REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Sep. 30, 2016
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 43,344	$ 54,286
Regulatory assets-noncurrent	396,924	441,294
Regulatory liability-current	5,747	9,469
Regulatory liabilities-noncurrent	28,457	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	5,747	0
Cost of removal obligation
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	17,224	30,549
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	10,269	10,657
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	964	205
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	23,834	36,957
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	6,239	14,232
Underrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	13,271	0
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	0	3,097
Regulatory assets-noncurrent	4,392	23,384
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	19,067	19,595
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	165,849	172,000
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	21,209	20,273
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	15,604	25,208
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	151,414	157,027
Deferred Superstorm Sandy costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	14,116	15,201
Other noncurrent regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	$ 5,273	$ 8,606