Regulatory Matters - FERC Matters (Details) - SCS	May 10, 2018
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity	11.25%
Public utilities, approved return on equity, maximum amount	8.65%

Public Utilities, Approved Return On Equity, Threshold, Percentage

No definition available.

Name:	so_PublicUtilitiesApprovedReturnOnEquityThresholdPercentage
Namespace Prefix:	so_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration