CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total		SJG		Common Stock	Common Stock SJG	Premium on Common Stock	Premium on Common Stock SJG	Treasury Stock	Accumulated Other Comprehensive Loss	Accumulated Other Comprehensive Loss SJG	Retained Earnings	Retained Earnings SJG	Postretirement Liability Adjustment	Postretirement Liability Adjustment SJG	Unrealized Gain (Loss) on Derivatives - Other		Unrealized Gain (Loss) on Derivatives - Other SJG		Unrealized Gain (Loss) on Available- for-Sale Securities	Other Comprehensive Income (Loss) of Affiliated Companies
Beginning Balance at Dec. 31, 2017	$ 1,192,409		$ 921,433		$ 99,436	$ 5,848	$ 709,658	$ 355,744	$ (271)	$ (36,765)	$ (25,997)	$ 420,351	$ 585,838	$ (36,262)	$ (25,507)	$ (396)	[1]	$ (490)	[2]	$ (10)	$ (97)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	111,240		66,747									111,240	66,747
Other Comprehensive Income, Net of Tax	9		9							9	9			0	0	9	[1]	9	[2]	0	0
Common Stock Issued or Granted Through Equity Offering or Stock Plans	175				80		98		(3)
Cash dividends	(22,336)											(22,336)
Ending Balance at Mar. 31, 2018	1,281,497		988,189		99,516	5,848	709,756	355,744	(274)	(36,756)	(25,988)	509,255	652,585	(36,262)	(25,507)	(387)	[1]	(481)	[2]	(10)	(97)
Beginning Balance at Dec. 31, 2017	1,192,409		921,433		99,436	5,848	709,658	355,744	(271)	(36,765)	(25,997)	420,351	585,838	(36,262)	(25,507)	(396)	[1]	(490)	[2]	(10)	(97)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income, Net of Tax	9		9							9	9			0	0	9	[1]	9	[2]	0	0
Ending Balance at Mar. 31, 2018	1,281,497		988,189		99,516	5,848	709,756	355,744	(274)	(36,756)	(25,988)	509,255	652,585	(36,262)	(25,507)	(387)	[1]	(481)	[2]	(10)	(97)
Beginning Balance at Dec. 31, 2017	1,192,409		921,433		99,436	5,848	709,658	355,744	(271)	(36,765)	(25,997)	420,351	585,838	(36,262)	(25,507)	(396)	[1]	(490)	[2]	(10)	(97)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other Comprehensive Income, Net of Tax	25	[3]	25	[4]
Ending Balance at Sep. 30, 2018	1,234,832		980,798		106,883	5,848	843,064	355,744	(287)	(36,740)	(25,972)	321,912	645,178	(36,262)	(25,507)	(371)	[1]	(465)	[2]	(10)	(97)
Beginning Balance at Dec. 31, 2017	1,192,409		921,433		99,436	5,848	709,658	355,744	(271)	(36,765)	(25,997)	420,351	585,838	(36,262)	(25,507)	(396)	[1]	(490)	[2]	(10)	(97)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income, Net of Tax	25	[3]	25	[4]
Ending Balance at Sep. 30, 2018	$ 1,234,832		$ 980,798		106,883	5,848	843,064	355,744	(287)	(36,740)	(25,972)	321,912	645,178	(36,262)	(25,507)	(371)	[1]	(465)	[2]	(10)	(97)
Additional Statement Information [Abstract]
Combined average statutory tax rate	27.00%		27.00%
Beginning Balance at Mar. 31, 2018	$ 1,281,497		$ 988,189		99,516	5,848	709,756	355,744	(274)	(36,756)	(25,988)	509,255	652,585	(36,262)	(25,507)	(387)	[1]	(481)	[2]	(10)	(97)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	(93,819)		1,569									(93,819)	1,569
Other Comprehensive Income, Net of Tax	8		8							8	8			0	0	8	[1]	8	[2]	0	0
Common Stock Issued or Granted Through Equity Offering or Stock Plans	139,929				7,366		132,571		(8)
Cash dividends	(23,898)											(23,898)
Ending Balance at Jun. 30, 2018	1,303,717		989,766		106,882	5,848	842,327	355,744	(282)	(36,748)	(25,980)	391,538	654,154	(36,262)	(25,507)	(379)		(473)		(10)	(97)
Beginning Balance at Mar. 31, 2018	1,281,497		988,189		99,516	5,848	709,756	355,744	(274)	(36,756)	(25,988)	509,255	652,585	(36,262)	(25,507)	(387)	[1]	(481)	[2]	(10)	(97)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income, Net of Tax	8		8							8	8			0	0	8	[1]	8	[2]	0	0
Ending Balance at Jun. 30, 2018	1,303,717		989,766		106,882	5,848	842,327	355,744	(282)	(36,748)	(25,980)	391,538	654,154	(36,262)	(25,507)	(379)		(473)		(10)	(97)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	(45,685)		(8,976)									(45,685)	(8,976)
Other Comprehensive Income, Net of Tax	8	[3]	8	[4]						8	8			0	0	8		8		0	0
Common Stock Issued or Granted Through Equity Offering or Stock Plans	733				1		737		(5)
Cash dividends	(23,941)											(23,941)
Ending Balance at Sep. 30, 2018	1,234,832		980,798		106,883	5,848	843,064	355,744	(287)	(36,740)	(25,972)	321,912	645,178	(36,262)	(25,507)	(371)	[1]	(465)	[2]	(10)	(97)
Beginning Balance at Jun. 30, 2018	1,303,717		989,766		106,882	5,848	842,327	355,744	(282)	(36,748)	(25,980)	391,538	654,154	(36,262)	(25,507)	(379)		(473)		(10)	(97)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income, Net of Tax	8	[3]	8	[4]						8	8			0	0	8		8		0	0
Ending Balance at Sep. 30, 2018	1,234,832		980,798		106,883	5,848	843,064	355,744	(287)	(36,740)	(25,972)	321,912	645,178	(36,262)	(25,507)	(371)	[1]	(465)	[2]	(10)	(97)
Beginning Balance at Dec. 31, 2018	1,267,022		1,008,022		106,883	5,848	843,268	355,744	(292)	(26,095)	(22,357)	343,258	668,787	(25,626)	(21,901)	(362)	[1]	(456)	[2]	(10)	(97)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	85,637		68,731									85,637	68,731
Other Comprehensive Income, Net of Tax	8		8							8	8			0	0	8	[1]	8	[2]	0	0
Common Stock Issued or Granted Through Equity Offering or Stock Plans	188,449				8,603		179,829		17
Cash dividends	(26,562)											(26,562)
Ending Balance at Mar. 31, 2019	1,514,554		1,076,761		115,486	5,848	1,023,097	355,744	(275)	(26,087)	(22,349)	402,333	737,518	(25,626)	(21,901)	(354)	[1]	(448)	[2]	(10)	(97)
Beginning Balance at Dec. 31, 2018	1,267,022		1,008,022		106,883	5,848	843,268	355,744	(292)	(26,095)	(22,357)	343,258	668,787	(25,626)	(21,901)	(362)	[1]	(456)	[2]	(10)	(97)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income, Net of Tax	8		8							8	8			0	0	8	[1]	8	[2]	0	0
Ending Balance at Mar. 31, 2019	1,514,554		1,076,761		115,486	5,848	1,023,097	355,744	(275)	(26,087)	(22,349)	402,333	737,518	(25,626)	(21,901)	(354)	[1]	(448)	[2]	(10)	(97)
Beginning Balance at Dec. 31, 2018	1,267,022		1,008,022		106,883	5,848	843,268	355,744	(292)	(26,095)	(22,357)	343,258	668,787	(25,626)	(21,901)	(362)	[1]	(456)	[2]	(10)	(97)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other Comprehensive Income, Net of Tax	25	[3]	25	[4]										0	0					0
Ending Balance at Sep. 30, 2019	1,416,787		1,067,150		115,491	5,848	1,026,643	355,744	(284)	(26,070)	(22,332)	301,007	727,890	(25,626)	(21,901)	(337)	[1]	(431)	[2]	(10)	(97)
Beginning Balance at Dec. 31, 2018	1,267,022		1,008,022		106,883	5,848	843,268	355,744	(292)	(26,095)	(22,357)	343,258	668,787	(25,626)	(21,901)	(362)	[1]	(456)	[2]	(10)	(97)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income, Net of Tax	25	[3]	25	[4]										0	0					0
Ending Balance at Sep. 30, 2019	$ 1,416,787		$ 1,067,150		115,491	5,848	1,026,643	355,744	(284)	(26,070)	(22,332)	301,007	727,890	(25,626)	(21,901)	$ (337)	[1]	(431)	[2]	(10)	(97)
Additional Statement Information [Abstract]
Combined average statutory tax rate	27.00%		27.00%													27.00%
Beginning Balance at Mar. 31, 2019	$ 1,514,554		$ 1,076,761		115,486	5,848	1,023,097	355,744	(275)	(26,087)	(22,349)	402,333	737,518	(25,626)	(21,901)	$ (354)	[1]	(448)	[2]	(10)	(97)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	(13,399)		1,976									(13,399)	1,976
Other Comprehensive Income, Net of Tax	8		8							8	8			0	0	8	[1]	8	[2]	0	0
Common Stock Issued or Granted Through Equity Offering or Stock Plans	1,871				2		1,877		(8)
Cash dividends	(26,562)											(26,562)
Ending Balance at Jun. 30, 2019	1,476,472		1,078,745		115,488	5,848	1,024,974	355,744	(283)	(26,079)	(22,341)	362,372	739,494	(25,626)	(21,901)	(346)		(440)		(10)	(97)
Beginning Balance at Mar. 31, 2019	1,514,554		1,076,761		115,486	5,848	1,023,097	355,744	(275)	(26,087)	(22,349)	402,333	737,518	(25,626)	(21,901)	(354)	[1]	(448)	[2]	(10)	(97)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income, Net of Tax	8		8							8	8			0	0	8	[1]	8	[2]	0	0
Ending Balance at Jun. 30, 2019	1,476,472		1,078,745		115,488	5,848	1,024,974	355,744	(283)	(26,079)	(22,341)	362,372	739,494	(25,626)	(21,901)	(346)		(440)		(10)	(97)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	(34,805)		(11,604)									(34,805)	(11,604)
Other Comprehensive Income, Net of Tax	9	[3]	9	[4]						9	9			0	0	9		9		0	0
Common Stock Issued or Granted Through Equity Offering or Stock Plans	1,671				3		1,669		(1)
Cash dividends	(26,560)											(26,560)
Ending Balance at Sep. 30, 2019	1,416,787		1,067,150		115,491	5,848	1,026,643	355,744	(284)	(26,070)	(22,332)	301,007	727,890	(25,626)	(21,901)	(337)	[1]	(431)	[2]	(10)	(97)
Beginning Balance at Jun. 30, 2019	1,476,472		1,078,745		115,488	5,848	1,024,974	355,744	(283)	(26,079)	(22,341)	362,372	739,494	(25,626)	(21,901)	(346)		(440)		(10)	(97)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Other Comprehensive Income, Net of Tax	9	[3]	9	[4]						9	9			0	0	9		9		0	0
Ending Balance at Sep. 30, 2019	$ 1,416,787		$ 1,067,150		$ 115,491	$ 5,848	$ 1,026,643	$ 355,744	$ (284)	$ (26,070)	$ (22,332)	$ 301,007	$ 727,890	$ (25,626)	$ (21,901)	$ (337)	[1]	$ (431)	[2]	$ (10)	$ (97)

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

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

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

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