Financing - Narrative (Details) $ / shares in Units, shares in Thousands			1 Months Ended						3 Months Ended		9 Months Ended
	Jun. 03, 2015 USD ($)	Feb. 20, 2014	Oct. 31, 2017 USD ($) shares	Sep. 30, 2017 USD ($) $ / shares shares	Aug. 31, 2017 USD ($)	Jun. 30, 2017 USD ($) agreement	May 31, 2017 USD ($) shares	Feb. 28, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($)	Nov. 30, 2017 USD ($)	Sep. 29, 2017 USD ($)	Sep. 28, 2017 USD ($)	Aug. 10, 2017 USD ($)	Jul. 31, 2017 USD ($)	Apr. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Short-term debt				$ 2,579,000,000							$ 2,579,000,000									$ 2,241,000,000
Variable rate pollution control revenue bonds outstanding				1,500,000,000							1,500,000,000
Maximum borrowing capacity				8,145,000,000							8,145,000,000
Value of stock redeemed											150,000,000
Line Of Credit Expire Year Four				530,000,000							530,000,000
Southern Company
Debt Instrument [Line Items]
Maximum borrowing capacity				2,000,000,000			$ 1,250,000,000.00				2,000,000,000
Line Of Credit Expire Year Four				0							0
Southern Company | Line of Credit | Uncommitted Bank Credit Arrangement
Debt Instrument [Line Items]
Increase in debt instrument					$ 250,000,000
Payable to bank by demand, term					30 days
Southern Company | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount						$ 500,000,000			$ 500,000,000
Debt stated interest rate						5.325%			5.325%
Southern Company | Senior Notes | Series 2017A Floating Rate Senior Notes
Debt Instrument [Line Items]
Debt face amount						$ 300,000,000			$ 300,000,000
Southern Company | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement
Debt Instrument [Line Items]
Number of loan agreements | agreement						2
Southern Company | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 4
Debt Instrument [Line Items]
Debt face amount						$ 100,000,000			100,000,000
Southern Company | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 5
Debt Instrument [Line Items]
Debt face amount						100,000,000			100,000,000
MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Working capital				769,000,000							769,000,000
Senior notes, current				4,000,000							4,000,000
Short-term debt				4,000,000							4,000,000									23,000,000
Long-term pollution control bond				50,000,000							50,000,000
Variable rate pollution control revenue bonds outstanding				40,000,000							40,000,000
Fixed rate pollution control revenue bonds outstanding				50,000,000							50,000,000
Maximum borrowing capacity				100,000,000							100,000,000
Promissory note	$ 301,000,000
Capital contributions from parent company						1,000,000,000					1,002,000,000	$ 227,000,000
Line Of Credit Expire Year Four				0							0
MISSISSIPPI POWER CO | Unsecured Debt
Debt Instrument [Line Items]
Loans payable to bank				935,000,000							935,000,000
MISSISSIPPI POWER CO | Tax-Exempt Variable Rate Demand Obligations
Debt Instrument [Line Items]
Short-term debt				40,000,000							40,000,000
MISSISSIPPI POWER CO | Short Term Note
Debt Instrument [Line Items]
Debt face amount																			$ 9,000,000
Debt stated interest rate																			5.00%
MISSISSIPPI POWER CO | Bank Loans | Loans Payable
Debt Instrument [Line Items]
Repayments of debt						10,000,000
MISSISSIPPI POWER CO | Notes Payable to Banks | Notes Payable to Banks
Debt Instrument [Line Items]
Repayments of debt					$ 12,500,000
MISSISSIPPI POWER CO | Promissory Notes | Promissory Notes
Debt Instrument [Line Items]
Debt face amount				150,000,000							150,000,000
Increase in debt instrument									40,000,000
Repayments of debt				109,000,000		591,000,000
MISSISSIPPI POWER CO | Unsecured Debt | Term Loan
Debt Instrument [Line Items]
Debt face amount						1,200,000,000.0			1,200,000,000.0
Repayments of debt						300,000,000
GEORGIA POWER CO
Debt Instrument [Line Items]
Short-term debt				400,000,000							400,000,000									391,000,000
Debt instrument, basis spread on variable rate		0.375%
Amortization period for line of credit facility		5 years
Variable rate pollution control revenue bonds outstanding				550,000,000							550,000,000
Variable rate pollution control revenue bonds outstanding, amount remarketed				318,000,000							318,000,000
Fixed rate pollution control revenue bonds outstanding				509,000,000							509,000,000
Maximum borrowing capacity				1,750,000,000							1,750,000,000
Capital contributions from parent company											412,000,000	294,000,000
Line Of Credit Expire Year Four				0							0
GEORGIA POWER CO | Loan Guarantee Agreement
Debt Instrument [Line Items]
Conditional borrowing commitment															$ 1,670,000,000
GEORGIA POWER CO | Line of Credit | Uncommitted Bank Credit Arrangement
Debt Instrument [Line Items]
Increase in debt instrument						$ 500,000,000
Repayments of debt					250,000,000
GEORGIA POWER CO | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount																			$ 450,000,000
Debt stated interest rate																			2.00%
GEORGIA POWER CO | Senior Notes | Series 2017B
Debt Instrument [Line Items]
Debt face amount																			$ 400,000,000
Debt stated interest rate																			3.25%
GEORGIA POWER CO | Senior Notes | Series 2017C
Debt Instrument [Line Items]
Debt face amount					$ 500,000,000
Debt stated interest rate					2.00%
GEORGIA POWER CO | Municipal Bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Fifth Series 1995
Debt Instrument [Line Items]
Debt face amount																		$ 27,000,000
GEORGIA POWER CO | Municipal Bonds | Development Authority of Bartow County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Bowen Project), First Series 1997
Debt Instrument [Line Items]
Debt face amount					$ 38,000,000
GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement
Debt Instrument [Line Items]
Number of loan agreements | agreement						3
Payable to bank by demand, term						30 days
GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 1
Debt Instrument [Line Items]
Debt face amount					50,000,000	$ 50,000,000			50,000,000
GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 2
Debt Instrument [Line Items]
Debt face amount						150,000,000			150,000,000
GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 3
Debt Instrument [Line Items]
Debt face amount					$ 100,000,000	$ 100,000,000			$ 100,000,000
GEORGIA POWER CO | Junior Subordinated Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount				$ 270,000,000							$ 270,000,000
Debt stated interest rate				5.00%							5.00%
ALABAMA POWER CO
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding				$ 854,000,000							$ 854,000,000
Maximum borrowing capacity				1,335,000,000							1,335,000,000
Capital contributions from parent company											337,000,000	253,000,000
Line Of Credit Expire Year Four				$ 500,000,000							$ 500,000,000
ALABAMA POWER CO | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount																			$ 550,000,000
Debt stated interest rate																			2.45%
ALABAMA POWER CO | Senior Notes | 5.00% Class A Preferred Stock
Debt Instrument [Line Items]
Preferred stock, shares issued (in shares) | shares				10,000
Preferred stock, value of shares issued				$ 250,000,000
Preferred stock, par value (in dollars per share) | $ / shares				$ 1							$ 1
Share price (in dollars per share) | $ / shares				$ 25							$ 25
Preferred stock, dividend rate				5.00%
GULF POWER CO
Debt Instrument [Line Items]
Short-term debt				$ 0							$ 0									268,000,000
Variable rate pollution control revenue bonds outstanding				82,000,000							82,000,000
Fixed rate pollution control revenue bonds outstanding				140,000,000							140,000,000
Maximum borrowing capacity				280,000,000							280,000,000
Capital contributions from parent company											7,000,000	10,000,000
Line Of Credit Expire Year Four				30,000,000							30,000,000
GULF POWER CO | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount							$ 300,000,000
Debt stated interest rate							3.30%
GULF POWER CO | Senior Notes | Series 2007A Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares							450
Value of stock redeemed							$ 45,000,000
Preferred stock, dividend rate							6.45%
GULF POWER CO | Senior Notes | Series 2007A
Debt Instrument [Line Items]
Debt stated interest rate							5.90%
Repayments of debt							$ 85,000,000
GULF POWER CO | Senior Notes | Short-term Floating Rate Bank Loan
Debt Instrument [Line Items]
Repayments of debt							$ 100,000,000
GULF POWER CO | Senior Notes | Series Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares							550
Value of stock redeemed							$ 55,000,000
Preferred stock, dividend rate							6.00%
GULF POWER CO | Senior Notes | Series 2013A Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares							500
Value of stock redeemed							$ 50,000,000
Preferred stock, dividend rate							5.60%
GULF POWER CO | Bank Loans
Debt Instrument [Line Items]
Debt face amount																			$ 100,000,000
Traditional Electric Operating Companies
Debt Instrument [Line Items]
Fixed rate pollution control revenue bonds outstanding				699,000,000							699,000,000
SOUTHERN POWER CO
Debt Instrument [Line Items]
Short-term debt				120,000,000							120,000,000									209,000,000
Maximum borrowing capacity				750,000,000			$ 600,000,000				750,000,000
Repayments of debt				40,000,000
Capital contributions from parent company											0	$ 800,000,000
Line Of Credit Expire Year Four				0							0
SOUTHERN POWER CO | Notes Payable to Banks | Floating Rate Bank Loan
Debt Instrument [Line Items]
Debt face amount				100,000,000							100,000,000			$ 60,000,000
Southern Company Gas Capital
Debt Instrument [Line Items]
Maximum borrowing capacity				1,200,000,000.0			1,200,000,000.0				1,200,000,000.0
Nicor Gas
Debt Instrument [Line Items]
Maximum borrowing capacity				700,000,000			700,000,000				700,000,000
Nicor Gas | Collateralized Mortgage Backed Securities | First Mortgage Bonds
Debt Instrument [Line Items]
Debt face amount																	$ 400,000,000
Nicor Gas | Collateralized Mortgage Backed Securities | First Mortgage Bonds 3.03% Series
Debt Instrument [Line Items]
Debt face amount																$ 100,000,000
Debt stated interest rate																3.03%
Nicor Gas | Collateralized Mortgage Backed Securities | First Mortgage Bonds 3.62% Series
Debt Instrument [Line Items]
Debt face amount																$ 100,000,000
Debt stated interest rate																3.62%
SOUTHERN Co GAS
Debt Instrument [Line Items]
Short-term debt				934,000,000							934,000,000									$ 1,257,000,000
Maximum borrowing capacity				1,900,000,000							1,900,000,000
Capital contributions from parent company										$ 1,089,000,000	79,000,000
Line Of Credit Expire Year Four				$ 0							$ 0
SOUTHERN Co GAS | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount							$ 450,000,000
Debt stated interest rate							4.40%
Southern Company | MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Promissory note								$ 551,000,000
Line of Credit | Southern Company | Line of Credit
Debt Instrument [Line Items]
Extinguishment of debt, amount							$ 1,000,000,000
Line of Credit | Southern Company Gas Capital
Debt Instrument [Line Items]
Termination of credit arrangement							1,300,000,000
Line of Credit | Nicor Gas | Line of Credit
Debt Instrument [Line Items]
Termination of credit arrangement							$ 700,000,000
Scenario, Forecast | Nicor Gas | Collateralized Mortgage Backed Securities | First Mortgage Bonds
Debt Instrument [Line Items]
Debt face amount													$ 200,000,000
Subsequent Event | GEORGIA POWER CO
Debt Instrument [Line Items]
Fixed rate pollution control revenue bonds outstanding, amount remarketed			$ 40,000,000
Subsequent Event | GEORGIA POWER CO | Senior Notes | Preferred Class A
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares			1,800
Value of stock redeemed			$ 45,000,000
Preferred stock, dividend rate			6.125%
Subsequent Event | GEORGIA POWER CO | Senior Notes | Series 2007A Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares			2,250
Value of stock redeemed			$ 225,000,000
Preferred stock, dividend rate			6.50%
Subsequent Event | ALABAMA POWER CO | Senior Notes | 6.50% Series Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares			2,000
Value of stock redeemed			$ 50,000,000
Preferred stock, dividend rate			6.50%
Subsequent Event | ALABAMA POWER CO | Senior Notes | 6.45% Series Preference Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares			6,000
Value of stock redeemed			$ 150,000,000
Preferred stock, dividend rate			6.45%
Subsequent Event | ALABAMA POWER CO | Senior Notes | 5.83% Class A Preferred Stock
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares			1,520
Value of stock redeemed			$ 38,000,000
Preferred stock, dividend rate			5.83%