Financing - Scheduled Maturities and Redemptions of Securities Due Within One Year (Details)	1 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Oct. 02, 2022 $ / shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2013
Debt Disclosure [Line Items]
Unused credit with banks		$ 8,094,000,000
2019		3,074,000,000		$ 3,076,000,000
2020		2,273,000,000		1,326,000,000
2021		2,643,000,000		2,655,000,000
Scheduled maturities and redemptions of securities due within one year
Senior notes		2,354,000,000		1,995,000,000
Other long-term debt		1,420,000,000		485,000,000
Revenue bonds		90,000,000		76,000,000
Capitalized leases		31,000,000		32,000,000
Unamortized debt issuance expense		(3,000,000)		(1,000,000)
Total		2,402,000,000		2,403,000,000
ALABAMA POWER CO
Debt Disclosure [Line Items]
Unused credit with banks		1,335,000,000
2019		200,000,000		200,000,000
2020		250,000,000		250,000,000
2021		$ 220,000,000		220,000,000
ALABAMA POWER CO | 4.92% Redeemable Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage		0.0492
Par value/stated capital per share (in dollars per share) | $ / shares		$ 100
Temporary equity, shares outstanding (in shares) | shares		80,000
Redemption Price Per Share (in dollars per share) | $ / shares		$ 103.23
ALABAMA POWER CO | 4.72% Redeemable Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage		0.0472
Par value/stated capital per share (in dollars per share) | $ / shares		$ 100
Temporary equity, shares outstanding (in shares) | shares		50,000
Redemption Price Per Share (in dollars per share) | $ / shares		$ 102.18
ALABAMA POWER CO | 4.64% Redeemable Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage		0.0464
Par value/stated capital per share (in dollars per share) | $ / shares		$ 100
Temporary equity, shares outstanding (in shares) | shares		60,000
Redemption Price Per Share (in dollars per share) | $ / shares		$ 103.14
ALABAMA POWER CO | 4.60% Redeemable Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage		0.0460
Par value/stated capital per share (in dollars per share) | $ / shares		$ 100
Temporary equity, shares outstanding (in shares) | shares		100,000
Redemption Price Per Share (in dollars per share) | $ / shares		$ 104.20
ALABAMA POWER CO | 4.52% Redeemable Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage		0.0452
Par value/stated capital per share (in dollars per share) | $ / shares		$ 100
Temporary equity, shares outstanding (in shares) | shares		50,000
Redemption Price Per Share (in dollars per share) | $ / shares		$ 102.93
ALABAMA POWER CO | 4.20% Redeemable Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage		0.0420
Par value/stated capital per share (in dollars per share) | $ / shares		$ 100
Temporary equity, shares outstanding (in shares) | shares		135,115
Redemption Price Per Share (in dollars per share) | $ / shares		$ 105.00
ALABAMA POWER CO | 5.00% Class A Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage	0.05	0.0500
Par value/stated capital per share (in dollars per share) | $ / shares	$ 25	$ 25
Temporary equity, shares outstanding (in shares) | shares	10,000,000	10,000,000
ALABAMA POWER CO | 5.83% Class A Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage	0.0583	0.0583
Temporary equity, shares outstanding (in shares) | shares	1,500,000
ALABAMA POWER CO | 6.450% Preference Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage		0.06450
ALABAMA POWER CO | 6.500% Preference Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage		0.06500
ALABAMA POWER CO | 5.30% Class A Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage	0.065	0.0530
Temporary equity, shares outstanding (in shares) | shares	2,000,000
ALABAMA POWER CO | 5.625% Preference Stock
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, dividend rate percentage	0.0645	0.05625
Temporary equity, shares outstanding (in shares) | shares	6,000,000
GEORGIA POWER CO
Debt Disclosure [Line Items]
Unused credit with banks		$ 1,732,000,000
2019		499,000,000		500,000,000
2020		950,000,000		0
2021		325,000,000		325,000,000
Scheduled maturities and redemptions of securities due within one year
Senior notes		750,000,000		450,000,000
Other long-term debt		100,000,000		0
Capitalized leases		11,000,000		10,000,000
Unamortized debt issuance expense		(1,000,000)		0
Total		860,000,000		460,000,000
Total		$ 747,000,000		748,000,000
Debt Instrument, Interest Rate, Stated Percentage		2.00%
MISSISSIPPI POWER CO
Debt Disclosure [Line Items]
Unused credit with banks		$ 100,000,000
2019		$ 125,000,000		$ 125,000,000
Redeemable Preferred/Preference Stock [Abstract]
Temporary equity, shares outstanding (in shares) | shares		334,210		334,210
Scheduled maturities and redemptions of securities due within one year
Long-term debt affiliated		$ 0		$ 551,000,000
Senior notes		0		35,000,000
Revenue bonds		90,000,000		40,000,000
Capitalized leases		0		3,000,000
Unamortized debt issuance expense		(1,000,000)		0
Total		50,000,000		50,000,000
Revenue bonds, current		50,000,000
Long term debt and capital lease obligation maturities repayments in year five		0
Debt Instrument, Interest Rate, Stated Percentage					7.13%
Long-term Pollution Control Bond, Current		$ 40,000,000
MISSISSIPPI POWER CO | 4.40% Redeemable Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Par value/stated capital per share (in dollars per share) | $ / shares		$ 100,000,000
Temporary equity, shares outstanding (in shares) | shares		8,867,000,000
Redemption Price Per Share (in dollars per share) | $ / shares		$ 104.32
MISSISSIPPI POWER CO | 4.72% Redeemable Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Par value/stated capital per share (in dollars per share) | $ / shares		$ 100,000,000
Temporary equity, shares outstanding (in shares) | shares		16,700,000,000
Redemption Price Per Share (in dollars per share) | $ / shares		$ 102.25
MISSISSIPPI POWER CO | 4.60% Redeemable Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Par value/stated capital per share (in dollars per share) | $ / shares		$ 100,000,000
Temporary equity, shares outstanding (in shares) | shares		8,643,000,000
Redemption Price Per Share (in dollars per share) | $ / shares		$ 107.00
MISSISSIPPI POWER CO | 5.25% Redeemable Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Par value/stated capital per share (in dollars per share) | $ / shares		$ 100,000,000
Temporary equity, shares outstanding (in shares) | shares		300,000,000,000
Redemption Price Per Share (in dollars per share) | $ / shares		$ 100.00
SOUTHERN POWER CO
Debt Disclosure [Line Items]
Unused credit with banks		$ 728,000,000		$ 522,000,000
Repayments of debt	$ 40,000,000
2019		600,000,000
2020		825,000,000
2021		300,000,000
Scheduled maturities and redemptions of securities due within one year
Total		770,000,000
Long term debt and capital lease obligation maturities repayments in year five		677,000,000
SOUTHERN Co GAS
Debt Disclosure [Line Items]
Unused credit with banks		1,890,000,000
2019		350,000,000
2020		0
2021		330,000,000
Scheduled maturities and redemptions of securities due within one year
Total		155,000,000
Medium-term Notes | SOUTHERN Co GAS
Scheduled maturities and redemptions of securities due within one year
Total		$ 157,000,000
Scenario, Forecast | ALABAMA POWER CO | 5.00% Class A Preferred Stock
Redeemable Preferred/Preference Stock [Abstract]
Redemption Price Per Share (in dollars per share) | $ / shares			$ 25.50