Derivatives - Foreign Currency Derivatives (Details) - 12 months ended Dec. 31, 2018 - Foreign currency derivatives - Cash Flow Hedges of Existing Debt € in Millions, $ in Millions |
USD ($) |
EUR (€) |
|---|---|---|
| Derivative [Line Items] | ||
| Pay Notional | $ 1,241 | |
| Receive Notional | € | € 1,100 | |
| Fair value gain (loss) at December 31, 2018 | 52 | |
| Southern Power | Maturity Date June 2022 | ||
| Derivative [Line Items] | ||
| Pay Notional | $ 677 | |
| Pay Rate | 2.95% | |
| Receive Notional | € | 600 | |
| Receive Rate | 1.00% | |
| Fair value gain (loss) at December 31, 2018 | $ 25 | |
| Southern Power | Maturity Date June 2026 | ||
| Derivative [Line Items] | ||
| Pay Notional | $ 564 | |
| Pay Rate | 3.78% | |
| Receive Notional | € | € 500 | |
| Receive Rate | 1.85% | |
| Fair value gain (loss) at December 31, 2018 | $ 27 |