Contingencies and Regulatory Matters - Alabama Power (Details) - ALABAMA POWER CO - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Deferred under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP Compliance	$ 15	$ 17
Rate CNP PPA	8	12
Retail Energy Cost Recovery	78	25
Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Natural Disaster Reserve	$ 38	$ 38