Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013
Long-term debt:
Long-term debt, Carrying Amount	$ 22,194
Long-term debt, Fair Value	22,822
Alabama Power [Member]
Long-term debt:
Long-term debt, Carrying Amount	6,179
Long-term debt, Fair Value	6,536
Georgia Power [Member]
Long-term debt:
Long-term debt, Carrying Amount	9,222
Long-term debt, Fair Value	9,400
Gulf Power [Member]
Long-term debt:
Long-term debt, Carrying Amount	1,246
Long-term debt, Fair Value	1,293
Mississippi Power [Member]
Long-term debt:
Long-term debt, Carrying Amount	2,140
Long-term debt, Fair Value	2,108
Southern Power [Member]
Long-term debt:
Long-term debt, Carrying Amount	1,628
Long-term debt, Fair Value	$ 1,667