Summary of Significant Accounting Policies - Schedule of Average AFUDC Composite Rates (Details)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Alabama Power
Capitalized Contract Cost [Line Items]
Composite rate	8.30%	8.30%	8.20%
Georgia Power
Capitalized Contract Cost [Line Items]
Composite rate	7.30%	5.60%			6.90%
Mississippi Power
Capitalized Contract Cost [Line Items]
Composite rate	3.30%	6.70%	6.50%
Atlanta Gas Light
Capitalized Contract Cost [Line Items]
Composite rate	7.90%	8.10%	4.10%
Atlanta Gas Light | Predecessor
Capitalized Contract Cost [Line Items]
Composite rate				4.10%
Chattanooga Gas
Capitalized Contract Cost [Line Items]
Composite rate	7.40%	7.40%	3.70%
Chattanooga Gas | Predecessor
Capitalized Contract Cost [Line Items]
Composite rate				3.70%
Nicor Gas
Capitalized Contract Cost [Line Items]
Composite rate	2.10%	1.20%	1.50%
Nicor Gas | Predecessor
Capitalized Contract Cost [Line Items]
Composite rate				1.50%