REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
SJG Utility Operations
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 278,827	$ 286,539
SJI
Regulatory Liabilities [Line Items]
Regulatory Liabilities	478,592	478,499
Excess Plant Removal Costs | SJG Utility Operations
Regulatory Liabilities [Line Items]
Regulatory Liabilities	19,849	20,805
Excess Plant Removal Costs | SJI
Regulatory Liabilities [Line Items]
Regulatory Liabilities	69,240	70,107
Excess Deferred Taxes | SJG Utility Operations
Regulatory Liabilities [Line Items]
Regulatory Liabilities	253,241	259,863
Excess Deferred Taxes | SJI
Regulatory Liabilities [Line Items]
Regulatory Liabilities	372,435	379,851
Deferred Revenues - Net | SJG Utility Operations
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	0
Deferred Revenues - Net | SJI
Regulatory Liabilities [Line Items]
Regulatory Liabilities	8,935	3,188
CIP Payable | SJG Utility Operations
Regulatory Liabilities [Line Items]
Regulatory Liabilities	5,737	5,871
CIP Payable | SJI
Regulatory Liabilities [Line Items]
Regulatory Liabilities	5,737	5,871
Amounts to be Refunded to Customers | SJG Utility Operations
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	0
Amounts to be Refunded to Customers | SJI
Regulatory Liabilities [Line Items]
Regulatory Liabilities	17,296	17,039
Other Regulatory Liabilities | SJG Utility Operations
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	0
Other Regulatory Liabilities | SJI
Regulatory Liabilities [Line Items]
Regulatory Liabilities	3,549	2,443
Elizabethtown Gas
Regulatory Liabilities [Line Items]
Regulatory Liabilities	197,139	189,336
Elizabethtown Gas | Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	47,996	47,909
Elizabethtown Gas | Excess Deferred Taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	117,963	118,757
Elizabethtown Gas | Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Regulatory Liabilities	8,935	3,188
Elizabethtown Gas | CIP Payable
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	0
Elizabethtown Gas | Amounts to be Refunded to Customers
Regulatory Liabilities [Line Items]
Regulatory Liabilities	17,296	17,039
Elizabethtown Gas | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities	3,549	2,443
Elkton Gas
Regulatory Liabilities [Line Items]
Regulatory Liabilities	2,626	2,624
Elkton Gas | Excess Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,395	1,393
Elkton Gas | Excess Deferred Taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,231	1,231
Elkton Gas | Deferred Revenues - Net
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	0
Elkton Gas | CIP Payable
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	0
Elkton Gas | Amounts to be Refunded to Customers
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	0
Elkton Gas | Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0	0
Weather Normalization | SJG Utility Operations
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0
Weather Normalization | SJI
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,400
Weather Normalization | Elizabethtown Gas
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,400	$ 3,200
Weather Normalization | Elkton Gas
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 0