REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 43,140	$ 54,286
Regulatory assets-noncurrent	408,481	441,294
Regulatory liability-current	12,120	9,469
Regulatory liabilities-noncurrent	38,726	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	12,120	0
Cost of removal obligation
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	25,318	30,549
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	12,659	10,657
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	749	205
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	28,662	36,957
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	13,180	14,232
Underrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	1,298	0
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	0	3,097
Regulatory assets-noncurrent	2,702	23,384
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	20,025	19,595
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	168,506	172,000
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	20,599	20,273
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	22,060	25,208
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	154,221	157,027
Deferred Superstorm Sandy costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	14,659	15,201
Other noncurrent regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	$ 5,709	$ 8,606