REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Sep. 30, 2017
Regulatory Assets [Line Items]
Regulatory assets-current		$ 18,297	$ 50,791
Regulatory assets-noncurrent		368,592	375,919
Regulatory Liabilities [Line Items]
Regulatory liability-current		8,185	78
Regulatory liabilities-noncurrent		209,139	14,507
Conservation Incentive Program
Regulatory Liabilities [Line Items]
Regulatory liability-current		6,994	0
Tax Act impact
Regulatory Liabilities [Line Items]
Regulatory liabilities-noncurrent	[1]	205,410	0
Cost of removal
Regulatory Liabilities [Line Items]
Regulatory liabilities-noncurrent		0	7,902
New Jersey Clean Energy Program
Regulatory Liabilities [Line Items]
Regulatory liabilities-noncurrent		1,902	5,795
Other noncurrent regulatory liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities-noncurrent		1,827	810
Derivatives at fair value, net
Regulatory Liabilities [Line Items]
Regulatory liability-current		1,191	78
New Jersey Clean Energy Program
Regulatory Assets [Line Items]
Regulatory assets-current		14,052	14,202
Underrecovered gas costs
Regulatory Assets [Line Items]
Regulatory assets-current		4,137	9,910
Derivatives at fair value, net
Regulatory Assets [Line Items]
Regulatory assets-current		108	9,010
Conservation Incentive Program
Regulatory Assets [Line Items]
Regulatory assets-current		0	17,669
Environmental remediation costs, Expended, net of recoveries
Regulatory Assets [Line Items]
Regulatory assets-noncurrent		33,017	28,547
Environmental remediation costs, Liability for future expenditures
Regulatory Assets [Line Items]
Regulatory assets-noncurrent		130,800	149,000
Deferred income taxes
Regulatory Assets [Line Items]
Regulatory assets-noncurrent		17,225	21,795
SAVEGREEN
Regulatory Assets [Line Items]
Regulatory assets-noncurrent		8,636	16,302
Postemployment and other benefit costs
Regulatory Assets [Line Items]
Regulatory assets-noncurrent		136,716	141,433
Deferred Superstorm Sandy costs
Regulatory Assets [Line Items]
Regulatory assets-noncurrent		10,858	13,030
Cost of removal
Regulatory Assets [Line Items]
Regulatory assets-noncurrent		22,339	0
Other noncurrent regulatory assets
Regulatory Assets [Line Items]
Regulatory assets-noncurrent		$ 9,001	$ 5,812

[1] Includes an adjustment related to the re-measurement of NJNG's net deferred tax liabilities to reflect the change in federal tax rates enacted in the Tax Act, which is net of sales tax collected from customers. For a more detailed discussion, see Note 12. Income Taxes.