Fair Value Measurements - Schedule of Increase (Decrease) In Fair Value Of Funds (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2019 |
Jun. 30, 2018 |
Jun. 30, 2019 |
Jun. 30, 2018 |
|
| Fair Value, Option, Quantitative Disclosures [Line Items] | ||||
| Increase (decrease) in fair value of funds | $ 75 | $ 14 | $ 227 | $ 4 |
| ALABAMA POWER CO | ||||
| Fair Value, Option, Quantitative Disclosures [Line Items] | ||||
| Increase (decrease) in fair value of funds | 38 | 15 | 125 | 10 |
| GEORGIA POWER CO | ||||
| Fair Value, Option, Quantitative Disclosures [Line Items] | ||||
| Increase (decrease) in fair value of funds | $ 37 | $ (1) | $ 102 | $ (6) |