REGULATION, REGULATORY ASSETS AND LIABILITIES (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Sep. 30, 2011
Regulatory Assets And Liabilities [Line Items]
Regulatory assets current	$ 80,920	$ 17,630
Regulatory assets noncurrent	410,901	434,185
Regulatory liability current	0	4,633
Regulatory liability noncurrent	63,391	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	63,133	59,752
Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liability noncurrent	258	85
Underrecovered gas costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets current	16,544	0
Conservation Incentive Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets current	20,871	9,178
Derivatives, net [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets current	43,505	8,452
Enviromental remediation costs expended, net of recoveries [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	61,259	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	10,879	10,879
Energy Efficiency Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	17,913	11,906
New Jersey Clean Energy Program [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	13,522	20,144
Post Employment Benefit Costs (Note 10) [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	120,247	123,827
Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets noncurrent	$ 4,181	$ 8,883