Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated future employer contributions over the next five years			5,000,000
OPEB | Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated future employer contributions over the next five years			8,000,000
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer discretionary contributions		$ 30,000,000	0.0	0
Employer contributions	[1]		137,000	$ 74,000
Special Termination Benefits | Voluntary Early Retirement Program
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension and other postretirement benefits cost			4,200,000
Other Severance Benefits | Voluntary Early Retirement Program
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension and other postretirement benefits cost			2,200,000
Natural Gas Distribution | Voluntary Early Retirement Program
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension and other postretirement benefits cost			5,100,000
Home Services and Other | Voluntary Early Retirement Program
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension and other postretirement benefits cost			$ 1,300,000

[1] Includes the Company’s PEP.