REVENUE - Accounts Receivable (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	$ 259,045	$ 231,434	$ 278,723	$ 253,661
Increase (decrease) in contracts with customers, asset	(19,678)	(22,227)
Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	30,800	16,148	85,423	84,821
Increase (decrease) in contracts with customers, asset	(54,623)	(68,673)
SJG | Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	96,824	91,842	88,657	84,940
Increase (decrease) in contracts with customers, asset	8,167	6,902
SJG | Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	13,276	3,915	$ 46,837	$ 45,016
Increase (decrease) in contracts with customers, asset	$ (33,561)	$ (41,101)