REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 23,527	$ 36,530
Overrecovered natural gas costs	717	26,188
Regulatory liabilities-noncurrent	193,817	196,450
Adelphia
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	116	158
Regulatory assets-noncurrent	1,514	997
Regulatory liabilities-noncurrent	164	0
NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	23,411	36,372
Regulatory assets-noncurrent	520,514	526,462
Overrecovered natural gas costs	717	26,188
Regulatory liabilities-noncurrent	193,654	196,286
Total current regulatory liabilities | NJNG
Regulatory Assets And Liabilities [Line Items]
Overrecovered natural gas costs	541	25,914
Derivatives at fair value, net | NJNG
Regulatory Assets And Liabilities [Line Items]
Overrecovered natural gas costs	176	274
Regulatory liabilities-noncurrent	0	352
Tax Act impact | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	192,895	195,425
Other noncurrent regulatory liabilities | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	759	509
New Jersey Clean Energy Program | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	6,147	15,570
Conservation Incentive Program | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	14,025	19,120
Derivatives at fair value, net | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	1,509	0
Regulatory assets-noncurrent	4	1
Other current regulatory assets | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	1,730	1,682
Regulatory assets-noncurrent	17,952	20,068
Expended, net of recoveries | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	37,476	36,516
Liability for future expenditures | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	143,495	150,590
Deferred income taxes | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	31,603	28,241
SAVEGREEN | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	21,842	21,281
Postemployment and other benefit costs | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	180,360	188,170
Deferred storm damage costs | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	5,429	6,515
Cost of removal | NJNG
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	$ 82,353	$ 75,080