Contingencies and Regulatory Matters - Recovery Balance of Each Regulatory Clause - Alabama Power (Details) - Alabama Power [Member] - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Deferred under recovered regulatory clause revenues [Member]
Loss Contingencies [Line Items]
Rate CNP Compliance – Under	$ 0	$ 2
Rate CNP PPA – Under	66	29
Under recovered regulatory clause revenues, current [Member]
Loss Contingencies [Line Items]
Rate CNP Compliance – Under	38	47
Rate CNP PPA – Under	30	27
Deferred over recovered regulatory clause revenues [Member]
Loss Contingencies [Line Items]
Retail Energy Cost Recovery – Over	128	47
Other regulatory liabilities, deferred [Member]
Loss Contingencies [Line Items]
Natural Disaster Reserve	$ 76	$ 84