ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Changes (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive loss before reclassifications	$ 0						$ 0
Amounts reclassified from AOCL	9						25
Other Comprehensive Income - Net of Tax	9	$ 8	$ 8	$ 9	$ 8	$ 8	25	$ 25
Reclassification of unrealized loss on derivatives, tax benefit (expense)	(3)			(3)			(10)	(10)
SJG
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive loss before reclassifications	0						0
Amounts reclassified from AOCL	9						25
Other Comprehensive Income - Net of Tax	9	8	8	9	8	8	25	25
Reclassification of unrealized loss on derivatives, tax benefit (expense)	(3)			(3)			(10)	$ (10)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(32,542)		(32,558)				(32,558)
Other Comprehensive Income - Net of Tax	9	8	8	9	8	8
Ending Balance	(32,533)	(32,542)					(32,533)
Total | SJG
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(27,859)		(27,875)				(27,875)
Other Comprehensive Income - Net of Tax	9	8	8	$ 9	$ 8	$ 8
Ending Balance	(27,850)	(27,859)					(27,850)
Postretirement Liability Adjustment
AOCI Attributable to Parent, 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 Attributable to Parent, 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 Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(311)		(327)				(327)
Other comprehensive loss before reclassifications	0						0
Amounts reclassified from AOCL	9						25
Other Comprehensive Income - Net of Tax	9						25
Ending Balance	(302)	(311)					(302)
Unrealized Gain (Loss) on Derivatives-Other | SJG
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(405)		(421)				(421)
Other comprehensive loss before reclassifications	0						0
Amounts reclassified from AOCL	9						25
Other Comprehensive Income - Net of Tax	9						25
Ending Balance	(396)	(405)					(396)
Unrealized Gain (Loss) on Available- for-Sale Securities
AOCI Attributable to Parent, 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 Attributable to Parent, 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)