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