Introduction - Schedule of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 2,078	$ 1,065
Restricted cash:
Total cash, cash equivalents, and restricted cash	2,101	1,068	$ 3,383	$ 1,978
Other current assets
Restricted cash:
Restricted cash	3	2
Other deferred charges and assets
Restricted cash:
Restricted cash	21	0
SOUTHERN POWER CO
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	192	182
Restricted cash:
Total cash, cash equivalents, and restricted cash	213	183	417	279
SOUTHERN POWER CO | Other current assets
Restricted cash:
Restricted cash	0
SOUTHERN POWER CO | Other deferred charges and assets
Restricted cash:
Restricted cash	21
SOUTHERN Co GAS
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	29	17
Restricted cash:
Total cash, cash equivalents, and restricted cash	32	19	$ 161	$ 49
SOUTHERN Co GAS | Other current assets
Restricted cash:
Restricted cash	3	2
SOUTHERN Co GAS | Other deferred charges and assets
Restricted cash:
Restricted cash	$ 0	$ 0