Summary of Significant Accounting Policies - Schedule of Goodwill Balances (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 |
|
| Goodwill [Roll Forward] | |||||
| Beginning balance | $ 6,268,000,000 | $ 6,268,000,000 | $ 6,251,000,000 | ||
| Impairment | $ 0 | (42,000,000) | |||
| Dispositions | (910,000,000) | ||||
| Ending balance | 5,315,000,000 | $ 6,251,000,000 | 5,315,000,000 | 6,268,000,000 | |
| Impairment charge | 0 | 42,000,000 | |||
| Southern Company Gas | |||||
| Goodwill [Roll Forward] | |||||
| Beginning balance | 5,967,000,000 | 5,967,000,000 | |||
| Impairment | 0 | (42,000,000) | 0 | (42,000,000) | 0 |
| Ending balance | 5,015,000,000 | 5,015,000,000 | 5,967,000,000 | ||
| Impairment charge | 0 | 42,000,000 | $ 0 | 42,000,000 | 0 |
| Gas Distribution Operations | Southern Company Gas | |||||
| Goodwill [Roll Forward] | |||||
| Beginning balance | 4,702,000,000 | 4,702,000,000 | |||
| Impairment | 0 | ||||
| Dispositions | (668,000,000) | ||||
| Ending balance | 4,034,000,000 | 4,034,000,000 | 4,702,000,000 | ||
| Impairment charge | 0 | ||||
| Gas marketing services | Southern Company Gas | |||||
| Goodwill [Roll Forward] | |||||
| Beginning balance | $ 1,265,000,000 | 1,265,000,000 | |||
| Impairment | (42,000,000) | ||||
| Dispositions | (242,000,000) | ||||
| Ending balance | $ 981,000,000 | 981,000,000 | $ 1,265,000,000 | ||
| Impairment charge | $ 42,000,000 | ||||