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Introduction - Narrative (Details) - USD ($)	3 Months Ended			9 Months Ended
Introduction - Narrative (Details) - USD ($)	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impairment charges	$ 18,000,000			$ 50,000,000
SOUTHERN Co GAS
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Inventory decrement	0		$ 0	10,000,000	$ 0
Nicor Gas
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Inventory decrement				0
ALABAMA POWER CO
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in ARO				$ 312,000,000
MISSISSIPPI POWER CO
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase in ARO		$ 58,000,000
Disposal Group, Held-for-sale, Not Discontinued Operations \| PowerSecure International, Inc.
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impairment charges		$ 32,000,000
Disposal Group, Held-for-sale, Not Discontinued Operations \| PowerSecure International, Inc. Lighting Business
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impairment charges	2,000,000
Intangible asset impairment	$ 3,000,000