Financing - Narrative (Details)	1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) loan	May 31, 2019 USD ($)	Mar. 31, 2019 USD ($) loan	Jan. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 USD ($) shares	Dec. 31, 2014	Oct. 30, 2019 USD ($)	Sep. 29, 2019 USD ($)	Aug. 31, 2019 USD ($)	Apr. 30, 2019 USD ($)	Aug. 31, 2018
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding					$ 1,400,000,000		$ 1,400,000,000
Borrowing capacity					$ 7,608,000,000		7,608,000,000
Repayments of short-term debt							$ 1,850,000,000	$ 1,800,000,000
Stock-based compensation awards not included in the diluted earnings per share calculation because they were anti-dilutive (in shares) | shares					0	0	0	0
Senior Notes
Debt Instrument [Line Items]
Repayments of debt				$ 1,200,000,000
Senior Notes | 1.85% Notes
Debt Instrument [Line Items]
Interest rate				1.85%
Repurchase amount				$ 522,000,000
Debt face amount				$ 1,000,000,000.0
Senior Notes | Series 2014B Notes
Debt Instrument [Line Items]
Interest rate				2.15%
Repurchase amount				$ 180,000,000
Debt face amount				350,000,000
Senior Notes | Series 2018A Notes
Debt Instrument [Line Items]
Repurchase amount				504,000,000
Debt face amount				750,000,000
Senior Notes | Series 2017A Notes
Debt Instrument [Line Items]
Redeemed aggregate principal amount					$ 300,000,000		$ 300,000,000
Junior Subordinated Debt | Series 2019A Remarketable Junior Subordinated Notes
Debt Instrument [Line Items]
Debt face amount												$ 1,725,000,000
Notes Payable to Banks | Uncommitted Bank Credit Arrangement
Debt Instrument [Line Items]
Repayments of short-term debt				250,000,000
Notes Payable to Banks | Floating Rate Bank Term Loan Agreement
Debt Instrument [Line Items]
Repayments of short-term debt				1,500,000,000
ALABAMA POWER CO
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding					854,000,000		854,000,000
Fixed rate pollution control revenue bonds outstanding					87,000,000		87,000,000
Borrowing capacity					1,328,000,000		1,328,000,000
ALABAMA POWER CO | Multi-year Credit Arrangement
Debt Instrument [Line Items]
Borrowing capacity					$ 525,000,000		$ 525,000,000				$ 500,000,000
ALABAMA POWER CO | Senior Notes | Series 2019A 3.45% Notes
Debt Instrument [Line Items]
Interest rate					3.45%		3.45%
Debt face amount					$ 600,000,000		$ 600,000,000
GEORGIA POWER CO
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding					550,000,000		550,000,000
Borrowing capacity					$ 1,750,000,000		1,750,000,000
Repayments of short-term debt							$ 0	$ 150,000,000
GEORGIA POWER CO | Senior Notes | Series 2019A 2.20% Notes
Debt Instrument [Line Items]
Interest rate					2.20%		2.20%
Debt face amount					$ 400,000,000		$ 400,000,000
GEORGIA POWER CO | Senior Notes | Series 2019B
Debt Instrument [Line Items]
Interest rate					2.65%		2.65%
Debt face amount					$ 350,000,000		$ 350,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project) First Series 1992
Debt Instrument [Line Items]
Redeemed aggregate principal amount				13,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Eighth Series 1994
Debt Instrument [Line Items]
Redeemed aggregate principal amount				20,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Second Series 1995
Debt Instrument [Line Items]
Redeemed aggregate principal amount				$ 75,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Bartow County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Bowen Project), First Series 2009
Debt Instrument [Line Items]
Debt face amount			$ 173,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), First Series 2013
Debt Instrument [Line Items]
Debt face amount			105,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Second Series 2008
Debt Instrument [Line Items]
Debt face amount			$ 65,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Fourth Series 1994
Debt Instrument [Line Items]
Debt face amount													$ 55,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Fourth Series 1995
Debt Instrument [Line Items]
Debt face amount													30,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Ninth Series 1994
Debt Instrument [Line Items]
Debt face amount													20,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Second Series 1994
Debt Instrument [Line Items]
Debt face amount													10,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Burke County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), Fifth Series 1994
Debt Instrument [Line Items]
Debt face amount	$ 55,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority of Bartow County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Bowen Project), First Series 2013
Debt Instrument [Line Items]
Debt face amount												72,000,000
GEORGIA POWER CO | Plant Vogtle Units 3 And 4
Debt Instrument [Line Items]
Undivided ownership interest									45.70%
GEORGIA POWER CO | Notes Payable to Banks
Debt Instrument [Line Items]
Number of short-term loans | loan	2
GEORGIA POWER CO | Notes Payable to Banks | First Short-Term Floating Rate Bank Loan
Debt Instrument [Line Items]
Debt face amount	$ 125,000,000
GEORGIA POWER CO | Notes Payable to Banks | Second Short-Term Floating Rate Bank Loan
Debt Instrument [Line Items]
Debt face amount					125,000,000		125,000,000
GEORGIA POWER CO | Term Loan
Debt Instrument [Line Items]
Number of term loan facilities | loan			2
GEORGIA POWER CO | 2014 FFB Credit Facility
Debt Instrument [Line Items]
Eligible project costs to be reimbursed			$ 5,130,000,000
Eligible project costs to be reimbursed, percentage (may not exceed)									70.00%
Borrowings made			$ 835,000,000
Interest rate			3.213%
Borrowings outstanding					3,460,000,000		3,460,000,000
Amortization period for line of credit facility									5 years
GEORGIA POWER CO | 2014 FFB Credit Facility | U.S. Treasury Rate
Debt Instrument [Line Items]
Basis spread									0.375%
GEORGIA POWER CO | Additional FFB Credit Facility
Debt Instrument [Line Items]
Eligible project costs to be reimbursed, percentage (may not exceed)			70.00%
Amounts received under Guarantee Settlement Agreement, less Customer Refunds			$ 1,492,000,000
MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding					40,000,000		40,000,000
Borrowing capacity					150,000,000		150,000,000
Repayments of short-term debt							0	300,000,000
MISSISSIPPI POWER CO | Municipal bonds | Mississippi Business Finance Corporation Pollution Control Revenue Refunding Bonds, Series 2002
Debt Instrument [Line Items]
Debt face amount			$ 43,000,000
MISSISSIPPI POWER CO | Credit Arrangement
Debt Instrument [Line Items]
Borrowing capacity					50,000,000		50,000,000
SOUTHERN Co GAS
Debt Instrument [Line Items]
Borrowing capacity					1,750,000,000		1,750,000,000
SOUTHERN POWER CO
Debt Instrument [Line Items]
Borrowing capacity		$ 600,000,000			600,000,000		600,000,000						750,000,000
Repayments of short-term debt							100,000,000	$ 0
SOUTHERN POWER CO | Notes Payable to Banks
Debt Instrument [Line Items]
Repayments of short-term debt		100,000,000
Southern Company Gas Capital
Debt Instrument [Line Items]
Borrowing capacity		$ 1,750,000,000			1,250,000,000		1,250,000,000						$ 1,900,000,000
Nicor Gas
Debt Instrument [Line Items]
Borrowing capacity					$ 500,000,000		$ 500,000,000
Nicor Gas | First Mortgage Bonds
Debt Instrument [Line Items]
Debt face amount												$ 200,000,000
Plant Vogtle Units 3 And 4 | GEORGIA POWER CO
Debt Instrument [Line Items]
Undivided ownership interest					45.70%		45.70%
Failure to vote, percentage of ownership interests									90.00%					90.00%
Scenario, Forecast | Nicor Gas | First Mortgage Bonds
Debt Instrument [Line Items]
Debt face amount										$ 100,000,000