Financing - Outstanding Classes of Capital Stock Narrative (Details)		1 Months Ended		3 Months Ended		12 Months Ended
	Oct. 23, 2018 USD ($) shares	Oct. 31, 2017 USD ($) shares	Sep. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) shares
Debt Instrument [Line Items]
Stock issued employee and director stock plans (in shares)						11,600,000
Proceeds from issuance of shares under share-based compensation plans | $						$ 442,000,000
Cash proceeds | $						$ 1,090,000,000	$ 793,000,000	$ 3,758,000,000
Common stock fees and commissions | $				$ 1,000,000	$ 5,000,000
Number of shares reserved for issuance to stock-based compensation plan (in shares)						92,000,000
Preference stock redemption | $							$ 609,000,000
Total redeemable preferred stock - percent capitalization				0.40%		0.40%	0.50%
Southern Company Common Stock
Debt Instrument [Line Items]
Remaining shares available for awards (in shares)				10,000,000		10,000,000
Alabama Power
Debt Instrument [Line Items]
Total redeemable preferred stock - percent capitalization				1.90%		1.90%	2.00%
Alabama Power | 5.00% Class A Preferred Stock
Debt Instrument [Line Items]
Shares outstanding (in shares)			10,000,000	10,000,000		10,000,000
Temporary equity, dividend rate percentage			0.05			0.05
Preferred stock, par value (in dollars per share) | $ / shares			$ 1
Alabama Power | 5.00% Class A Preferred Stock | Aggregate Stated Capital
Debt Instrument [Line Items]
Debt instrument, face amount | $			$ 250,000,000
Alabama Power | 5.30% Class A Preferred Stock
Debt Instrument [Line Items]
Shares outstanding (in shares)		2,000,000
Temporary equity, dividend rate percentage		0.065
Alabama Power | 5.30% Class A Preferred Stock | Aggregate Stated Capital
Debt Instrument [Line Items]
Debt instrument, face amount | $		$ 50,000,000
Alabama Power | 5.625% Preference Stock
Debt Instrument [Line Items]
Shares outstanding (in shares)		6,000,000
Temporary equity, dividend rate percentage		0.0645
Alabama Power | 5.625% Preference Stock | Aggregate Stated Capital
Debt Instrument [Line Items]
Debt instrument, face amount | $		$ 150,000,000
Alabama Power | 5.83% Class A Preferred Stock
Debt Instrument [Line Items]
Shares outstanding (in shares)		1,520,000
Temporary equity, dividend rate percentage		0.0583
Alabama Power | 5.83% Class A Preferred Stock | Aggregate Stated Capital
Debt Instrument [Line Items]
Debt instrument, face amount | $		$ 38,000,000
Georgia Power | Senior Notes | Preferred Class A
Debt Instrument [Line Items]
Preferred stock redemption (in shares)		1,800,000
Preference stock redemption | $		$ 45,000,000
Preferred stock, dividend rate		6.125%
Georgia Power | Senior Notes | Series 2007A Preference Stock
Debt Instrument [Line Items]
Preferred stock redemption (in shares)		2,250,000
Preference stock redemption | $		$ 225,000,000
Preferred stock, dividend rate		6.50%
Mississippi Power
Debt Instrument [Line Items]
Shares outstanding (in shares)				0		0	334,210
Preferred stock, par value (in dollars per share) | $ / shares				$ 100		$ 100	$ 100
Total redeemable preferred stock - percent capitalization				0.00%		0.00%	1.30%
Mississippi Power | 4.40% Series Preferred Stock
Debt Instrument [Line Items]
Temporary equity, dividend rate percentage	0.044
Par value/stated capital per share | $	$ 886,700
Preferred stock redemption (in shares)	8,867
Mississippi Power | 4.60% Series Preferred Stock
Debt Instrument [Line Items]
Temporary equity, dividend rate percentage	0.046
Par value/stated capital per share | $	$ 864,300
Preferred stock redemption (in shares)	8,643
Mississippi Power | 4.72% Series Preferred Stock
Debt Instrument [Line Items]
Temporary equity, dividend rate percentage	0.0472
Par value/stated capital per share | $	$ 1,670,000
Preferred stock redemption (in shares)	16,700
Mississippi Power | 5.25% Series Preferred Stock
Debt Instrument [Line Items]
Temporary equity, dividend rate percentage	0.0525
Par value/stated capital per share | $	$ 30,000,000
Preferred stock redemption (in shares)	1,200,000
Total redeemable preferred stock - percent capitalization	25.00%
Common Stock
Debt Instrument [Line Items]
Stock issued (in shares)						26,209,000	17,319,000	76,140,000
At-The-Market | Common Stock
Debt Instrument [Line Items]
Stock issued (in shares)				2,500,000	12,100,000
Cash proceeds | $				$ 108,000,000	$ 540,000,000