REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Sep. 30, 2018
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		$ 18,509	$ 18,297
Regulatory assets-noncurrent		370,024	368,592
Regulatory liabilities-current		12,822	8,185
Regulatory liabilities-noncurrent		210,580	209,139
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-current		7,861	6,994
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-current		4,961	1,191
Tax Act impact
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	[1]	204,161	205,410
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent		4,722	1,902
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent		1,697	1,827
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		12,359	14,052
Underrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		6,149	4,137
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		1	108
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		32,764	33,017
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		129,556	130,800
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		17,842	17,225
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		5,888	8,636
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		138,027	136,716
Deferred storm damage costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		10,315	10,858
Cost of removal
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		26,587	22,339
Other noncurrent regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		$ 9,045	$ 9,001

[1] Reflects the re-measurement and subsequent amortization of NJNG's net deferred tax liabilities as a result of the change in federal tax rates enacted in the Tax Act.