Summary of Significant Accounting Policies - Schedule of Goodwill Balances (Details) - USD ($)	3 Months Ended		6 Months Ended	12 Months Ended
	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Beginning balance		$ 6,268,000,000		$ 6,268,000,000	$ 6,251,000,000
Impairment	$ 0			(42,000,000)
Dispositions				(910,000,000)
Ending balance	5,315,000,000		$ 6,251,000,000	5,315,000,000	6,268,000,000
Impairment charge	0			42,000,000
Southern Company Gas
Goodwill [Roll Forward]
Beginning balance		5,967,000,000		5,967,000,000
Impairment	0	(42,000,000)	0	(42,000,000)	0
Ending balance	5,015,000,000			5,015,000,000	5,967,000,000
Impairment charge	0	42,000,000	$ 0	42,000,000	0
Gas Distribution Operations | Southern Company Gas
Goodwill [Roll Forward]
Beginning balance		4,702,000,000		4,702,000,000
Impairment				0
Dispositions				(668,000,000)
Ending balance	4,034,000,000			4,034,000,000	4,702,000,000
Impairment charge				0
Gas marketing services | Southern Company Gas
Goodwill [Roll Forward]
Beginning balance		$ 1,265,000,000		1,265,000,000
Impairment				(42,000,000)
Dispositions				(242,000,000)
Ending balance	$ 981,000,000			981,000,000	$ 1,265,000,000
Impairment charge				$ 42,000,000