ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Changes (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 1,423,785	$ 1,267,022
Other comprehensive loss before reclassifications	0
Amounts reclassified from AOCL	8
Other Comprehensive Income - Net of Tax*	8	8
Ending Balance	1,497,283	1,514,554
Reclassification of unrealized loss on derivatives, tax benefit (expense)	(4)	(4)
SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	1,089,898	1,008,022
Other comprehensive loss before reclassifications	0
Amounts reclassified from AOCL	8
Other Comprehensive Income - Net of Tax*	8	8
Ending Balance	1,160,428	1,076,761
Reclassification of unrealized loss on derivatives, tax benefit (expense)	(4)	(4)
Postretirement Liability Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(32,124)
Other comprehensive loss before reclassifications	0
Amounts reclassified from AOCL	0
Other Comprehensive Income - Net of Tax*	0
Ending Balance	(32,124)
Postretirement Liability Adjustment | SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(27,454)
Other comprehensive loss before reclassifications	0
Amounts reclassified from AOCL	0
Other Comprehensive Income - Net of Tax*	0
Ending Balance	(27,454)
Unrealized Gain (Loss) on Derivatives-Other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(327)
Other comprehensive loss before reclassifications	0
Amounts reclassified from AOCL	8
Other Comprehensive Income - Net of Tax*	8
Ending Balance	(319)
Unrealized Gain (Loss) on Derivatives-Other | SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(421)
Other comprehensive loss before reclassifications	0
Amounts reclassified from AOCL	8
Other Comprehensive Income - Net of Tax*	8
Ending Balance	(413)
Unrealized Gain (Loss) on Available- for-Sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(10)
Other comprehensive loss 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
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(97)
Other comprehensive loss before reclassifications	0
Amounts reclassified from AOCL	0
Other Comprehensive Income - Net of Tax*	0
Ending Balance	(97)
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(32,558)	(26,095)
Other Comprehensive Income - Net of Tax*	8	8
Ending Balance	(32,550)	(26,087)
Total | SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(27,875)	(22,357)
Other Comprehensive Income - Net of Tax*	8	8
Ending Balance	$ (27,867)	$ (22,349)