REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Sep. 30, 2018
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		$ 32,920	$ 18,297
Regulatory assets-noncurrent		388,769	368,592
Regulatory liabilities-current		0	8,185
Regulatory liabilities-noncurrent		203,660	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		0	1,191
Regulatory liabilities-noncurrent		0	123
Tax Act impact
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	[1]	201,664	205,410
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent		207	1,902
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent		1,789	1,704
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		15,252	14,052
Underrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		9,949	4,137
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		2,065	0
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current		5,654	108
Regulatory assets-noncurrent		750	0
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		34,340	33,017
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		125,566	130,800
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		19,419	17,225
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		7,684	8,636
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		132,605	136,716
Deferred storm damage costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		9,229	10,858
Cost of removal
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		49,929	22,339
Other noncurrent regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent		$ 9,247	$ 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.