ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Changes (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ 1,192,409
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		9
Other Comprehensive Income - Net of Tax	[1]	9	$ 1,515
Ending balance		$ 1,281,497
Combined statutory tax rate		25.00%	40.00%
South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ 921,433
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		9
Other Comprehensive Income - Net of Tax	[2]	9	$ 7
Ending balance		$ 988,188
Combined statutory tax rate		25.00%	40.00%
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (36,765)
Ending balance		(36,756)
Postretirement Liability Adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(36,262)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		0
Other Comprehensive Income - Net of Tax		0
Ending balance		(36,262)
Unrealized Gain (Loss) on Derivatives-Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(396)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		9
Other Comprehensive Income - Net of Tax		9
Ending balance		(387)
Unrealized Gain (Loss) on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(10)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		0
Other Comprehensive Income - Net of Tax		0
Ending balance		(10)
Other Comprehensive Income (Loss) of Affiliated Companies
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(97)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		0
Other Comprehensive Income - Net of Tax		0
Ending balance		(97)
Total | South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(25,997)
Ending balance		(25,988)
Postretirement Liability Adjustment | South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(25,507)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		0
Other Comprehensive Income - Net of Tax		0
Ending balance		(25,507)
Unrealized Gain (Loss) on Derivatives-Other | South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(490)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		9
Other Comprehensive Income - Net of Tax		9
Ending balance		$ (481)

[1] Determined using a combined average statutory tax rate of approximately 25% and 40% in 2018 and 2017, respectively.

[2] Determined using a combined average statutory tax rate of approximately 25% and 40% in 2018 and 2017, respectively.