Commitments - Rent Expense (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Recorded Unconditional Purchase Obligation [Line Items]
Operating leases rent expense			$ 192	$ 176	$ 169
Alabama Power
Recorded Unconditional Purchase Obligation [Line Items]
Operating leases rent expense			23	25	18
Georgia Power
Recorded Unconditional Purchase Obligation [Line Items]
Operating leases rent expense			34	31	28
Mississippi Power
Recorded Unconditional Purchase Obligation [Line Items]
Operating leases rent expense			4	3	3
Southern Power
Recorded Unconditional Purchase Obligation [Line Items]
Operating leases rent expense			31	29	$ 22
Southern Company Gas
Recorded Unconditional Purchase Obligation [Line Items]
Operating leases rent expense	$ 8		$ 15	$ 15
Predecessor | Southern Company Gas
Recorded Unconditional Purchase Obligation [Line Items]
Operating leases rent expense		$ 6