Contingencies and Regulatory Matters - Alabama Power (Details) - USD ($) $ in Millions	Aug. 06, 2018	Jun. 30, 2018	Dec. 31, 2017
ALABAMA POWER CO | Deferred under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP Compliance		$ 30	$ 17
Rate CNP PPA		11	12
Retail Energy Cost Recovery		80	25
ALABAMA POWER CO | Under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Retail Energy Cost Recovery		28	0
ALABAMA POWER CO | Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Natural Disaster Reserve		$ 30	$ 38
Plant Greene County Units 1 and 2 | Reserve Margin Plan | Subsequent Event | MISSISSIPPI POWER CO
Loss Contingencies [Line Items]
Accelerated retirement period	4 years