Summary of Significant Accounting Policies - Goodwill and Other Intangible Assets and Liabilities (Narrative) (Details) - USD ($) |
3 Months Ended | 6 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|---|
Dec. 31, 2018 |
Mar. 31, 2018 |
Dec. 31, 2016 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Finite-Lived Intangible Assets [Line Items] | |||||
| Impairment charge | $ 0 | $ 42,000,000 | |||
| Intangible liabilities included in other deferred credits and liabilities | 465,000,000 | 465,000,000 | $ 691,000,000 | ||
| Southern Company Gas | |||||
| Finite-Lived Intangible Assets [Line Items] | |||||
| Impairment charge | 0 | $ 42,000,000 | $ 0 | 42,000,000 | 0 |
| Intangible liabilities included in other deferred credits and liabilities | 105,000,000 | 105,000,000 | $ 118,000,000 | ||
| Accumulated amortization of intangible liabilities | 74,000,000 | 74,000,000 | |||
| Remaining amortization to be recorded in 2019 | 17,000,000 | 17,000,000 | |||
| Intangible liabilities | Southern Company Gas | |||||
| Finite-Lived Intangible Assets [Line Items] | |||||
| Intangible liabilities included in other deferred credits and liabilities | $ 91,000,000 | $ 91,000,000 | |||