Contingencies and Regulatory Matters - Recovery Balance of Each Regulatory Clause - Alabama Power (Details) - ALABAMA POWER CO - USD ($) $ in Millions	Sep. 30, 2017	Feb. 17, 2017	Dec. 31, 2016
Deferred over recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP Compliance	$ 9		$ 0
Deferred under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP Compliance	0		9
Rate CNP PPA	17		142
Other regulatory liabilities, current
Loss Contingencies [Line Items]
Retail Energy Cost Recovery()	0		76
Over (under) recovered environmental clause		$ (11)
Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Natural Disaster Reserve	$ 51		$ 69
Regulatory assets, deferred
Loss Contingencies [Line Items]
Over (under) recovered environmental clause		$ 36