Financing - Schedule of Long-Term Debt Financing Activities (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Debt Instrument [Line Items]
Senior Note Issuances	$ 1,350
Senior Note Maturities, Redemptions, and Repurchases	2,900
Revenue Bond Issuances and Reofferings of Purchased Bonds	627
Revenue Bond Maturities, Redemptions, and Repurchases	248
Other Long-Term Debt Issuances	2,760
Other Long-Term Debt Redemptions and Maturities	69
Parent Company | Southern Company
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	2,400
Revenue Bond Issuances and Reofferings of Purchased Bonds	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	1,725
Other Long-Term Debt Redemptions and Maturities	0
Subsidiaries | ALABAMA POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	600
Senior Note Maturities, Redemptions, and Repurchases	200
Revenue Bond Issuances and Reofferings of Purchased Bonds	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	1
Subsidiaries | GEORGIA POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	750
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Issuances and Reofferings of Purchased Bonds	584
Revenue Bond Maturities, Redemptions, and Repurchases	223
Other Long-Term Debt Issuances	835
Other Long-Term Debt Redemptions and Maturities	11
Subsidiaries | MISSISSIPPI POWER CO
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Issuances and Reofferings of Purchased Bonds	43
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	0
Subsidiaries | SOUTHERN Co GAS
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	300
Revenue Bond Issuances and Reofferings of Purchased Bonds	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	200
Other Long-Term Debt Redemptions and Maturities	50
Subsidiaries | Other
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Issuances and Reofferings of Purchased Bonds	0
Revenue Bond Maturities, Redemptions, and Repurchases	25
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	14
Eliminations | Subsidiaries
Debt Instrument [Line Items]
Senior Note Issuances	0
Senior Note Maturities, Redemptions, and Repurchases	0
Revenue Bond Issuances and Reofferings of Purchased Bonds	0
Revenue Bond Maturities, Redemptions, and Repurchases	0
Other Long-Term Debt Issuances	0
Other Long-Term Debt Redemptions and Maturities	$ (7)