REGULATION - REGULATORY ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	$ 24,246	$ 32,871	$ 32,871
Regulatory assets-noncurrent	490,283	496,637	496,637
Regulatory liabilities-noncurrent	203,313	$ 202,435	202,435
Tax Act impact
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	199,169		200,417
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	3,598		197
Other noncurrent regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities-noncurrent	546		1,821
New Jersey Clean Energy Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	13,757		15,468
Underrecovered gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	1,481		9,506
Conservation Incentive Program
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	1,427		3,371
Derivatives at fair value, net
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	5,562		4,526
Regulatory assets-noncurrent	0		486
Expended, net of recoveries
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	38,170		38,351
Liability for future expenditures
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	129,241		131,080
Deferred income taxes
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	19,714		19,631
SAVEGREEN
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	11,918		10,201
Postemployment and other benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	208,371		212,461
Deferred storm damage costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	8,144		8,687
Cost of removal
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-noncurrent	64,777		65,660
Other regulatory assets (liabilities)
Regulatory Assets And Liabilities [Line Items]
Regulatory assets-current	2,019		0
Regulatory assets-noncurrent	$ 9,948		$ 10,080