Retirement Benefits - 401K Plan Matching Contributions (Details) - Employee Savings Plan - USD ($) $ in Millions |
6 Months Ended | 12 Months Ended | |||
|---|---|---|---|---|---|
Dec. 31, 2016 |
Jun. 30, 2016 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Defined Benefit Plan Disclosure [Line Items] | |||||
| Defined contribution plan, cost | $ 119 | $ 118 | $ 105 | ||
| Alabama Power | |||||
| Defined Benefit Plan Disclosure [Line Items] | |||||
| Defined contribution plan, cost | 24 | 23 | 23 | ||
| Georgia Power | |||||
| Defined Benefit Plan Disclosure [Line Items] | |||||
| Defined contribution plan, cost | 26 | 26 | 27 | ||
| Mississippi Power | |||||
| Defined Benefit Plan Disclosure [Line Items] | |||||
| Defined contribution plan, cost | 5 | 5 | $ 5 | ||
| Southern Power | |||||
| Defined Benefit Plan Disclosure [Line Items] | |||||
| Defined contribution plan, cost | 3 | ||||
| Southern Company Gas | |||||
| Defined Benefit Plan Disclosure [Line Items] | |||||
| Defined contribution plan, cost | $ 8 | $ 18 | $ 19 | ||
| Predecessor | Southern Company Gas | |||||
| Defined Benefit Plan Disclosure [Line Items] | |||||
| Defined contribution plan, cost | $ 12 | ||||