Summary of Significant Accounting Policies - Income and Other Taxes (Narrative) (Details) - USD ($) $ in Millions |
6 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|
Dec. 31, 2016 |
Jun. 30, 2016 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Southern Power | ||||
| Tax Credit Carryforward [Line Items] | ||||
| Reduction in tax basis of assets (percent) | 50.00% | |||
| Southern Company Gas | ||||
| Tax Credit Carryforward [Line Items] | ||||
| Excise taxes collected | $ 32 | $ 114 | $ 100 | |
| Included In Operating Expenses | Southern Company Gas | ||||
| Tax Credit Carryforward [Line Items] | ||||
| Excise taxes collected | $ 31 | $ 111 | $ 98 | |
| Predecessor | Southern Company Gas | ||||
| Tax Credit Carryforward [Line Items] | ||||
| Excise taxes collected | $ 57 | |||
| Predecessor | Included In Operating Expenses | Southern Company Gas | ||||
| Tax Credit Carryforward [Line Items] | ||||
| Excise taxes collected | $ 56 | |||