Summary of Significant Accounting Policies - Schedule of Storm Damage Reserves (Details) - Major storms - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Liability for Catastrophe Claims [Line Items] | |||
| Storm damage reserves | $ 74.0 | $ 41.0 | $ 40.0 |
| Alabama Power | |||
| Liability for Catastrophe Claims [Line Items] | |||
| Storm damage reserves | 16.0 | 4.0 | 3.0 |
| Georgia Power | |||
| Liability for Catastrophe Claims [Line Items] | |||
| Storm damage reserves | 30.0 | 30.0 | 30.0 |
| Mississippi Power | |||
| Liability for Catastrophe Claims [Line Items] | |||
| Storm damage reserves | 1.0 | 3.0 | 4.0 |
| Gulf Power | |||
| Liability for Catastrophe Claims [Line Items] | |||
| Storm damage reserves | $ 26.9 | $ 3.5 | $ 3.5 |