REVENUE - Accounts Receivable (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	$ 231,434	$ 223,314	$ 253,661	$ 337,502
Increase (Decrease) in contracts with customers, asset	(22,227)	(114,188)
Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	16,148	21,253	84,821	79,538
Increase (Decrease) in contracts with customers, asset	(68,673)	(58,285)
SJG | Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	91,842	90,190	84,940	101,572
Increase (Decrease) in contracts with customers, asset	6,902	(11,382)
SJG | Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	3,915	8,415	$ 45,016	$ 43,271
Increase (Decrease) in contracts with customers, asset	$ (41,101)	$ (34,856)