The excise taxes collected by a public utility and reported in revenues if the amount of excise taxes is equal to 1 percent or more of such total.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(1)(e))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

Name:	us-gaap_ExciseTaxesCollected
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Income and Other Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Southern Power
Tax Credit Carryforward [Line Items]
Percentage reduction in tax basis of assets	50.00%
Southern Company Gas
Tax Credit Carryforward [Line Items]
Excise taxes collected	$ 107	$ 117	$ 114
Included In Operating Expenses \| Southern Company Gas
Tax Credit Carryforward [Line Items]
Excise taxes collected	$ 104	$ 114	$ 111