REVENUE - Accounts Receivable (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable
Disaggregation of Revenue [Line Items]
Contract with Customer, Asset, Net, Current	$ 179,108	$ 220,561	$ 337,502	$ 202,379
Increase (Decrease) in Contract with Customer, Asset	(158,394)	18,182
Unbilled Revenues
Disaggregation of Revenue [Line Items]
Contract with Customer, Asset, Net, Current	18,714	29,313	79,538	73,377
Increase (Decrease) in Contract with Customer, Asset	(60,824)	(44,064)
SJG | Accounts Receivable
Disaggregation of Revenue [Line Items]
Contract with Customer, Asset, Net, Current	65,511	72,010	101,572	78,571
Increase (Decrease) in Contract with Customer, Asset	(36,061)	(6,561)
SJG | Unbilled Revenues
Disaggregation of Revenue [Line Items]
Contract with Customer, Asset, Net, Current	7,831	7,995	$ 43,271	$ 54,980
Increase (Decrease) in Contract with Customer, Asset	$ (35,440)	$ (46,985)