PENSION AND OTHER POSTRETIREMENT BENEFITS - ESTIMATED COSTS THAT WILL BE AMORTIZED (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Pension Benefits
Regulatory Assets into Net Periodic Benefit Costs
Prior Service Cost/(Credit)	$ 124
Net Actuarial Loss	5,847
Accumulated Other Comprehensive Income (Loss) into Net Periodic Benefit Costs
Prior Service Cost/(Credit)	6
Net Actuarial Loss	5,665
Other Postretirement Benefits
Regulatory Assets into Net Periodic Benefit Costs
Prior Service Cost/(Credit)	(257)
Net Actuarial Loss	743
Accumulated Other Comprehensive Income (Loss) into Net Periodic Benefit Costs
Prior Service Cost/(Credit)	(87)
Net Actuarial Loss	213
South Jersey Gas Company | Pension Benefits
Accumulated Other Comprehensive Income (Loss) into Net Periodic Benefit Costs
Prior Service Cost/(Credit)	0
Net Actuarial Loss	4,143
South Jersey Gas Company | Other Postretirement Benefits
Accumulated Other Comprehensive Income (Loss) into Net Periodic Benefit Costs
Prior Service Cost/(Credit)	0
Net Actuarial Loss	$ 0