Retirement Benefits - Accumulated Benefit Obligation (Details) - Pension plans - USD ($) $ in Millions |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Defined Benefit Plan Disclosure [Line Items] | ||
| Total accumulated benefit obligation for the pension plans | $ 11,683 | $ 12,577 |
| Alabama Power | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Total accumulated benefit obligation for the pension plans | 2,550 | 2,696 |
| Georgia Power | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Total accumulated benefit obligation for the pension plans | 3,613 | 3,847 |
| Mississippi Power | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Total accumulated benefit obligation for the pension plans | 513 | 541 |
| Southern Power | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Total accumulated benefit obligation for the pension plans | 101 | 111 |
| Southern Company Gas | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Total accumulated benefit obligation for the pension plans | $ 842 | $ 1,139 |