Regulatory Matters - Schedule of GAS Unrecognized Ratemaking Amounts (Details) - Regulatory Asset Off Balance Sheet - USD ($) $ in Millions |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Atlanta Gas Light | ||
| Schedule of Regulatory Assets and Liabilities [Line Items] | ||
| Regulatory asset | $ 95 | $ 104 |
| Virginia Natural Gas | ||
| Schedule of Regulatory Assets and Liabilities [Line Items] | ||
| Regulatory asset | 11 | 11 |
| Nicor Gas | ||
| Schedule of Regulatory Assets and Liabilities [Line Items] | ||
| Regulatory asset | 4 | 2 |
| Southern Company Gas | ||
| Schedule of Regulatory Assets and Liabilities [Line Items] | ||
| Regulatory asset | $ 110 | $ 117 |