ACQUISITIONS AND DISPOSITIONS - Pre-tax Profit (Details) - SOUTHERN POWER CO - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Plant Nacogdoches
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Earnings before income taxes	$ 0		$ 16
Plant Mankato
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Earnings before income taxes	$ 12	$ 2	$ 20

Amount before tax of income (loss) from an individually significant component disposed of or held-for-sale attributable to parent. Excludes discontinued operations.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3A
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=109226691&loc=SL51724579-110230

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration