Summary of Significant Accounting Policies - Schedule of Costs for Affiliate Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Alabama Power \| Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	$ 508	$ 479	$ 460
Alabama Power \| Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	8	9	2
Georgia Power \| Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	653	625	606
Georgia Power \| Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	101	102	35
Georgia Power \| Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	21	22	10
Mississippi Power \| Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	104	140	231
Mississippi Power \| Alabama Power \| Power Pool
Related Party Transaction [Line Items]
Costs for services	15	16	29
Southern Power \| Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	98	218	193
Southern Power \| Alabama Power \| Power Pool
Related Party Transaction [Line Items]
Costs for services	41	27	21
Southern Power \| Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	25	25	7
Southern Power \| Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	119	119	17
Southern Company Gas \| Southern Company Services, Inc.
Related Party Transaction [Line Items]
Costs for services	194	63	17
Southern Company Gas \| Southern Natural Gas Company, LLC
Related Party Transaction [Line Items]
Costs for services	$ 32	$ 32	$ 15