REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Sep. 30, 2015
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 32,638	$ 24,258
Regulatory assets-noncurrent	436,178	410,155
Regulatory liability-current	0	12,154
Regulatory liabilities-noncurrent	61,333	67,533
Conservation Incentive Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liability-current	0	5,167
Overrecovered Gas Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liability-current	0	6,987
Cost of Removal Obligation [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	46,348	54,880
New Jersey Clean Energy Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	14,087	11,956
Other Noncurrent Regulatory Liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	898	697
Conservation Incentive Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	3,190	0
Regulatory assets-noncurrent	28,686	0
New Jersey Clean Energy Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	5,586	14,293
Underrecovered Gas Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	5,052	0
Derivatives at Fair Value, Net [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	18,810	9,965
Regulatory assets-noncurrent	12,932	5,153
Environmental Remediation Costs Expended, Net of Recoveries [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	16,122	18,886
Environmental Remediation Costs Liability for Future Expenditures [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	178,512	180,400
Deferred Income Taxes [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	18,956	17,460
SAVEGREEN [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	23,009	26,882
Postemployment and Other Benefit Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	136,346	140,636
Deferred Superstorm Sandy Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	15,201	15,201
Other Noncurrent Regulatory Assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	$ 6,414	$ 5,537