Financing - Narrative (Details)	1 Months Ended						3 Months Ended		9 Months Ended
	Nov. 06, 2018 USD ($) shares	Aug. 31, 2018 USD ($)	Jun. 30, 2018 USD ($) agreement	May 31, 2018 USD ($) agreement	Apr. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jul. 31, 2018 USD ($)	Jan. 31, 2018 USD ($)	Jan. 04, 2018 USD ($)
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding							$ 1,500,000,000		$ 1,500,000,000
Maximum borrowing capacity							8,143,000,000		8,143,000,000
Repayments of short-term debt									1,800,000,000	$ 409,000,000
Stock redeemed during period, par value										150,000,000
Floating rate Promissory Note Due July 2018
Debt Instrument [Line Items]
Debt face amount													$ 100,000,000
Southern Company
Debt Instrument [Line Items]
Maximum borrowing capacity							2,000,000,000		2,000,000,000
Southern Company | Floating Rate Bank Term Loan Agreement 1 | Notes Payable to Banks
Debt Instrument [Line Items]
Debt face amount						$ 900,000,000
Southern Company | Uncommitted Bank Credit Arrangement | Notes Payable to Banks
Debt Instrument [Line Items]
Debt face amount					$ 250,000,000
Debt instrument, payable by demand from bank, term					30 days
Repayments of short-term debt		$ 250,000,000
Southern Company | Floating Rate Bank Term Loan Agreement 6 | Notes Payable to Banks
Debt Instrument [Line Items]
Debt face amount		1,500,000,000.0
Southern Company | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 1
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
Southern Company | Senior Notes | Series 2018A Floating Rate Senior Notes Due February 14, 2020
Debt Instrument [Line Items]
Debt face amount		750,000,000
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							120,000,000		120,000,000
Maximum borrowing capacity							1,333,000,000		1,333,000,000
ALABAMA POWER CO | Senior Notes | Series 2018A Floating Rate Senior Notes
Debt Instrument [Line Items]
Debt face amount			$ 500,000,000					$ 500,000,000
Debt stated interest rate			4.30%					4.30%
ALABAMA POWER CO | Senior Notes | Series 2008B 5.40% Senior Notes due June 1, 2018
Debt Instrument [Line Items]
Debt stated interest rate					5.40%
Repurchased face amount					$ 250,000,000
ALABAMA POWER CO | Subsequent Event | Pollution Control Revenue Bonds
Debt Instrument [Line Items]
Debt instrument, repurchase amount	$ 120,000,000
GEORGIA POWER CO
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding							550,000,000		550,000,000
Fixed rate pollution control revenue bonds outstanding							345,000,000		345,000,000
Maximum borrowing capacity							1,750,000,000		$ 1,750,000,000
Amortization period for line of credit facility									5 years
Percent of eligible project costs to be reimbursed									70.00%
Repayments of short-term debt									$ 150,000,000	300,000,000
GEORGIA POWER CO | Loan Guarantee Agreement
Debt Instrument [Line Items]
Conditional borrowing commitment										1,670,000,000
GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 2
Debt Instrument [Line Items]
Debt face amount												$ 150,000,000
GEORGIA POWER CO | Notes Payable to Banks | Floating Rate Bank Term Loan Agreement 3
Debt Instrument [Line Items]
Debt face amount												$ 100,000,000
GEORGIA POWER CO | Senior Notes
Debt Instrument [Line Items]
Repayments of debt				$ 902,000,000
GEORGIA POWER CO | Senior Notes | Series 2007A 5.65% Senior Notes
Debt Instrument [Line Items]
Debt instrument, repurchase amount				89,000,000
Debt face amount				$ 250,000,000
Debt stated interest rate				5.65%
GEORGIA POWER CO | Senior Notes | Series 2009A 5.95% Senior Notes
Debt Instrument [Line Items]
Debt instrument, repurchase amount				$ 326,000,000
Debt face amount				$ 500,000,000
Debt stated interest rate				5.95%
GEORGIA POWER CO | Senior Notes | Series 2010B 5.40% Senior Notes
Debt Instrument [Line Items]
Debt instrument, repurchase amount				$ 335,000,000
Debt face amount				$ 600,000,000
Debt stated interest rate				5.40%
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							104,600,000		104,600,000
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							173,000,000		173,000,000
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		55,000,000
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		65,000,000
GEORGIA POWER CO | Municipal bonds | Development Authority Of Bartow County (Georgia) Pollution Control Revenue Bonds (Georgia Power Company Plant Vogtle Project), First Series 2013
Debt Instrument [Line Items]
Debt face amount							71,735,000.000		71,735,000.000
GEORGIA POWER CO | FFB Loan
Debt Instrument [Line Items]
Conditional borrowing commitment										1,670,000,000
Maximum borrowing capacity							2,600,000,000		2,600,000,000
GULF POWER CO
Debt Instrument [Line Items]
Variable rate pollution control revenue bonds outstanding							82,000,000		82,000,000
Fixed rate pollution control revenue bonds outstanding							58,000,000		58,000,000
Maximum borrowing capacity							280,000,000		280,000,000
MISSISSIPPI POWER CO
Debt Instrument [Line Items]
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
Repayments of short-term debt									300,000,000	$ 109,000,000
MISSISSIPPI POWER CO | Notes Payable to Banks
Debt Instrument [Line Items]
Debt face amount						300,000,000
MISSISSIPPI POWER CO | Senior Notes | Series 2018A Floating Rate Senior Notes
Debt Instrument [Line Items]
Debt face amount						300,000,000
MISSISSIPPI POWER CO | Senior Notes | Series 2018B 3.95% Senior Notes
Debt Instrument [Line Items]
Debt face amount						$ 300,000,000
Debt stated interest rate						3.95%
MISSISSIPPI POWER CO | Municipal bonds | Pollution Control Revenue Bonds
Debt Instrument [Line Items]
Debt instrument, repurchase amount											$ 43,000,000
MISSISSIPPI POWER CO | Promissory Notes | Promissory Notes
Debt Instrument [Line Items]
Repayments of debt							100,000,000	$ 200,000,000
MISSISSIPPI POWER CO | Unsecured Debt | Unsecured Term Loan
Debt Instrument [Line Items]
Repayments of debt						$ 900,000,000
MISSISSIPPI POWER CO | Subsequent Event | Senior Notes | Series G 5.40% Senior Notes Due July 1, 2035
Debt Instrument [Line Items]
Debt stated interest rate	5.40%
Aggregate principal amount outstanding	$ 30,000,000
MISSISSIPPI POWER CO | Subsequent Event | Senior Notes | Series 2009A 5.55% Senior Notes Due March 1, 2019
Debt Instrument [Line Items]
Debt stated interest rate	5.55%
Aggregate principal amount outstanding	$ 125,000,000
MISSISSIPPI POWER CO | 4.40% Series Preferred Stock | Subsequent Event
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares	8,867
Stock redeemed during period, par value	$ 886,700
Preferred stock, dividend rate	4.40%
MISSISSIPPI POWER CO | 4.60% Series Preferred Stock | Subsequent Event
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares	8,643
Stock redeemed during period, par value	$ 864,300
Preferred stock, dividend rate	4.60%
MISSISSIPPI POWER CO | 4.72% Series Preferred Stock | Subsequent Event
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares	16,700
Stock redeemed during period, par value	$ 1,670,000
Preferred stock, dividend rate	4.72%
MISSISSIPPI POWER CO | Depository Shares | Subsequent Event
Debt Instrument [Line Items]
Stock redeemed during period (in shares) | shares	1,200,000
Stock redeemed during period, par value	$ 30,000,000
Interest in shares	25.00%
MISSISSIPPI POWER CO | 5.25% Series Preferred Stock | Subsequent Event
Debt Instrument [Line Items]
Preferred stock, dividend rate	5.25%
Traditional Electric Operating Companies
Debt Instrument [Line Items]
Fixed rate pollution control revenue bonds outstanding							573,000,000		573,000,000
SOUTHERN POWER CO
Debt Instrument [Line Items]
Maximum borrowing capacity							750,000,000		750,000,000
Repayments of debt				$ 770,000,000
Proceeds from third-party tax equity									148,000,000
SOUTHERN POWER CO | Floating Rate Bank Term Loan Agreement 1 | Notes Payable to Banks
Debt Instrument [Line Items]
Debt face amount				$ 100,000,000
Number of loan agreements | agreement				2
Repurchased face amount			$ 420,000,000					$ 420,000,000
SOUTHERN POWER CO | Senior Notes | Series 2015A 1.50% Senior Notes
Debt Instrument [Line Items]
Debt stated interest rate			1.50%					1.50%
Repurchased face amount			$ 350,000,000					$ 350,000,000
SOUTHERN Co GAS
Debt Instrument [Line Items]
Maximum borrowing capacity							1,900,000,000		1,900,000,000
SOUTHERN Co GAS | Uncommitted Bank Credit Arrangement | Notes Payable to Banks
Debt Instrument [Line Items]
Debt face amount				$ 95,000,000
Debt instrument, payable by demand from bank, term				30 days
SOUTHERN Co GAS | Senior Notes | Series 2017A
Debt Instrument [Line Items]
Debt face amount			$ 200,000,000					$ 200,000,000
Nicor Gas
Debt Instrument [Line Items]
Maximum borrowing capacity							$ 500,000,000		$ 500,000,000
Nicor Gas | First Mortgage Bonds
Debt Instrument [Line Items]
Debt face amount											$ 300,000,000
Nicor Gas | First Mortgage Bonds | First Mortgage Bonds Issued August 2018
Debt Instrument [Line Items]
Debt face amount		$ 100,000,000
Nicor Gas | Subsequent Event | First Mortgage Bonds | First Mortgage Bonds Issued November 2018
Debt Instrument [Line Items]
Debt face amount	$ 200,000,000