REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Regulatory Assets [Line Items]
Regulatory assets-current	$ 50,791	$ 54,286
Regulatory assets-noncurrent	375,919	441,294
Regulatory Liabilities [Line Items]
Regulatory liability-current	78	9,469
Regulatory liabilities-noncurrent	14,507	41,411
Overrecovered gas costs
Regulatory Liabilities [Line Items]
Regulatory liability-current	0	9,469
Cost of removal obligation
Regulatory Liabilities [Line Items]
Regulatory liabilities-noncurrent	7,902	30,549
New Jersey Clean Energy Program
Regulatory Liabilities [Line Items]
Regulatory liabilities-noncurrent	5,795	10,657
Other noncurrent regulatory liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities-noncurrent	664	205
Derivatives at fair value, net
Regulatory Liabilities [Line Items]
Regulatory liability-current	78	0
Regulatory liabilities-noncurrent	146	0
Conservation Incentive Program
Regulatory Assets [Line Items]
Regulatory assets-current	17,669	36,957
New Jersey Clean Energy Program
Regulatory Assets [Line Items]
Regulatory assets-current	14,202	14,232
Underrecovered gas costs
Regulatory Assets [Line Items]
Regulatory assets-current	9,910	0
Derivatives at fair value, net
Regulatory Assets [Line Items]
Regulatory assets-current	9,010	3,097
Regulatory assets-noncurrent	0	23,384
Environmental remediation costs, Expended, net of recoveries
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	28,547	19,595
Environmental remediation costs, Liability for future expenditures
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	149,000	172,000
Deferred income taxes
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	21,795	20,273
SAVEGREEN
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	16,302	25,208
Postemployment and other benefit costs
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	141,433	157,027
Deferred Superstorm Sandy costs
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	13,030	15,201
Other noncurrent regulatory assets
Regulatory Assets [Line Items]
Regulatory assets-noncurrent	$ 5,812	$ 8,606