REVENUE - Accounts Receivable (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	$ 262,850	$ 317,917	$ 253,661	$ 337,502
Increase (Decrease) in contracts with customers, asset	9,189	(19,585)
Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	51,973	68,155	84,821	79,538
Increase (Decrease) in contracts with customers, asset	(32,848)	(11,383)
SJG | Accounts Receivable
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	112,606	146,185	84,940	101,572
Increase (Decrease) in contracts with customers, asset	27,666	44,613
SJG | Unbilled Revenue
Disaggregation of Revenue [Line Items]
Contracts with customers, asset	24,916	38,292	$ 45,016	$ 43,271
Increase (Decrease) in contracts with customers, asset	$ (20,100)	$ (4,979)