REVENUE - Accounts Receivable (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	$ 327,832	$ 262,850	$ 278,723	$ 253,661
Increase (Decrease) in contracts with customers, asset	49,109	9,189
Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	68,155	51,973	85,423	84,821
Increase (Decrease) in contracts with customers, asset	(17,268)	(32,848)
SJG | Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	130,286	112,606	88,657	84,940
Increase (Decrease) in contracts with customers, asset	41,629	27,666
SJG | Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	35,362	24,916	$ 46,837	$ 45,016
Increase (Decrease) in contracts with customers, asset	$ (11,475)	$ (20,100)