Derivatives - Schedule of Foreign Currency Derivatives (Details) - 9 months ended Sep. 30, 2021 - Foreign currency derivatives € in Millions, $ in Millions	USD ($)	EUR (€)
Derivative [Line Items]
Pay Notional	$ 2,717
Receive Notional | €		€ 2,350
Fair Value Gain (Loss) at September 30, 2021	(23)
Fair Value Hedges of Existing Debt | Southern Company parent
Derivative [Line Items]
Pay Notional	$ 1,476
Pay Rate	3.39%
Receive Notional | €		1,250
Receive Rate	1.88%
Fair Value Gain (Loss) at September 30, 2021	$ (32)
Cash Flow Hedges of Existing Debt | SOUTHERN POWER CO
Derivative [Line Items]
Pay Notional	1,241
Receive Notional | €		1,100
Fair Value Gain (Loss) at September 30, 2021	9
Cash Flow Hedges of Existing Debt | SOUTHERN POWER CO | June 2022
Derivative [Line Items]
Pay Notional	$ 677
Pay Rate	2.95%
Receive Notional | €		600
Receive Rate	1.00%
Fair Value Gain (Loss) at September 30, 2021	$ 9
Cash Flow Hedges of Existing Debt | SOUTHERN POWER CO | June 2026
Derivative [Line Items]
Pay Notional	$ 564
Pay Rate	3.78%
Receive Notional | €		€ 500
Receive Rate	1.85%
Fair Value Gain (Loss) at September 30, 2021	$ 0