ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Changes (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				$ 1,289,240
Other comprehensive income before reclassifications		$ 0		0
Amounts reclassified from AOCL		7		1,529
Other Comprehensive Income - Net of Tax	[1]	7	$ 203	1,529	$ 407
Ending balance		$ 1,221,350		$ 1,221,350
Combined statutory tax rate		40.00%	40.00%	40.00%	40.00%
South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance				$ 839,900
Other comprehensive income before reclassifications		$ 0		0
Amounts reclassified from AOCL		7		21
Other Comprehensive Income - Net of Tax	[2]	7	$ 45	21	$ 66
Ending balance		$ 922,931		$ 922,931
Combined statutory tax rate		40.00%	40.00%	40.00%	40.00%
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (25,859)		$ (27,381)
Ending balance		(25,852)		(25,852)
Postretirement Liability Adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(25,342)		(25,342)
Other comprehensive income before reclassifications		0		0
Amounts reclassified from AOCL		0		0
Other Comprehensive Income - Net of Tax		0		0
Ending balance		(25,342)		(25,342)
Unrealized Gain (Loss) on Derivatives-Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(410)		(1,932)
Other comprehensive income before reclassifications		0		0
Amounts reclassified from AOCL		7		1,529
Other Comprehensive Income - Net of Tax		7		1,529
Ending balance		(403)		(403)
Unrealized Gain (Loss) on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(10)		(10)
Other comprehensive income before reclassifications		0		0
Amounts reclassified from AOCL		0		0
Other Comprehensive Income - Net of Tax		0		0
Ending balance		(10)		(10)
Other Comprehensive Income (Loss) of Affiliated Companies
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(97)		(97)
Other comprehensive income before reclassifications		0		0
Amounts reclassified from AOCL		0		0
Other Comprehensive Income - Net of Tax		0		0
Ending balance		(97)		(97)
Total | South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(14,920)		(14,934)
Ending balance		(14,913)		(14,913)
Postretirement Liability Adjustment | South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(14,417)		(14,417)
Other comprehensive income before reclassifications		0		0
Amounts reclassified from AOCL		0		0
Other Comprehensive Income - Net of Tax		0		0
Ending balance		(14,417)		(14,417)
Unrealized Gain (Loss) on Derivatives-Other | South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(503)		(517)
Other comprehensive income before reclassifications		0		0
Amounts reclassified from AOCL		7		21
Other Comprehensive Income - Net of Tax		7		21
Ending balance		$ (496)		$ (496)

[1] Determined using a combined average statutory tax rate of approximately 40%.

[2] Determined using a combined average statutory tax rate of approximately 40%.