Income Taxes - Impact of Unrecognized Tax Benefits on Effective Tax Rate, If Recognized (Details) - USD ($) |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|---|---|---|---|---|
| Impact on effective tax rate | ||||
| Tax positions impacting the effective tax rate | $ 18,000,000 | $ 20,000,000 | ||
| Tax positions not impacting the effective tax rate | 0 | 464,000,000 | ||
| Balance of unrecognized tax benefits | $ 0 | 18,000,000 | 484,000,000 | $ 433,000,000 |
| Mississippi Power | ||||
| Impact on effective tax rate | ||||
| Tax positions impacting the effective tax rate | 0 | 1,000,000 | ||
| Tax positions not impacting the effective tax rate | 0 | 464,000,000 | ||
| Balance of unrecognized tax benefits | 0 | 0 | 465,000,000 | 421,000,000 |
| Southern Power | ||||
| Impact on effective tax rate | ||||
| Tax positions impacting the effective tax rate | 0 | 17,000,000 | ||
| Tax positions not impacting the effective tax rate | 0 | 0 | ||
| Balance of unrecognized tax benefits | $ 0 | $ 0 | $ 17,000,000 | $ 8,000,000 |