Summary of Significant Accounting Policies - Schedule of Average AFUDC Composite Rates (Details) |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
Jun. 30, 2016 |
Dec. 31, 2015 |
|---|---|---|---|---|---|
| Alabama Power | |||||
| Capitalized Contract Cost [Line Items] | |||||
| Composite rate | 8.30% | 8.30% | 8.20% | ||
| Georgia Power | |||||
| Capitalized Contract Cost [Line Items] | |||||
| Composite rate | 7.30% | 5.60% | 6.90% | ||
| Mississippi Power | |||||
| Capitalized Contract Cost [Line Items] | |||||
| Composite rate | 3.30% | 6.70% | 6.50% | ||
| Atlanta Gas Light | |||||
| Capitalized Contract Cost [Line Items] | |||||
| Composite rate | 7.90% | 8.10% | 4.10% | ||
| Atlanta Gas Light | Predecessor | |||||
| Capitalized Contract Cost [Line Items] | |||||
| Composite rate | 4.10% | ||||
| Chattanooga Gas | |||||
| Capitalized Contract Cost [Line Items] | |||||
| Composite rate | 7.40% | 7.40% | 3.70% | ||
| Chattanooga Gas | Predecessor | |||||
| Capitalized Contract Cost [Line Items] | |||||
| Composite rate | 3.70% | ||||
| Nicor Gas | |||||
| Capitalized Contract Cost [Line Items] | |||||
| Composite rate | 2.10% | 1.20% | 1.50% | ||
| Nicor Gas | Predecessor | |||||
| Capitalized Contract Cost [Line Items] | |||||
| Composite rate | 1.50% |