ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Changes (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ 1,267,022	$ 1,192,409
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		8
Other Comprehensive Income - Net of Tax	[1]	8	9
Ending balance		$ 1,514,554	$ 1,281,497
Combined statutory tax rate		27.00%	25.00%
SJG Utility Operations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ 1,008,022	$ 921,433
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		8
Other Comprehensive Income - Net of Tax	[2]	8	9
Ending balance		$ 1,076,761	$ 988,189
Combined statutory tax rate		27.00%	25.00%
Unrealized Gain (Loss) on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		$ (10)	$ (10)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		0
Other Comprehensive Income - Net of Tax		0	0
Ending balance		(10)	(10)
Other Comprehensive Income (Loss) of Affiliated Companies
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(97)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		0
Other Comprehensive Income - Net of Tax		0
Ending balance		(97)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(26,095)	(36,765)
Other Comprehensive Income - Net of Tax		8	9
Ending balance		(26,087)	(36,756)
Total | SJG Utility Operations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(22,357)	(25,997)
Other Comprehensive Income - Net of Tax		8	9
Ending balance		(22,349)	(25,988)
Postretirement Liability Adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(25,626)	(36,262)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		0
Other Comprehensive Income - Net of Tax		0	0
Ending balance		(25,626)	(36,262)
Postretirement Liability Adjustment | SJG Utility Operations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(21,901)	(25,507)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		0
Other Comprehensive Income - Net of Tax		0	0
Ending balance		(21,901)	$ (25,507)
Unrealized Gain (Loss) on Derivatives-Other
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(362)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		8
Other Comprehensive Income - Net of Tax		8
Ending balance		(354)
Unrealized Gain (Loss) on Derivatives-Other | SJG Utility Operations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(456)
Other comprehensive income before reclassifications		0
Amounts reclassified from AOCL		8
Other Comprehensive Income - Net of Tax		8
Ending balance		$ (448)

[1] Determined using a combined average statutory tax rate of approximately 27% and 25% in 2019 and 2018, respectively.

[2] Determined using a combined average statutory tax rate of approximately 27% and 25% in 2019 and 2018, respectively.