REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 15,051	$ 50,791
Regulatory assets-noncurrent	352,152	375,919
Regulatory liability-current	29,169	78
Regulatory liabilities-noncurrent	217,510	14,507
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liability-current	229	0
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	210,500	0
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory liability-current	62	78
Regulatory liabilities-noncurrent	0	146
Customer refund payable
Regulatory Assets And Liabilities [Line Items]
Regulatory liability-current	28,878	0
Cost of removal obligation
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	0	7,902
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	5,914	5,795
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	1,096	664
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	5,690	14,202
Underrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	7,549	9,910
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	1,812	9,010
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	0	17,669
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	28,868	28,547
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	142,474	149,000
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	16,773	21,795
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	7,549	16,302
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	135,930	141,433
Deferred storm damage costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	11,944	13,030
Other noncurrent regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	$ 8,614	$ 5,812