REGULATION, REGULATORY ASSETS AND LIABILITIES (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Sep. 30, 2011
Regulatory Assets And Liabilities [Line Items]
Regulatory assets current	$ 52,940	$ 17,630
Regulatory assets noncurrent	411,528	434,185
Regulatory liability current	0	4,633
Regulatory liability noncurrent	66,100	59,837
Overrecovered gas costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liability current	0	4,633
Cost of removal obligation [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liability noncurrent	66,007	59,752
Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liability noncurrent	93	85
Conservation Incentive Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets current	26,240	9,178
Underrecovered gas costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets current	8,772	0
Derivatives, net [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets current	17,928	8,452
Enviromental remediation costs expended, net of recoveries [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	59,683	75,646
Enviromental remediation costs liability for future expenditures [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	182,900	182,900
Deferred income and other taxes [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	11,574	10,879
Energy Efficiency Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	21,944	11,906
New Jersey Clean Energy Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	7,434	20,144
Post Employment Benefit Costs (Note 10) [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	118,456	123,827
Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	$ 9,537	$ 8,883