REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands		Mar. 31, 2019	Sep. 30, 2018
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		$ 17,906	$ 18,297
Regulatory assets-noncurrent		376,420	368,592
Regulatory liabilities-current		3,717	8,185
Regulatory liabilities-noncurrent		208,869	209,139
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-current		0	6,994
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-current		3,717	1,191
Tax Act impact
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	[1]	202,912	205,410
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent		4,198	1,902
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent		1,759	1,827
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		5,772	14,052
Underrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		12,093	4,137
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		40	0
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		1	108
Regulatory assets-noncurrent		9	0
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		31,436	33,017
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		128,897	130,800
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		18,499	17,225
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		5,570	8,636
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		135,316	136,716
Deferred storm damage costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		9,772	10,858
Cost of removal
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		37,847	22,339
Other noncurrent regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		$ 9,074	$ 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.