SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 1,798	$ 1,065
Restricted cash:
Total cash, cash equivalents, and restricted cash	1,829	1,068	$ 1,978	$ 1,519
Other current assets
Restricted cash:
Restricted cash	2	2
Other Noncurrent Assets
Restricted cash:
Restricted cash	29	0
Southern Power
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	107	182
Restricted cash:
Total cash, cash equivalents, and restricted cash	135	182	279	181
Southern Power | Other current assets
Restricted cash:
Restricted cash	0
Southern Power | Other Noncurrent Assets
Restricted cash:
Restricted cash	29
Southern Power | Tax Equity Contributions | Garland
Restricted cash:
Restricted cash	19
Southern Power | Estimated Construction Completion Costs | Deuel Harvest
Restricted cash:
Restricted cash	10
SOUTHERN Co GAS
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	45	17
Restricted cash:
Total cash, cash equivalents, and restricted cash	48	19	$ 49	$ 70
SOUTHERN Co GAS | Other current assets
Restricted cash:
Restricted cash	2	2
SOUTHERN Co GAS | Other Noncurrent Assets
Restricted cash:
Restricted cash	$ 0	$ 0