Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

Name:	us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Income Taxes - Summary of Valuation Allowances (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Valuation Allowance [Line Items]
Valuation allowances	$ 190	$ 148
Federal
Valuation Allowance [Line Items]
Valuation allowances	19
State (net of federal benefit)
Valuation Allowance [Line Items]
Valuation allowances	171
GEORGIA POWER CO
Valuation Allowance [Line Items]
Valuation allowances	39
GEORGIA POWER CO \| Federal
Valuation Allowance [Line Items]
Valuation allowances	6
GEORGIA POWER CO \| State (net of federal benefit)
Valuation Allowance [Line Items]
Valuation allowances	33
MISSISSIPPI POWER CO
Valuation Allowance [Line Items]
Valuation allowances	124
MISSISSIPPI POWER CO \| Federal
Valuation Allowance [Line Items]
Valuation allowances	0
MISSISSIPPI POWER CO \| State (net of federal benefit)
Valuation Allowance [Line Items]
Valuation allowances	124
SOUTHERN Co GAS
Valuation Allowance [Line Items]
Valuation allowances	12
SOUTHERN Co GAS \| Federal
Valuation Allowance [Line Items]
Valuation allowances	11
SOUTHERN Co GAS \| State (net of federal benefit)
Valuation Allowance [Line Items]
Valuation allowances	$ 1