Commitments - Fuel Expenses (Details) - Fuel - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Long-term Purchase Commitment [Line Items]
Cost of goods and services sold	$ 4,637	$ 4,400	$ 4,361
Alabama Power
Long-term Purchase Commitment [Line Items]
Cost of goods and services sold	1,301	1,225	1,297
Georgia Power
Long-term Purchase Commitment [Line Items]
Cost of goods and services sold	1,698	1,671	1,807
Mississippi Power
Long-term Purchase Commitment [Line Items]
Cost of goods and services sold	405	395	343
Southern Power
Long-term Purchase Commitment [Line Items]
Cost of goods and services sold	$ 699	$ 621	$ 456