ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Changes (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 1,497,283	$ 1,423,785	$ 1,514,554	$ 1,267,022	$ 1,423,785	$ 1,267,022
Other comprehensive loss before reclassifications	0				0
Amounts reclassified from AOCL	8				16
Other Comprehensive Income - Net of Tax	8	8	8	8	16	16
Ending Balance	1,666,329	1,497,283	1,476,472	1,514,554	1,666,329	1,476,472
Reclassification of unrealized loss on derivatives, tax benefit (expense)	(4)		(4)		(8)	(8)
SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	1,160,428	1,089,898	1,076,761	1,008,022	1,089,898	1,008,022
Other comprehensive loss before reclassifications	0				0
Amounts reclassified from AOCL	8				16
Other Comprehensive Income - Net of Tax	8	8	8	8	16	16
Ending Balance	1,173,614	1,160,428	1,078,745	1,076,761	1,173,614	1,078,745
Reclassification of unrealized loss on derivatives, tax benefit (expense)	(4)		(4)		(8)	(8)
Postretirement Liability Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(32,124)	(32,124)			(32,124)
Other comprehensive loss before reclassifications	0				0
Amounts reclassified from AOCL	0				0
Other Comprehensive Income - Net of Tax	0				0
Ending Balance	(32,124)	(32,124)			(32,124)
Postretirement Liability Adjustment | SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(27,454)	(27,454)			(27,454)
Other comprehensive loss before reclassifications	0				0
Amounts reclassified from AOCL	0				0
Other Comprehensive Income - Net of Tax	0				0
Ending Balance	(27,454)	(27,454)			(27,454)
Unrealized Gain (Loss) on Derivatives-Other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(319)	(327)			(327)
Other comprehensive loss before reclassifications	0				0
Amounts reclassified from AOCL	8				16
Other Comprehensive Income - Net of Tax	8				16
Ending Balance	(311)	(319)			(311)
Unrealized Gain (Loss) on Derivatives-Other | SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(413)	(421)			(421)
Other comprehensive loss before reclassifications	0				0
Amounts reclassified from AOCL	8				16
Other Comprehensive Income - Net of Tax	8				16
Ending Balance	(405)	(413)			(405)
Unrealized Gain (Loss) on Available- for-Sale Securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(10)	(10)			(10)
Other comprehensive loss before reclassifications	0				0
Amounts reclassified from AOCL	0				0
Other Comprehensive Income - Net of Tax	0				0
Ending Balance	(10)	(10)			(10)
Other Comprehensive Income (Loss) of Affiliated Companies
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(97)	(97)			(97)
Other comprehensive loss before reclassifications	0				0
Amounts reclassified from AOCL	0				0
Other Comprehensive Income - Net of Tax	0				0
Ending Balance	(97)	(97)			(97)
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(32,550)	(32,558)	(26,087)	(26,095)	(32,558)	(26,095)
Other Comprehensive Income - Net of Tax	8	8	8	8
Ending Balance	(32,542)	(32,550)	(26,079)	(26,087)	(32,542)	(26,079)
Total | SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(27,867)	(27,875)	(22,349)	(22,357)	(27,875)	(22,357)
Other Comprehensive Income - Net of Tax	8	8	8	8
Ending Balance	$ (27,859)	$ (27,867)	$ (22,341)	$ (22,349)	$ (27,859)	$ (22,341)