Income Taxes - Accrued Interest for Unrecognized Tax Benefits (Details) (Mississippi Power [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Mississippi Power [Member]
Reconciliation of Accrued Interest For Unrecognized Tax Benefits [Roll Forward]
Interest accrued at beginning of year	$ 1,171	$ 772	$ 680
Interest accrued during the period	1,698	399	92
Balance at end of year	$ 2,869	$ 1,171	$ 772