Contingencies and Regulatory Matters - Recovery Balance of Each Regulatory Clause - Alabama Power (Details) - Alabama Power [Member] - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Under recovered regulatory clause revenues [Member]
Loss Contingencies [Line Items]
Rate CNP Compliance	$ 0	$ 43
Rate CNP PPA	52	99
Deferred under recovered regulatory clause revenues [Member]
Loss Contingencies [Line Items]
Rate CNP Compliance	23	0
Rate CNP PPA	87	0
Other regulatory liabilities, current [Member]
Loss Contingencies [Line Items]
Retail Energy Cost Recovery	0	238
Deferred over recovered regulatory clause revenues [Member]
Loss Contingencies [Line Items]
Retail Energy Cost Recovery	134	0
Other regulatory liabilities, deferred [Member]
Loss Contingencies [Line Items]
Natural Disaster Reserve	$ 71	$ 75