Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions	Mar. 31, 2017 USD ($)
Southern Company [Member]
Long-term debt:
Carrying Amount	$ 45,881
Fair Value	46,828
Alabama Power [Member]
Long-term debt:
Carrying Amount	7,439
Fair Value	7,807
Georgia Power [Member]
Long-term debt:
Carrying Amount	11,362
Fair Value	11,777
Gulf Power [Member]
Long-term debt:
Carrying Amount	1,079
Fair Value	1,110
Mississippi Power [Member]
Long-term debt:
Carrying Amount	2,977
Fair Value	2,909
Southern Power [Member]
Long-term debt:
Carrying Amount	5,648
Fair Value	5,694
Southern Company Gas [Member]
Long-term debt:
Carrying Amount	5,268
Fair Value	$ 5,487