ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Changes (Details) - USD ($) $ in Thousands	3 Months Ended								9 Months Ended
	Sep. 30, 2019		Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018		Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019		Sep. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 1,476,472		$ 1,514,554	$ 1,267,022	$ 1,303,717		$ 1,281,497	$ 1,192,409	$ 1,267,022		$ 1,192,409
Other comprehensive loss before reclassifications	0								0
Amounts reclassified from AOCL	9								25
Other Comprehensive Income - Net of Tax*	9	[1]	8	8	8	[1]	8	9	25	[1]	25	[1]
Ending Balance	1,416,787		1,476,472	1,514,554	1,234,832		1,303,717	1,281,497	$ 1,416,787		$ 1,234,832
Combined average statutory tax rate									27.00%		27.00%
SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	1,078,745		1,076,761	1,008,022	989,766		988,189	921,433	$ 1,008,022		$ 921,433
Other comprehensive loss before reclassifications	0								0
Amounts reclassified from AOCL	9								25
Other Comprehensive Income - Net of Tax*	9	[2]	8	8	8	[2]	8	9	25	[2]	25	[2]
Ending Balance	1,067,150		1,078,745	1,076,761	980,798		989,766	988,189	$ 1,067,150		$ 980,798
Combined average statutory tax rate									27.00%		27.00%
Postretirement Liability Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(25,626)		(25,626)	(25,626)	(36,262)		(36,262)	(36,262)	$ (25,626)		$ (36,262)
Other comprehensive loss before reclassifications	0								0
Amounts reclassified from AOCL	0								0
Other Comprehensive Income - Net of Tax*	0		0	0	0		0	0	0
Ending Balance	(25,626)		(25,626)	(25,626)	(36,262)		(36,262)	(36,262)	(25,626)		(36,262)
Postretirement Liability Adjustment | SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(21,901)		(21,901)	(21,901)	(25,507)		(25,507)	(25,507)	(21,901)		(25,507)
Other comprehensive loss before reclassifications	0								0
Amounts reclassified from AOCL	0								0
Other Comprehensive Income - Net of Tax*	0		0	0	0		0	0	0
Ending Balance	(21,901)		(21,901)	(21,901)	(25,507)		(25,507)	(25,507)	(21,901)		(25,507)
Unrealized Gain (Loss) on Derivatives-Other
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(346)			(362)					(362)
Other comprehensive loss before reclassifications	0								0
Amounts reclassified from AOCL	9								25
Other Comprehensive Income - Net of Tax*	9								25
Ending Balance	(337)		(346)						(337)
Unrealized Gain (Loss) on Derivatives-Other | SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(440)			(456)					(456)
Other comprehensive loss before reclassifications	0								0
Amounts reclassified from AOCL	9								25
Other Comprehensive Income - Net of Tax*	9								25
Ending Balance	(431)		(440)						(431)
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)	(10)		(10)	(10)	(10)		(10)
Other comprehensive loss before reclassifications	0								0
Amounts reclassified from AOCL	0								0
Other Comprehensive Income - Net of Tax*	0		0	0	0		0	0	0
Ending Balance	(10)		(10)	(10)	(10)		(10)	(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	(26,079)		(26,087)	(26,095)	(36,748)		(36,756)	(36,765)	(26,095)		(36,765)
Other Comprehensive Income - Net of Tax*	9		8	8	8		8	9
Ending Balance	(26,070)		(26,079)	(26,087)	(36,740)		(36,748)	(36,756)	(26,070)		(36,740)
Total | SJG
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(22,341)		(22,349)	(22,357)	(25,980)		(25,988)	(25,997)	(22,357)		(25,997)
Other Comprehensive Income - Net of Tax*	9		8	8	8		8	9
Ending Balance	$ (22,332)		$ (22,341)	$ (22,349)	$ (25,972)		$ (25,980)	$ (25,988)	$ (22,332)		$ (25,972)

[1] Determined using a combined average statutory tax rate of approximately 27% in each of 2019 and 2018.

[2] Determined using a combined average statutory tax rate of approximately 27% for both 2019 and 2018.