COMMITMENTS AND CONTINGENT LIABILITIES - LEGAL PROCEEDINGS (Details)	6 Months Ended
	Mar. 31, 2018 USD ($) site	Sep. 30, 2017 USD ($)
Site Contingency [Line Items]
Number of MGP sites | site	5
Recovery of expenditures, including carrying costs, rolling period approved by the BPU	7 years
Regulatory assets	$ 352,152,000	$ 375,919,000
Manufactured gas plant remediation	142,474,000	149,000,000
Minimum
Site Contingency [Line Items]
Product liability contingency, loss exposure in excess of accrual, best estimate	117,600,000
Loss contingency, estimate of possible loss	600,000
Maximum
Site Contingency [Line Items]
Product liability contingency, loss exposure in excess of accrual, best estimate	205,200,000
Loss contingency, estimate of possible loss	3,200,000
Expended, net of recoveries
Site Contingency [Line Items]
Regulatory assets	28,868,000	28,547,000
Liability for future expenditures
Site Contingency [Line Items]
Regulatory assets	142,474,000	$ 149,000,000
RAC
Site Contingency [Line Items]
Public utilities, approved rate, amount	9,400,000
Public utilities, requested rate, amount	$ 7,000,000