SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration of Revenue (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Feb. 19, 2020 USD ($)	Jan. 29, 2019 facility MW	Dec. 31, 2018 USD ($)
Concentration Risk [Line Items]
Outstanding accounts receivable	$ 2,413			$ 2,630
Mississippi Power
Concentration Risk [Line Items]
Wholesale customers representing total operating revenues (percent)	15.70%
Period of cancellation notices	10 years
Outstanding accounts receivable	$ 79			100
PG&E
Concentration Risk [Line Items]
Number of solar facilities | facility			4
Capacity of natural gas facility (mw) | MW			207
Southern Power
Concentration Risk [Line Items]
Outstanding accounts receivable	97			$ 118
PPAs | Southern Power
Concentration Risk [Line Items]
Outstanding accounts receivable	2
Transmission Interconnections | Southern Power
Concentration Risk [Line Items]
Outstanding accounts receivable	33
Other accounts and notes receivable | Transmission Interconnections | Southern Power
Concentration Risk [Line Items]
Outstanding accounts receivable	27
Other Noncurrent Assets | Transmission Interconnections | Southern Power
Concentration Risk [Line Items]
Outstanding accounts receivable	$ 6
Subsequent Event | Transmission Interconnections | Southern Power
Concentration Risk [Line Items]
Bankruptcy claims, amount received		$ 15