Income Taxes - Impact of Unrecognized Tax Benefits on Effective Tax Rate, If Recognized (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Impact on effective tax rate
Tax positions impacting the effective tax rate		$ 18,000,000	$ 20,000,000
Tax positions not impacting the effective tax rate		0	464,000,000
Balance of unrecognized tax benefits	$ 0	18,000,000	484,000,000	$ 433,000,000
Mississippi Power
Impact on effective tax rate
Tax positions impacting the effective tax rate		0	1,000,000
Tax positions not impacting the effective tax rate		0	464,000,000
Balance of unrecognized tax benefits	0	0	465,000,000	421,000,000
Southern Power
Impact on effective tax rate
Tax positions impacting the effective tax rate		0	17,000,000
Tax positions not impacting the effective tax rate		0	0
Balance of unrecognized tax benefits	$ 0	$ 0	$ 17,000,000	$ 8,000,000