Financing - Outstanding Classes of Capital Stock (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Temporary Equity [Line Items]
Redeemable preferred stock	$ 118	$ 118	$ 375
Preferred and preference stock	$ 0	$ 412	$ 0
Mississippi Power [Member]
Temporary Equity [Line Items]
Temporary Equity, Shares Outstanding	334,210	334,210
Preferred Stock Dividend Rate Stated Percentage Rate Range Maximum	5.25%	5.25%
Redeemable Preferred Stock [Member]
Temporary Equity [Line Items]
Redeemable preferred stock	$ 118	$ 118		$ 375
Preferred and preference stock		262
Temporary Equity, Other Changes		$ 5
Depositary Shares [Member] | Mississippi Power [Member]
Temporary Equity [Line Items]
Temporary Equity, Shares Outstanding	1,200,000,000,000