REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 21,456	$ 50,791
Regulatory assets-noncurrent	348,079	375,919
Regulatory liabilities-current	7,482	78
Regulatory liabilities-noncurrent	211,431	14,507
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-current	6,637	0
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	206,832	0
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-current	845	78
Regulatory liabilities-noncurrent	466	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	2,484	5,795
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	1,649	664
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	15,533	14,202
Underrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	5,693	9,910
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	230	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	29,647	28,547
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	140,821	149,000
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	16,796	21,795
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	7,410	16,302
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	133,179	141,433
Deferred storm damage costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	11,401	13,030
Other noncurrent regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	$ 8,825	$ 5,812