STOCK-BASED COMPENSATION PLANS (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Vesting period of shares			3 years	3 years
Shares granted (in shares)			26,266
Shares Outstanding (in shares)		46,475	36,794	36,794		46,475
Fair Value Per Share (in USD per share)		$ 26.67	$ 24.10	$ 24.10		$ 26.67
Expected volatility, measurement period			3 years
Cost of restricted stock awards			$ 0.3	$ 0.2	$ 0.4
Amount to be capitalized to Utlity Plant (as a percent)			50.00%	50.00%	50.00%
Unrecognized compensation cost of awards granted under the plan						$ 0.7
Weighted average period over which unrecognized compensation cost is to be recognized (in years)			1 year 9 months 18 days
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward}
Nonvested Shares Outstanding, beginning balance (in shares)		46,475	36,794
Shares granted (in shares)			26,266
Vested (in shares)	[1]		(15,948)
Cancelled/Forfeited (in shares)			(637)
Nonvested Shares Outstanding, ending balance (in shares)			46,475	36,794
Weighted Average Grant Date Fair Value
Weighted Average Grant Date Fair Value, Nonvested Shares Outstanding, beginning balance (in USD per share)		$ 26.67	$ 24.10
Weighted Average Grant Date Fair Value, Granted (in USD per share)			28.58
Weighted Average Grant Date Fair Value, Vested (in USD per share)	[1]		23.89
Weighted Average Grant Date Fair Value, Cancelled/Forfeited (in USD per share)			26.71
Weighted Average Grant Date Fair Value, Nonvested Shares Outstanding, ending balance (in USD per share)			$ 26.67	$ 24.10
Number of shares awarded during the period					12,901
Market value of shares awarded					$ 0.6
Cash payments to SJI relating to stock awards			$ 0.2	$ 0.4	$ 0.4
Officers and key employees
Weighted Average Grant Date Fair Value
Number of shares awarded during the period			0	0
Officers and key employees | Forecast
Weighted Average Grant Date Fair Value
Number of shares awarded during the period		0
2014 - TSR
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Shares Outstanding (in shares)		10,287	10,287			10,287
Fair Value Per Share (in USD per share)		$ 21.31	$ 21.31			$ 21.31
Expected Volatility			20.00%
Risk-Free Interest Rate			0.80%
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward}
Nonvested Shares Outstanding, beginning balance (in shares)		10,287
Nonvested Shares Outstanding, ending balance (in shares)			10,287
Weighted Average Grant Date Fair Value
Weighted Average Grant Date Fair Value, Nonvested Shares Outstanding, beginning balance (in USD per share)		$ 21.31
Weighted Average Grant Date Fair Value, Nonvested Shares Outstanding, ending balance (in USD per share)			$ 21.31
2014 - EPS
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Shares Outstanding (in shares)		10,287	10,287			10,287
Fair Value Per Share (in USD per share)		$ 27.22	$ 27.22			$ 27.22
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward}
Nonvested Shares Outstanding, beginning balance (in shares)		10,287
Nonvested Shares Outstanding, ending balance (in shares)			10,287
Weighted Average Grant Date Fair Value
Weighted Average Grant Date Fair Value, Nonvested Shares Outstanding, beginning balance (in USD per share)		$ 27.22
Weighted Average Grant Date Fair Value, Nonvested Shares Outstanding, ending balance (in USD per share)			$ 27.22
2015 - TSR
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Shares Outstanding (in shares)		7,250	7,250			7,250
Fair Value Per Share (in USD per share)		$ 26.31	$ 26.31			$ 26.31
Expected Volatility			16.00%
Risk-Free Interest Rate			1.10%
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward}
Nonvested Shares Outstanding, beginning balance (in shares)		7,250
Nonvested Shares Outstanding, ending balance (in shares)			7,250
Weighted Average Grant Date Fair Value
Weighted Average Grant Date Fair Value, Nonvested Shares Outstanding, beginning balance (in USD per share)		$ 26.31
Weighted Average Grant Date Fair Value, Nonvested Shares Outstanding, ending balance (in USD per share)			$ 26.31
2015 - EPS, ROE, Time
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Shares Outstanding (in shares)		18,651	18,651			18,651
Fair Value Per Share (in USD per share)		$ 29.47	$ 29.47			$ 29.47
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward}
Nonvested Shares Outstanding, beginning balance (in shares)		18,651
Nonvested Shares Outstanding, ending balance (in shares)			18,651
Weighted Average Grant Date Fair Value
Weighted Average Grant Date Fair Value, Nonvested Shares Outstanding, beginning balance (in USD per share)		$ 29.47
Weighted Average Grant Date Fair Value, Nonvested Shares Outstanding, ending balance (in USD per share)			$ 29.47
Restricted stock
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Vesting period of shares			3 years
Amount of payout limit for one-third of the time-based shares of restricted stock			100.00%
Minimum return on equity requirement			7.00%
Shares granted (in shares)			7,878
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward}
Shares granted (in shares)			7,878
Minimum
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Percentage of actual amount of shares that ultimately vest of original share units granted			0.00%
Maximum
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Percentage of actual amount of shares that ultimately vest of original share units granted			200.00%

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