COMMITMENTS AND CONTINGENT LIABILITIES - LEGAL PROCEEDINGS (Details)	3 Months Ended	6 Months Ended	9 Months Ended
	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2019 USD ($) site	Sep. 30, 2018 USD ($)
Site Contingency [Line Items]
Number of MGP sites | site			5
Environmental remediation expense			$ 600,000
Environmental remediation expense, incurred to date			1,500,000
Manufactured gas plant remediation	$ 125,566,000		$ 125,566,000	$ 130,800,000
Recovery from third party of environmental remediation cost, period			7 years
Regulatory assets	388,769,000		$ 388,769,000	368,592,000
Minimum
Site Contingency [Line Items]
Product liability contingency, loss exposure in excess of accrual, best estimate			117,700,000
Maximum
Site Contingency [Line Items]
Product liability contingency, loss exposure in excess of accrual, best estimate			204,100,000
Expended, net of recoveries
Site Contingency [Line Items]
Regulatory assets	34,340,000		$ 34,340,000	$ 33,017,000
RAC
Site Contingency [Line Items]
Approved rate, amount	$ 8,500,000	$ 7,100,000
NJDEP
Site Contingency [Line Items]
Number of MGP sites | site			2