DERIVATIVES - Location and Amount of Gain (Loss) Recognized in Income on Cash Flow and Fair Value Hedging Relationships (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative [Line Items]
Total cost of sales	$ 71	$ 79	$ 654	$ 956
Depreciation and amortization	889	760	2,619	2,267
Total interest expense, net of amounts capitalized	(443)	(434)	(1,343)	(1,294)
Total other income (expense), net	113	61	319	239
Cash Flow Hedging | Energy-related derivatives | Total cost of natural gas
Derivative [Line Items]
Gain (loss) on hedge	0	0	(8)	0
Cash Flow Hedging | Energy-related derivatives | Total depreciation and amortization
Derivative [Line Items]
Gain (loss) on hedge	(1)	(1)	(3)	(5)
Cash Flow Hedging | Interest rate derivatives | Total interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) on hedge	(6)	(5)	(19)	(14)
Cash Flow Hedging | Foreign currency derivatives | Total interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) on hedge	(6)	(6)	(18)	(18)
Cash Flow Hedging | Foreign currency derivatives | Total other income (expense), net
Derivative [Line Items]
Gain (loss) on hedge	56	(54)	52	(62)
Fair Value Hedging | Interest rate derivatives | Total interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) on hedge	(3)	10	27	43
SOUTHERN POWER CO
Derivative [Line Items]
Depreciation and amortization	129	120	367	357
Total interest expense, net of amounts capitalized	(36)	(43)	(114)	(127)
Total other income (expense), net	13	6	19	48
SOUTHERN POWER CO | Cash Flow Hedging | Energy-related derivatives | Total depreciation and amortization
Derivative [Line Items]
Gain (loss) on hedge	(1)	(1)	(3)	(5)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Total interest expense, net of amounts capitalized
Derivative [Line Items]
Gain (loss) on hedge	(6)	(6)	(18)	(18)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Total other income (expense), net
Derivative [Line Items]
Gain (loss) on hedge	$ 56	$ (54)	$ 52	$ (62)