REVENUE - Accounts Receivable (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	$ 272,286	$ 167,534	$ 278,723	$ 253,661
Increase (decrease) in contracts with customers, asset	(6,437)	(86,127)
Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	26,783	22,712	85,423	84,821
Increase (decrease) in contracts with customers, asset	(58,640)	(62,109)
SJG | Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	83,031	62,447	88,657	84,940
Increase (decrease) in contracts with customers, asset	(5,626)	(22,493)
SJG | Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	8,288	9,985	$ 46,837	$ 45,016
Increase (decrease) in contracts with customers, asset	$ (38,549)	$ (35,031)