COMMITMENTS AND CONTINGENT LIABILITIES - LEGAL PROCEEDINGS (Details)		12 Months Ended
	Apr. 01, 2018 USD ($)	Sep. 30, 2017 USD ($) site	Sep. 30, 2016 USD ($)
Loss Contingencies [Line Items]
Number of manufactured gas plant sites (in sites) | site		5
Recovery period		7 years
Regulatory assets		$ 375,919,000	$ 441,294,000
Minimum
Loss Contingencies [Line Items]
Litigation settlement, gross		117,600,000
Loss contingency, estimate of possible loss		600,000
Maximum
Loss Contingencies [Line Items]
Litigation settlement, gross		205,200,000
Loss contingency, estimate of possible loss		3,200,000.0
Environmental remediation costs, Expended, net of recoveries
Loss Contingencies [Line Items]
Regulatory assets		28,547,000	19,595,000
Environmental remediation costs, Liability for future expenditures
Loss Contingencies [Line Items]
Regulatory assets		149,000,000	$ 172,000,000
RAC
Loss Contingencies [Line Items]
Approved rate, amount		$ 9,400,000
Scenario, Forecast | RAC
Loss Contingencies [Line Items]
Approved rate, amount	$ 7,000,000