REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 57,039	$ 54,286
Regulatory assets-noncurrent	388,767	441,294
Regulatory liability-current	76	9,469
Regulatory liabilities-noncurrent	20,050	41,411
Overrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory liability-current	0	9,469
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory liability-current	76	0
Cost of removal obligation
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	13,324	30,549
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	6,017	10,657
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	709	205
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	20,623	36,957
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	15,363	14,232
Underrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	11,476	0
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	9,577	3,097
Regulatory assets-noncurrent	112	23,384
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	23,076	19,595
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	161,016	172,000
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	21,919	20,273
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	15,286	25,208
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	148,494	157,027
Deferred Superstorm Sandy costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	13,573	15,201
Other noncurrent regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	$ 5,291	$ 8,606