Contingencies and Regulatory Matters - Cost Recovery Clauses (Details) - ALABAMA POWER CO - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP Compliance	$ 0	$ 17
Rate CNP PPA	30	12
Retail Energy Cost Recovery	58	25
Under recovered regulatory clause revenues
Loss Contingencies [Line Items]
Rate CNP Compliance	7	0
Retail Energy Cost Recovery	41	0
Other regulatory liabilities, deferred
Loss Contingencies [Line Items]
Natural Disaster Reserve	$ 24	$ 38