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	AOCL	AOCL SJG	Retained Earnings	Retained Earnings SJG
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss)	85,637	68,731								85,637	68,731
Other Comprehensive Income, Net of Tax	8	8						8	8
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
Additional Statement Information [Abstract]
Cash Dividends Declared (in dollars per share)	$ 0.29
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other Comprehensive Income, Net of Tax	16	16
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
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss)	(13,399)	1,976								(13,399)	1,976
Other Comprehensive Income, Net of Tax	8	8						8	8
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
Additional Statement Information [Abstract]
Cash Dividends Declared (in dollars per share)	$ 0.29
Beginning Balance at Dec. 31, 2019	$ 1,423,785	1,089,898	115,493	5,848	1,027,902	355,744	(289)	(32,558)	(27,875)	313,237	756,181
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss)	101,041	70,522								101,041	70,522
Other Comprehensive Income, Net of Tax	8	8						8	8
Common Stock Issued or Granted Through Equity Offering or Stock Plans	(275)		62		(352)		15
Cash dividends	(27,276)									(27,276)
Ending Balance at Mar. 31, 2020	$ 1,497,283	1,160,428	115,555	5,848	1,027,550	355,744	(274)	(32,550)	(27,867)	387,002	826,703
Additional Statement Information [Abstract]
Cash Dividends Declared (in dollars per share)	$ 0.30
Beginning Balance at Dec. 31, 2019	$ 1,423,785	1,089,898	115,493	5,848	1,027,902	355,744	(289)	(32,558)	(27,875)	313,237	756,181
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other Comprehensive Income, Net of Tax	16	16
Ending Balance at Jun. 30, 2020	1,666,329	1,173,614	125,733	5,848	1,216,363	365,244	(311)	(32,542)	(27,859)	357,086	830,381
Beginning Balance at Mar. 31, 2020	1,497,283	1,160,428	115,555	5,848	1,027,550	355,744	(274)	(32,550)	(27,867)	387,002	826,703
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income (Loss)	(2,639)	3,678								(2,639)	3,678
Other Comprehensive Income, Net of Tax	8	8						8	8
Common Stock Issued or Granted Through Equity Offering or Stock Plans	198,954		10,178		188,813		(37)
Cash dividends	(27,277)									(27,277)
Additional Investment by Shareholder		9,500				9,500
Ending Balance at Jun. 30, 2020	$ 1,666,329	$ 1,173,614	$ 125,733	$ 5,848	$ 1,216,363	$ 365,244	$ (311)	$ (32,542)	$ (27,859)	$ 357,086	$ 830,381
Additional Statement Information [Abstract]
Cash Dividends Declared (in dollars per share)	$ 0.30