Income Taxes - Accrued Interest for Unrecognized Tax Benefits (Details) - MISSISSIPPI POWER CO - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Accrued Interest For Unrecognized Tax Benefits [Roll Forward]
Interest accrued at beginning of year	$ 28	$ 13	$ 3
Interest accrued during the period	(28)	15	10
Balance at end of year	$ 0	$ 28	$ 13