REGULATORY MATTERS - GAS Natural Gas Cost Recovery (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Southern Company Gas
Public Utilities, General Disclosures [Line Items]
Over recovered fuel cost	$ 88	$ 74

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_PublicUtilitiesGeneralDisclosuresLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration