SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Asset Management Agreement (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Jul. 01, 2018
Accounting Policies [Abstract]
Natural Gas in Storage			$ 9,685
Intangible Asset	$ 6,400	$ 11,500	19,200
Profit Sharing - Other Liabilities	$ (7,000)	$ (10,600)	(17,546)
Total Consideration			$ 11,339

Asset Management Agreement, Recognizable Identifiable Assets Acquired and Liabilities Assumed, Profit Sharing Liabilities

No definition available.

Name:	sji_AssetManagementAgreementRecognizableIdentifiableAssetsAcquiredandLiabilitiesAssumedProfitSharingLiabilities
Namespace Prefix:	sji_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant