REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
SJG
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 257,539	$ 274,482
SJG | Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	14,963	16,333
SJG | Excess Deferred Taxes
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	242,576	251,355
SJG | Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
SJG | CIP Payable
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		6,794
SJG | Weather Normalization
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		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	430,498	442,918
SJI | Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	52,864	52,676
SJI | Excess Deferred Taxes
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	359,319	369,050
SJI | Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	6,183	52
SJI | CIP Payable
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		6,794
SJI | Weather Normalization
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		2,684
SJI | Amounts to be Refunded to Customers
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	10,252	10,625
SJI | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,880	1,037
ETG Utility Operations
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	172,959	168,436
ETG Utility Operations | Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	37,901	36,343
ETG Utility Operations | Excess Deferred Taxes
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	116,743	117,695
ETG Utility Operations | Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	6,183	52
ETG Utility Operations | CIP Payable
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		0
ETG Utility Operations | Weather Normalization
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		2,684
ETG Utility Operations | Amounts to be Refunded to Customers
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	10,252	10,625
ETG Utility Operations | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	1,880	1,037
ELK Utility Operations
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
ELK Utility Operations | Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
ELK Utility Operations | Excess Deferred Taxes
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
ELK Utility Operations | Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
ELK Utility Operations | CIP Payable
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		0
ELK Utility Operations | Weather Normalization
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities		0
ELK Utility Operations | Amounts to be Refunded to Customers
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	0	0
ELK Utility Operations | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Total Regulatory Liabilities	$ 0	$ 0