Summary of Significant Accounting Policies - Concentration of Revenue (Narrative) (Details) $ in Millions |
12 Months Ended | |
|---|---|---|
|
Dec. 31, 2018
USD ($)
|
Jan. 29, 2019
Facility
MW
|
|
| Concentration Risk [Line Items] | ||
| Outstanding accounts receivable | $ 2,630 | |
| Mississippi Power | ||
| Concentration Risk [Line Items] | ||
| Wholesale customers representing total operating revenues (percent) | 17.30% | |
| Period of cancellation notices | 10 years | |
| Outstanding accounts receivable | $ 100 | |
| Southern Power | ||
| Concentration Risk [Line Items] | ||
| Outstanding accounts receivable | 118 | |
| Subsequent Event | PG&E | ||
| Concentration Risk [Line Items] | ||
| Number of solar facilities | Facility | 4 | |
| Capacity of natural gas facility (mw) | MW | 207 | |
| PPAs | Southern Power | ||
| Concentration Risk [Line Items] | ||
| Outstanding accounts receivable | 1 | |
| Transmission Interconnections | Southern Power | ||
| Concentration Risk [Line Items] | ||
| Outstanding accounts receivable | 36 | |
| Outstanding accounts receivable classified in other deferred charges and assets | $ 17 |