REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
SJG
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 237,738	$ 245,360
SJG | Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	12,730	12,666
SJG | Excess Deferred Taxes
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	225,008	232,694
SJG | Deferred Gas Costs - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
SJG | Amounts to be Refunded to Customers
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
SJG | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
SJI
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	418,089	420,577
SJI | Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	46,502	50,619
SJI | Excess Deferred Taxes
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	338,174	346,582
SJI | Deferred Gas Costs - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	22,689	15,322
SJI | Amounts to be Refunded to Customers
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	9,257	6,969
SJI | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,467	1,085
ETG Utility Operations
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	180,351	175,217
ETG Utility Operations | Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	33,772	37,953
ETG Utility Operations | Excess Deferred Taxes
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	113,166	113,888
ETG Utility Operations | Deferred Gas Costs - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	22,689	15,322
ETG Utility Operations | Amounts to be Refunded to Customers
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	9,257	6,969
ETG Utility Operations | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 1,467	$ 1,085