STOCK-BASED COMPENSATION PLAN (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 30, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2019
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of options granted (in shares)		0	0
Number of options outstanding (in shares)			0			0
Vesting period of shares				3 years
Service period of shares		3 years
Fair value per share (in dollars per share)	$ 31.37	$ 29.57				$ 31.37
Expected volatility, measurement period		3 years
Total Cost		$ 952	$ 1,306
Capitalized		(81)	(101)
Net Expense		$ 871	1,205
Unrecognized compensation cost of awards granted under the plan						$ 5,300
Weighted average period over which unrecognized compensation cost is to be recognized		1 year 4 months 24 days
Additional disclosures [Abstract]
Weighted average fair value nonvested shares-beginning balance (in dollars per share)	$ 31.37	$ 29.57
Weighted average fair value nonvested shares-granted during the period (in dollars per share)		26.89
Weighted average fair value nonvested shares- forfeited during the period (in dollars per share)		31.73
Weighted average fair value nonvested shares-vested during the period (in dollars per share)		26.04
Weighted average fair value nonvested shares-ending balance (in dollars per share)		$ 31.37
SJG Utility Operations
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Costs capitalized		60.00%
Officers and Key Employees
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares granted (in shares)		0
Shares outstanding (in shares)	250,095	411,809				250,095
Total Cost		$ 750	$ 1,100
Number of shares awarded		122,265	63,030
Fair value of shares issued		$ 3,600	$ 1,900
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Nonvested shares outstanding, beginning balance (in shares)	250,095	411,809
Granted (in shares)		0
Cancelled/Forfeited (in shares)		(14,181)
Vested (in shares)		(147,533)
Nonvested shares outstanding, ending balance (in shares)		250,095
Officers and Key Employees | 2017 - TSR
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Shares outstanding (in shares)	42,214	42,214				42,214
Fair value per share (in dollars per share)	$ 32.17	$ 32.17				$ 32.17
Expected volatility		20.80%
Risk-free interest rate		1.47%
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Nonvested shares outstanding, beginning balance (in shares)	42,214
Nonvested shares outstanding, ending balance (in shares)		42,214
Additional disclosures [Abstract]
Weighted average fair value nonvested shares-beginning balance (in dollars per share)	$ 32.17
Weighted average fair value nonvested shares-ending balance (in dollars per share)		$ 32.17
Officers and Key Employees | 2017 - CEGR, Time
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Shares outstanding (in shares)	56,741	56,741				56,741
Fair value per share (in dollars per share)	$ 33.69	$ 33.69				$ 33.69
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Nonvested shares outstanding, beginning balance (in shares)	56,741
Nonvested shares outstanding, ending balance (in shares)		56,741
Additional disclosures [Abstract]
Weighted average fair value nonvested shares-beginning balance (in dollars per share)	$ 33.69
Weighted average fair value nonvested shares-ending balance (in dollars per share)		$ 33.69
Officers and Key Employees | 2018 - TSR
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Shares outstanding (in shares)	56,411	56,411				56,411
Fair value per share (in dollars per share)	$ 31.05	$ 31.05				$ 31.05
Expected volatility		21.90%
Risk-free interest rate		2.00%
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Nonvested shares outstanding, beginning balance (in shares)	56,411
Nonvested shares outstanding, ending balance (in shares)		56,411
Additional disclosures [Abstract]
Weighted average fair value nonvested shares-beginning balance (in dollars per share)	$ 31.05
Weighted average fair value nonvested shares-ending balance (in dollars per share)		$ 31.05
Officers and Key Employees | 2018 - CEGR, Time
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Shares outstanding (in shares)	94,729	94,729				94,729
Fair value per share (in dollars per share)	$ 31.23	$ 31.23				$ 31.23
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Nonvested shares outstanding, beginning balance (in shares)	94,729
Nonvested shares outstanding, ending balance (in shares)		94,729
Additional disclosures [Abstract]
Weighted average fair value nonvested shares-beginning balance (in dollars per share)	$ 31.23
Weighted average fair value nonvested shares-ending balance (in dollars per share)		$ 31.23
Director
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares granted (in shares)		30,028	30,394
Vesting period of shares		12 months
Director shares vested						100.00%
Shares outstanding (in shares)	30,028	26,416				30,028
Total Cost		$ 202	$ 206
Fair value of shares issued		$ 800	$ 1,000
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Nonvested shares outstanding, beginning balance (in shares)	30,028	26,416
Granted (in shares)		30,028	30,394
Cancelled/Forfeited (in shares)		0
Vested (in shares)		(26,416)
Nonvested shares outstanding, ending balance (in shares)		30,028
Director | 2018
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Shares outstanding (in shares)	30,028	30,028				30,028
Fair value per share (in dollars per share)	$ 26.89	$ 26.89				$ 26.89
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Nonvested shares outstanding, beginning balance (in shares)	30,028
Nonvested shares outstanding, ending balance (in shares)		30,028
Additional disclosures [Abstract]
Weighted average fair value nonvested shares-beginning balance (in dollars per share)	$ 26.89
Weighted average fair value nonvested shares-ending balance (in dollars per share)		$ 26.89
Stock Appreciation Rights
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares granted (in shares)		0
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Granted (in shares)		0
Restricted Stock | Year one
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Vesting period of shares		1 year
Restricted Stock | Year two
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Vesting period of shares		2 years
Service period of shares					2 years
Restricted Stock | Year three
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Vesting period of shares		3 years
Service period of shares					3 years
Restricted Stock | Officers and Key Employees
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares granted (in shares)		0	185,214
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Granted (in shares)		0	185,214
Restricted Stock | Officers and Key Employees | 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%
Restricted Stock | Officers and Key Employees | 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%
Restricted Stock | South Jersey Gas Company's officers and other key employees
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares granted (in shares)		0	26,652
Net Expense		$ 100	$ 200
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Granted (in shares)		0	26,652
Time-based Restricted Stock | Minimum
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Return on equity award threshold					7.00%
Time-based Restricted Stock | Officers and Key Employees
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares granted (in shares)		0	56,634
Vesting period of shares		3 years			3 years
Payout limit					100.00%
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Granted (in shares)		0	56,634
Time-based Restricted Stock | Officers and Key Employees | Year one
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Percentage of shares vested					33.33%
Time-based Restricted Stock | Officers and Key Employees | Year two
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Percentage of shares vested					33.33%
Time-based Restricted Stock | Officers and Key Employees | Year three
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Percentage of shares vested					33.33%
Total Shareholder Return
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Vesting period of shares		3 years
Subsequent event | Restricted Stock | Officers and Key Employees
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares granted (in shares)	96,240
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Granted (in shares)	96,240
Subsequent event | Time-based Restricted Stock | Officers and Key Employees
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares granted (in shares)	81,631
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Granted (in shares)	81,631
Restricted Stock Units (RSUs) | Director
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Number of shares granted (in shares)			26,416
Restricted stock award activity, excluding accrued dividend equivalents [Roll Forward]
Granted (in shares)			26,416