Financing - Schedule of Long-Term Debt Financing Activities (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Debt Instrument [Line Items]
Senior Note Issuances	$ 1,850
Senior Note Maturities, Redemptions, and Repurchases	2,350
Revenue Bond Maturities, Redemptions, and Repurchases	712
Other Long-Term Debt Issuances	100
Other Long-Term Debt Redemptions and Maturities	1,436
Elimination
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	(1)
ALABAMA POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	500
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	0
GEORGIA POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	1,000
Revenue Bond Maturities, Redemptions, and Repurchases	469
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	107
MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	600
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Maturities, Redemptions, and Repurchases	43
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	900
SOUTHERN POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	350
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	420
SOUTHERN Co GAS
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Maturities, Redemptions, and Repurchases	200
Other Long-Term Debt Issuances	100
Other Long-Term Debt Redemptions and Maturities	0
Other
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	10
Southern Company
Debt Instrument [Line Items]
Senior Note Issuances	750
Senior Note Maturities, Redemptions, and Repurchases	1,000
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	$ 0