REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 52,251	$ 32,871
Regulatory assets-noncurrent	451,101	496,637
Regulatory liabilities-current	6,774	0
Regulatory liabilities-noncurrent	197,281	202,435
Over-recovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-current	6,774	0
Tax Act impact
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	196,705	200,417
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	128	197
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	448	1,821
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	17,062	15,468
Under-recovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	0	9,506
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	20,836	3,371
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	9,845	4,526
Regulatory assets-noncurrent	0	486
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	36,053	38,351
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	128,886	131,080
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	22,477	19,631
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	15,229	10,201
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	161,847	212,461
Deferred storm damage costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	7,058	8,687
Cost of removal
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	62,831	65,660
Other regulatory assets (liabilities)
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	4,508	0
Regulatory assets-noncurrent	$ 16,720	$ 10,080