REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 56,866	$ 32,871
Regulatory assets-noncurrent	450,039	496,637
Regulatory liabilities-noncurrent	199,737	202,435
Tax Act impact
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	197,928	200,417
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	1,318	197
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	491	1,821
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	5,868	15,468
Under-recovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	9,246	9,506
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	23,481	3,371
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	16,231	4,526
Regulatory assets-noncurrent	0	486
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	35,856	38,351
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	128,868	131,080
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	21,049	19,631
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	12,101	10,201
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	165,577	212,461
Deferred storm damage costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	7,601	8,687
Cost of removal
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	62,488	65,660
Other regulatory assets (liabilities)
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	2,040	0
Regulatory assets-noncurrent	$ 16,499	$ 10,080