SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Asset Management Agreement (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Jul. 01, 2018
Accounting Policies [Abstract]
Natural Gas in Storage			$ 9,685
Intangible Asset	$ 12,800	$ 16,600	19,200
Profit Sharing - Other Liabilities	$ (11,700)	$ (17,000)	(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