Financing - Notes Payable Narrative (Details)	1 Months Ended				3 Months Ended		12 Months Ended
	Aug. 31, 2018 USD ($)	Jun. 30, 2018 USD ($) agreement	May 31, 2018 USD ($) agreement	Apr. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2018 USD ($)	Jan. 31, 2018 USD ($)
Short-term Debt [Line Items]
Ratio of indebtedness to capitalization, debt covenant, required							70.00%
Repayments of short-term debt							$ 1,900,000,000	$ 659,000,000	$ 478,000,000
Floating rate Promissory Note Due July 2018
Short-term Debt [Line Items]
Debt instrument, face amount											$ 100,000,000
Notes Payable to Banks | Floating Rate Bank Term Loan Agreement
Short-term Debt [Line Items]
Debt instrument, face amount										$ 900,000,000
Notes Payable to Banks | Uncommitted Bank Credit Arrangement
Short-term Debt [Line Items]
Debt instrument, face amount				$ 250,000,000
Payable by demand from bank, term				30 days
Repayments of short-term debt	$ 250,000,000
Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 6
Short-term Debt [Line Items]
Debt instrument, face amount	$ 1,500,000,000.0
Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 4
Short-term Debt [Line Items]
Debt instrument, face amount		$ 100,000,000				$ 100,000,000
Number of loan agreements | agreement		2
Alabama Power and Southern Power
Short-term Debt [Line Items]
Ratio of indebtedness to capitalization, debt covenant, required							65.00%
Georgia Power
Short-term Debt [Line Items]
Repayments of short-term debt							$ 150,000,000	550,000,000	0
Georgia Power | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 2
Short-term Debt [Line Items]
Debt instrument, face amount											$ 150,000,000
Georgia Power | Senior Notes
Short-term Debt [Line Items]
Repayments of debt			$ 902,000,000
Mississippi Power
Short-term Debt [Line Items]
Repayments of short-term debt							300,000,000	109,000,000	478,000,000
Mississippi Power | Senior Notes | Series 2018A Floating Rate Senior Notes
Short-term Debt [Line Items]
Debt instrument, face amount										$ 300,000,000
Mississippi Power | Promissory Notes | Promissory Notes
Short-term Debt [Line Items]
Repayments of debt					$ 100,000,000	$ 200,000,000
Southern Power
Short-term Debt [Line Items]
Repayments of short-term debt							$ 100,000,000	$ 0	$ 0
Southern Power | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement
Short-term Debt [Line Items]
Debt instrument, face amount			$ 100,000,000
Number of loan agreements | agreement			2
Southern Company Gas | Notes Payable to Banks | Uncommitted Bank Credit Arrangement
Short-term Debt [Line Items]
Debt instrument, face amount			$ 95,000,000
Payable by demand from bank, term			30 days
Southern Company Gas | Senior Notes | Series 2017A
Short-term Debt [Line Items]
Debt instrument, face amount				$ 181,000,000