REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Sep. 30, 2015
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 25,122	$ 24,258
Regulatory assets-noncurrent	447,226	410,155
Regulatory liability-current	6,572	12,154
Regulatory liabilities-noncurrent	50,664	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	6,572	6,987
Cost of Removal Obligation [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	38,179	54,880
Derivatives at Fair Value, Net [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	293	0
New Jersey Clean Energy Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	12,020	11,956
Other Noncurrent Regulatory Liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	172	697
Conservation Incentive Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	3,795	0
Regulatory assets-noncurrent	30,018	0
New Jersey Clean Energy Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	16,507	14,293
Derivatives at Fair Value, Net [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	4,820	9,965
Regulatory assets-noncurrent	22,632	5,153
Environmental Remediation Costs Expended, Net of Recoveries [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	18,543	18,886
Environmental Remediation Costs Liability for Future Expenditures [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	175,310	180,400
Deferred Income Taxes [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	19,635	17,460
SAVEGREEN [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	24,108	26,882
Postemployment and Other Benefit Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	134,202	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	$ 7,577	$ 5,537