Financing - Dividend Restrictions and Structural Considerations Narrative (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items]
Undistributed retained earnings of subsidiaries	$ 4,900
Southern Company Gas
Variable Interest Entity [Line Items]
Retained earnings, unappropriated	$ 814
Southern Company Gas \| Southern Company Gas Capital \| Southern Company Gas Capital
Variable Interest Entity [Line Items]
Noncontrolling ownership percentage held by parent	100.00%

A segregation of retained earnings which is available for dividend distribution. Earnings not paid out as dividends but instead reinvested in the core business or used to pay off debt. Unappropriated profit is part of shareholder equity. Also called cumulative distributions or earned surplus or accumulated earnings or unappropriated profit.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(a)(4)(ii))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(30)(a)(3)(ii))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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