INCOME TAXES AND CREDITS (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Tax at Statutory Rate									$ 36,233,000	$ 35,482,000	$ 33,974,000
Increase (Decrease) Resulting from:
State Income Taxes									4,584,000	1,935,000	4,833,000
Amortization of Investment Tax Credits									(149,000)	(211,000)	(258,000)
ESOP Dividend									(1,168,000)	(1,109,000)	(1,058,000)
AFUDC									(1,109,000)	(1,481,000)	(916,000)
Research and Development Credits									(1,400,000)	0	0
Other - Net									(46,000)	279,000	(1,742,000)
Total Income Tax Expense	$ 10,352,000	$ (2,871,000)	$ 3,292,000	$ 26,172,000	$ 9,455,000	$ 534,000	$ 2,379,000	$ 22,527,000	36,945,000	34,895,000	34,833,000
Current:
Federal									0	60,000	(53,000)
State									(2,203,000)	2,642,000	2,946,000
Total Current									(2,203,000)	2,702,000	2,893,000
Deferred:
Federal									30,042,000	32,069,000	27,707,000
State									9,255,000	335,000	4,491,000
Total Deferred									39,297,000	32,404,000	32,198,000
Investment Tax Credits									(149,000)	(211,000)	(258,000)
Total Income Tax Expense	10,352,000	$ (2,871,000)	$ 3,292,000	26,172,000	9,455,000	$ 534,000	$ 2,379,000	22,527,000	$ 36,945,000	34,895,000	34,833,000
Annual amortization rate of investment tax credits									3.00%
Deferred Tax Assets:
Net Operating Loss and Tax Credits	64,978,000				53,460,000				$ 64,978,000	53,460,000
Deferred State tax	20,825,000				17,390,000				20,825,000	17,390,000
Provision for Uncollectibles	4,030,000				2,676,000				4,030,000	2,676,000
Conservation Incentive Program	0				2,027,000				0	2,027,000
Investment Tax Basis Gross-Up	0				77,000				0	77,000
Pension & Other Post Retirement Benefits	28,464,000				27,782,000				28,464,000	27,782,000
Deferred Revenues	4,844,000				11,452,000				4,844,000	11,452,000
Other	3,985,000				3,624,000				3,985,000	3,624,000
Total Deferred Tax Assets	127,126,000				118,488,000				127,126,000	118,488,000
Deferred Tax Liabilities:
Book Versus Tax Basis of Property	480,682,000				417,178,000				480,682,000	417,178,000
Deferred Fuel Costs - Net	3,998,000				22,959,000				3,998,000	22,959,000
Environmental Remediation	20,408,000				13,500,000				20,408,000	13,500,000
Deferred Regulatory Costs	566,000				6,333,000				566,000	6,333,000
Deferred Pension & Other Post Retirement Benefits	42,216,000				39,891,000				42,216,000	39,891,000
Budget Billing - Customer Accounts	830,000				1,138,000				830,000	1,138,000
Section 461 Prepayments	1,017,000				1,026,000				1,017,000	1,026,000
Conservation Incentive Program	1,132,000				0				1,132,000	0
Other	8,951,000				7,421,000				8,951,000	7,421,000
Total Deferred Tax Liabilities	559,800,000				509,446,000				559,800,000	509,446,000
Current Deferred Tax (Assets)	0				44,064,000				0	44,064,000
Noncurrent Deferred Tax Liabilities	432,674,000				435,022,000				432,674,000	435,022,000
Deferred Tax Liability - Net	432,674,000				390,958,000				432,674,000	390,958,000
Research and development credits	1,400,000								1,400,000
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at January 1,				$ 552,000				$ 547,000	552,000	547,000	503,000
Increase as a result of tax position taken in prior years									7,000	5,000	44,000
Decrease due to a lapse in the statue of limitations									0	0	0
Settlements									0	0	0
Balance at December 31,	559,000				552,000				559,000	552,000	547,000
Accrued interest and penalties on unrecognized tax benefits	700,000				700,000				700,000	700,000	$ 600,000
Internal Revenue Service (IRS)
Deferred Tax Liabilities:
Operating loss carryforwards	179,300,000								179,300,000
Internal Revenue Service (IRS) | 2031
Deferred Tax Liabilities:
Operating loss carryforwards	63,000,000								63,000,000
Internal Revenue Service (IRS) | 2032
Deferred Tax Liabilities:
Operating loss carryforwards	4,200,000								4,200,000
Internal Revenue Service (IRS) | 2034
Deferred Tax Liabilities:
Operating loss carryforwards	49,100,000								49,100,000
Internal Revenue Service (IRS) | 2035
Deferred Tax Liabilities:
Operating loss carryforwards	63,000,000								63,000,000
SJI | Prepaid taxes
Deferred Tax Liabilities:
Income taxes due from parent company	$ 0				$ 0				$ 0	$ 0