Fair Value Measurements - Financial Instruments for which Carrying Amount did not Equal Fair Value (Details) $ in Millions |
Jun. 30, 2019
USD ($)
|
|---|---|
| Long-term debt, including securities due within one year: | |
| Carrying amount | $ 42,596 |
| Fair value | 45,394 |
| ALABAMA POWER CO | |
| Long-term debt, including securities due within one year: | |
| Carrying amount | 7,922 |
| Fair value | 8,717 |
| GEORGIA POWER CO | |
| Long-term debt, including securities due within one year: | |
| Carrying amount | 10,969 |
| Fair value | 11,749 |
| MISSISSIPPI POWER CO | |
| Long-term debt, including securities due within one year: | |
| Carrying amount | 1,618 |
| Fair value | 1,657 |
| SOUTHERN POWER CO | |
| Long-term debt, including securities due within one year: | |
| Carrying amount | 5,011 |
| Fair value | 5,261 |
| SOUTHERN Co GAS | |
| Long-term debt, including securities due within one year: | |
| Carrying amount | 5,916 |
| Fair value | $ 6,420 |