Common Stock and Stock Compensation - Textual Dividend Restrictions (Details) $ in Billions	12 Months Ended
Dec. 31, 2017 USD ($)
Equity [Abstract]
Undistributed retained earnings of the subsidiaries	$ 5.3

Undistributed retained earnings of the subsidiaries.

No definition available.

Name:	so_UndistributedRetainedEarningsOfSubsidiaries
Namespace Prefix:	so_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration