ACCUMULATED OTHER COMPREHENSIVE LOSS (AOCL) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		$ 1,289,240	$ 1,037,539	$ 932,432
Other comprehensive income before reclassifications		(10,920)
Amounts reclassified from AOCL		1,536
Other Comprehensive (Loss) Income - Net of Tax		(9,384)	(2,882)	5,759
Balance		1,192,409	1,289,240	1,037,539
Postretirement Liability Adjustment (A)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		(25,342)
Other comprehensive income before reclassifications		(10,920)
Amounts reclassified from AOCL		0
Other Comprehensive (Loss) Income - Net of Tax	[1]	(10,920)	(3,197)	5,518
Balance		(36,262)	(25,342)
Unrealized Gain (Loss) on Derivatives-Other (B)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		(1,932)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		1,536
Other Comprehensive (Loss) Income - Net of Tax		1,536
Balance		(396)	(1,932)
Unrealized Gain (Loss) on Available- for-Sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		(10)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		0
Other Comprehensive (Loss) Income - Net of Tax		0
Balance		(10)	(10)
Other Comprehensive Income (Loss) of Affiliated Companies (B)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		(97)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		0
Other Comprehensive (Loss) Income - Net of Tax		0
Balance		(97)	(97)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		(27,381)	(24,499)	(30,258)
Other Comprehensive (Loss) Income - Net of Tax		(9,384)	(2,882)	5,759
Balance		(36,765)	(27,381)	(24,499)
South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		839,900	707,927	680,568
Other comprehensive income before reclassifications		(11,090)
Amounts reclassified from AOCL		27
Other Comprehensive (Loss) Income - Net of Tax		(11,063)	(2,072)	1,617	[1]
Balance		921,433	839,900	707,927
South Jersey Gas Company | Postretirement Liability Adjustment (A)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		(14,417)
Other comprehensive income before reclassifications		(11,090)
Amounts reclassified from AOCL		0
Other Comprehensive (Loss) Income - Net of Tax	[1]	(11,090)	(2,197)	1,617
Balance		(25,507)	(14,417)
South Jersey Gas Company | Unrealized Gain (Loss) on Derivatives-Other (B)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		(517)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		27
Other Comprehensive (Loss) Income - Net of Tax		27
Balance		(490)	(517)
South Jersey Gas Company | Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance		(14,934)	(12,862)	(14,479)
Other Comprehensive (Loss) Income - Net of Tax		(11,063)	(2,072)	1,617	[1]
Balance		$ (25,997)	$ (14,934)	$ (12,862)
Post-Retirement Liability
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Combined average statutory tax rate		27.00%	40.00%	40.00%
Post-Retirement Liability | South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Combined average statutory tax rate		27.00%	40.00%	40.00%
Unrealized Gain (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Combined average statutory tax rate		39.00%	40.00%	40.00%
Unrealized Gain (Loss) | South Jersey Gas Company
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Combined average statutory tax rate		39.00%	40.00%	40.00%

[1] Determined using a combined average statutory tax rate of 27% for 2017 and 40% for 2016 and 2015.