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 |