Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Sep. 30, 2015 USD ($)
Southern Company [Member]
Long-term debt:
Long-term debt, including securities due within one year, Carrying Amount	$ 25,489
Long-term debt, including securities due within one year, Fair Value	26,099
Alabama Power [Member]
Long-term debt:
Long-term debt, including securities due within one year, Carrying Amount	7,295
Long-term debt, including securities due within one year, Fair Value	7,558
Georgia Power [Member]
Long-term debt:
Long-term debt, including securities due within one year, Carrying Amount	9,887
Long-term debt, including securities due within one year, Fair Value	10,231
Gulf Power [Member]
Long-term debt:
Long-term debt, including securities due within one year, Carrying Amount	1,310
Long-term debt, including securities due within one year, Fair Value	1,338
Mississippi Power [Member]
Long-term debt:
Long-term debt, including securities due within one year, Carrying Amount	2,273
Long-term debt, including securities due within one year, Fair Value	2,228
Southern Power [Member]
Long-term debt:
Long-term debt, including securities due within one year, Carrying Amount	2,142
Long-term debt, including securities due within one year, Fair Value	$ 2,149