Income Taxes - Accrued Interest for Unrecognized Tax Benefits (Details) - Mississippi Power [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Accrued Interest For Unrecognized Tax Benefits [Roll Forward]
Interest accrued at beginning of year	$ 13	$ 3	$ 1
Interest accrued during the period	15	10	2
Balance at end of year	$ 28	$ 13	$ 3