REVENUE - Accounts Receivable (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	$ 167,534	$ 179,108	$ 253,661	$ 337,502
Increase (Decrease) in contracts with customers, asset	(86,127)	(158,394)
Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	22,712	18,714	84,821	79,538
Increase (Decrease) in contracts with customers, asset	(62,109)	(60,824)
SJG | Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	62,447	65,511	84,940	101,572
Increase (Decrease) in contracts with customers, asset	(22,493)	(36,061)
SJG | Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	9,985	7,831	$ 45,016	$ 43,271
Increase (Decrease) in contracts with customers, asset	$ (35,031)	$ (35,440)