Schedule I (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended								12 Months Ended
		May 31, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Expenses:
Operations											$ 174,200	$ 151,957	$ 148,672
Depreciation											100,718	90,389	72,451
Energy and Other Taxes			$ 1,348	$ 1,517	$ 1,551	$ 2,071	$ 1,725	$ 1,449	$ 1,243	$ 1,925	6,487	6,342	6,026
Operating Income											4,410	189,276	156,894
Other Income:
Equity in (Losses) Earnings of Subsidiaries (See Note 1)											5,794	5,396	(27,812)
Total Other Income											15,474	9,989	9,510
Tax Reform Adjustments											(13,521)	0	0
Interest Charges											54,019	31,449	31,622
Income Taxes			(16,498)	(24,765)	(5,544)	21,870	19,266	2,807	(7,189)	39,267	(24,937)	54,151	1,360
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent			4,009	(37,548)	(7,612)	37,747	46,008	9,664	(4,798)	68,187	(3,404)	119,061	105,610
Equity in Undistributed Earnings of Discontinued Operations			36	(45)	(47)	(30)	(75)	(29)	(29)	(118)	(86)	(251)	(503)
Net Income (Loss)			4,045	(37,593)	(7,659)	37,717	45,933	9,635	(4,827)	68,069	(3,490)	118,810	105,107
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net Income			4,045	(37,593)	(7,659)	37,717	45,933	9,635	(4,827)	68,069	(3,490)	118,810	105,107
Other Comprehensive (Loss) Income - Net of Tax
Postretirement Liability Adjustment (A)	[1]										(10,920)	(3,197)	5,518
Other Comprehensive (Loss) Income - Net of Tax											(9,384)	(2,882)	5,759
Retained Earnings [Roll Forward]
Retained Earnings - Beginning						510,597					510,597
Net Income			4,045	$ (37,593)	$ (7,659)	37,717	45,933	$ 9,635	$ (4,827)	68,069	(3,490)	118,810	105,107
Dividends Declared - Common Stock											(87,308)	(82,380)	(70,158)
Retained Earnings - Ending							510,597					510,597
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of Subsidiary, Net of Cash & Restricted Cash Acquired (See Note 1)											0	0	3,133
Capital Expenditures (See Note 1)											(272,965)	(279,423)	(343,883)
Purchase of Company-Owned Life Insurance											(9,180)	(2,398)	(2,328)
Net Cash Used in Investing Activities (See Note 1)											(287,264)	(280,298)	(387,345)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from Issuance of Long Term Debt											450,000	61,000	130,000
Principal Repayments of Long Term Debt											(293,309)	(49,366)	(125,009)
Payments for Issuance of Long Term Debt											(14,204)	(147)	(64)
Net Borrowings from (Repayments of) Short-Term Credit Facilities											50,300	(135,600)	186,000
Dividends on Common Stock											(87,308)	(82,380)	(70,158)
Proceeds from Sale of Common Stock		$ 203,600									0	214,426	63,192
Net Cash Provided by (Used in) Financing Activities											104,728	(3,054)	183,628
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect											7,785	(20,725)	(16,987)
Cash, Cash Equivalents and Restricted Cash at Beginning of Year (See Note 1)						18,282					18,282
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents														$ 39,695	$ 31,910	$ 52,635	$ 69,622
Cash, Cash Equivalents and Restricted Cash at End of Year (See Note 1)			7,819				18,282				7,819	18,282
Property Plant and Equipment:
Nonutility Property, Plant and Equipment, at cost														741,027	821,942
Accumulated Depreciation														(194,913)	(151,084)
Property, Plant and Equipment - Net														2,700,197	2,623,770
Investments:
Available-for-Sale Securities														36	32
Total Investments														94,204	42,566
Current Assets:
Cash and Cash Equivalents			7,819			18,282	18,282				18,282	18,282		7,819	18,282
Other														28,247	31,369
Total Current Assets														438,993	473,313
Other Noncurrent Assets														98,426	83,429
Total Assets														3,865,086	3,730,567
Equity:
Common Stock SJI, Par Value $1.25, Authorized 120,000,000 shares, Outstanding - 79,549,080 (2017) shares and 79,478,055 (2016)														99,436	99,347	88,707
Premium on Common Stock														709,658	706,943
Treasury Stock (at par)														(271)	(266)
Accumulated Other Comprehensive Loss														(36,765)	(27,381)
Retained Earnings						510,597	510,597				510,597	510,597			510,597
Total Equity														1,192,409	1,289,240	$ 1,037,539	$ 932,432
Long-Term Debt														1,204,173	1,047,482
Current Liabilities:
Notes Payable - Banks														346,400	296,100
Current Portion of Long-Term Debt														63,809	231,909
Accounts Payable														284,899	243,669
Other Current Liabilities														15,860	15,219
Total Current Liabilities														883,082	952,624
Other Noncurrent Liabilities														9,436	10,584
Total Capitalization and Liabilities														$ 3,865,086	$ 3,730,567
Common stock, par value (in USD per share)		$ 1.25												$ 1.25	$ 1.25
Common stock, authorized (in shares)														120,000,000	120,000,000
Common Stock, outstanding (in shares)														79,549,080	79,478,055	70,965,622	68,334,860
Accounting Standards Update 2016-09, Statutory Tax Withholding Component
CASH FLOWS FROM FINANCING ACTIVITIES:
Net Cash Provided by (Used in) Financing Activities												(400)	(300)
Parent Company
Income Statement [Abstract]
Management Service Fee Revenues											34,321	25,463	20,990
Operating Expenses:
Operations											45,182	23,852	17,979
Depreciation											311	377	406
Energy and Other Taxes											1,324	1,033	806
Total Operating Expenses											46,817	25,262	19,191
Operating Income											(12,496)	201	1,799
Other Income:
Equity in (Losses) Earnings of Subsidiaries (See Note 1)											(2,793)	119,061	105,610
Other											16,752	11,953	10,145
Acquisition Costs, net of tax (A)											(12,031)	0	0
Total Other Income											13,959	131,014	115,755
Tax Reform Adjustments											11,420	0	0
Interest Charges											23,818	12,148	11,822
Income Taxes											(18,951)	6	122
(Loss) Income from Continuing Operations											(3,404)	119,061	105,610
Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent											(3,404)	119,061	105,610
Equity in Undistributed Earnings of Discontinued Operations											(86)	(251)	(503)
Net Income (Loss)											(3,490)	118,810	105,107
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net Income											(3,490)	118,810	105,107
Other Comprehensive (Loss) Income - Net of Tax
Postretirement Liability Adjustment (A)											(10,920)	(3,197)	5,518
Unrealized Gain (Loss) on Available-for-Sale Securities (B)											0	118	(53)
Unrealized Gain on Derivatives - Other (B)											1,536	197	294
Other Comprehensive (Loss) Income - Net of Tax											(9,384)	(2,882)	5,759
Comprehensive Income											(12,874)	115,928	110,866
Retained Earnings [Roll Forward]
Retained Earnings - Beginning						510,597				474,167	510,597	474,167	439,218
Net Income											(3,490)	118,810	105,107
Retained Earnings After Net Income Before Dividends Declared														$ 507,107	$ 592,977	$ 544,325
Dividends Declared - Common Stock											(87,308)	(82,380)	(70,158)
Excess Tax Benefit on Restricted Stock											552	0	0
Retained Earnings - Ending			420,351				510,597				420,351	510,597	474,167
Statement of Cash Flows [Abstract]
CASH PROVIDED BY OPERATING ACTIVITIES (See Note 1)											17,339	20,507	38,926
CASH FLOWS FROM INVESTING ACTIVITIES:
Acquisition of Subsidiary, Net of Cash & Restricted Cash Acquired (See Note 1)											(16,096)	32,300	(118,802)
Capital Expenditures (See Note 1)											(801)	(345)	(150)
Purchase of Company-Owned Life Insurance											(9,180)	(2,398)	(2,328)
Investment in Affiliate											(40,000)	(65,000)	0
Net Cash Used in Investing Activities (See Note 1)											(66,077)	(35,443)	(121,280)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from Issuance of Long Term Debt											50,000	0	50,000
Principal Repayments of Long Term Debt											(16,000)	0	(114,000)
Payments for Issuance of Long Term Debt											(12,174)	(84)	(55)
Net Borrowings from (Repayments of) Short-Term Credit Facilities											102,600	(105,500)	153,000
Dividends on Common Stock											(87,308)	(82,380)	(70,158)
Net Settlement of Restricted Stock (See Note 1)											(751)	(387)	(333)
Proceeds from Sale of Common Stock											0	214,426	63,192
Net Cash Provided by (Used in) Financing Activities											36,367	26,075	81,646
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect											(12,371)	11,139	(708)
Cash, Cash Equivalents and Restricted Cash at Beginning of Year (See Note 1)						12,847					12,847
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents														476	12,847	1,708	$ 2,416
Cash, Cash Equivalents and Restricted Cash at End of Year (See Note 1)			476				12,847				476	12,847
Property Plant and Equipment:
Nonutility Property, Plant and Equipment, at cost														3,318	2,685
Accumulated Depreciation														(2,194)	(2,026)
Property, Plant and Equipment - Net														1,124	659
Investments:
Investments in Subsidiaries														1,209,308	1,233,856
Available-for-Sale Securities														36	32
Total Investments														1,209,344	1,233,888
Current Assets:
Cash and Cash Equivalents			476			12,847	12,847				12,847	12,847		476	12,847
Receivable from Associated Companies														636,327	550,227
Accounts Receivable														52	49
Other														5,017	0
Total Current Assets														641,872	563,123
Other Noncurrent Assets														50,735	44,974
Total Assets														1,903,075	1,842,644
Equity:
Common Stock SJI, Par Value $1.25, Authorized 120,000,000 shares, Outstanding - 79,549,080 (2017) shares and 79,478,055 (2016)														99,436	99,347
Premium on Common Stock														709,658	706,943
Treasury Stock (at par)														(271)	(266)
Accumulated Other Comprehensive Loss														(36,765)	(27,381)
Retained Earnings			$ 420,351			$ 510,597	$ 510,597			$ 474,167	510,597	474,167	439,218	420,351	510,597	$ 474,167	$ 439,218
Total Equity														1,192,409	1,289,240
Long-Term Debt														364,946	323,971
Current Liabilities:
Notes Payable - Banks														294,400	191,800
Current Portion of Long-Term Debt														0	16,000
Payable to Associated Companies														404	382
Accounts Payable														17,316	211
Other Current Liabilities														7,763	7,707
Total Current Liabilities														319,883	216,100
Other Noncurrent Liabilities														25,837	13,333
Total Capitalization and Liabilities														$ 1,903,075	$ 1,842,644
Common stock, par value (in USD per share)														$ 1.25	$ 1.25
Common stock, authorized (in shares)														120,000,000	120,000,000
Common Stock, outstanding (in shares)														79,549,080	79,478,055
Dividends received from subsidiaries											$ 20,000	0	40,800
Parent Company | Accounting Standards Update 2016-09, Statutory Tax Withholding Component
Statement of Cash Flows [Abstract]
CASH PROVIDED BY OPERATING ACTIVITIES (See Note 1)												$ 400	$ 300
Post-Retirement Liability
Other Comprehensive (Loss) Income - Net of Tax
Combined average statutory tax rate											27.00%	40.00%	40.00%
Unrealized Gain (Loss)
Other Comprehensive (Loss) Income - Net of Tax
Combined average statutory tax rate											39.00%	40.00%	40.00%

[1] Determined using a combined average statutory tax rate of 27% for 2017 and 40% for 2016 and 2015.