Income Taxes - Impact of Unrecognized Tax Benefits on Effective Tax Rate, If Recognized (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Impact on effective tax rate
Tax positions impacting the effective tax rate	$ 18	$ 20	$ 10
Tax positions not impacting the effective tax rate	0	464	423
Balance of unrecognized tax benefits	18	484	433	$ 170
MISSISSIPPI POWER CO
Impact on effective tax rate
Tax positions impacting the effective tax rate	0	1	(2)
Tax positions not impacting the effective tax rate	0	464	423
Balance of unrecognized tax benefits	$ 0	$ 465	$ 421	$ 421	$ 165