Fair Value Measurements - Financial Instruments, Carrying Amount Not Equal to Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, Carrying Amount	$ 45,080	$ 27,216
Long-term debt, Fair Value	46,286	27,913
Alabama Power [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, Carrying Amount	7,092	6,849
Long-term debt, Fair Value	7,544	7,192
Georgia Power [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, Carrying Amount	10,516	10,145
Long-term debt, Fair Value	11,034	10,480
Gulf Power [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, Carrying Amount	1,074	1,303
Long-term debt, Fair Value	1,097	1,339
Mississippi Power [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, Carrying Amount	2,979	2,537
Long-term debt, Fair Value	2,922	2,413
Southern Power [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, Carrying Amount	5,628	3,122
Long-term debt, Fair Value	5,691	3,117
Successor [Member] | Southern Company Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, Carrying Amount	5,281
Long-term debt, Fair Value	$ 5,491
Predecessor [Member] | Southern Company Gas [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, Carrying Amount		3,820
Long-term debt, Fair Value		$ 4,066