REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 41,574	$ 50,791
Regulatory assets-noncurrent	362,150	375,919
Regulatory liability-current	434	78
Regulatory liabilities-noncurrent	237,838	14,507
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	229,315	0
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory liability-current	434	78
Regulatory liabilities-noncurrent	0	146
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	7,527	5,795
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	996	664
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	12,626	14,202
Underrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	490	9,910
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	18,471	9,010
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	9,987	17,669
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	31,236	28,547
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	144,046	149,000
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	16,356	21,795
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	13,419	16,302
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	138,681	141,433
Deferred Post-Tropical Cyclone Sandy costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	12,487	13,030
Other noncurrent regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	$ 5,925	$ 5,812