REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 39,342	$ 36,530
Overrecovered natural gas costs	16,941	26,188
Regulatory liabilities-noncurrent	195,282	196,450
Adelphia
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	329	158
Regulatory assets-noncurrent	1,213	997
Regulatory liabilities-noncurrent	166	0
NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	39,013	36,372
Regulatory assets-noncurrent	520,124	526,462
Overrecovered natural gas costs	16,941	26,188
Regulatory liabilities-noncurrent	195,116	196,286
Total current regulatory liabilities | NJNG
Regulatory Assets And Liabilities [Line Items]
Overrecovered natural gas costs	16,941	25,914
Derivatives at fair value, net | NJNG
Regulatory Assets And Liabilities [Line Items]
Overrecovered natural gas costs	0	274
Regulatory liabilities-noncurrent	0	352
Tax Act impact | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	194,155	195,425
Other noncurrent regulatory liabilities | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	961	509
New Jersey Clean Energy Program | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	13,508	15,570
Conservation Incentive Program | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	20,689	19,120
Derivatives at fair value, net | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	3,194	0
Regulatory assets-noncurrent	8	1
Other current regulatory assets | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	1,622	1,682
Regulatory assets-noncurrent	17,517	20,068
Expended, net of recoveries | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	36,892	36,516
Liability for future expenditures | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	148,002	150,590
Deferred income taxes | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	32,406	28,241
SAVEGREEN | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	21,825	21,281
Postemployment and other benefit costs | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	184,265	188,170
Deferred storm damage costs | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	5,972	6,515
Cost of removal | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	$ 73,237	$ 75,080