Commitments - Capacity Expenses (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Recorded Unconditional Purchase Obligation [Line Items] | |||
| Expense under purchase power agreements accounted for as operating leases | $ 231 | $ 235 | $ 232 |
| Alabama Power | |||
| Recorded Unconditional Purchase Obligation [Line Items] | |||
| Expense under purchase power agreements accounted for as operating leases | 44 | 41 | 42 |
| Georgia Power | |||
| Recorded Unconditional Purchase Obligation [Line Items] | |||
| Expense under purchase power agreements accounted for as operating leases | $ 113 | $ 118 | $ 113 |