INTRODUCTION - Restricted Cash (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 3,379	$ 1,975
Restricted cash:
Total cash, cash equivalents, and restricted cash	3,383	1,978	$ 2,935	$ 1,519
Other accounts and notes receivable
Restricted cash:
Restricted cash	0	3
Other current assets
Restricted cash:
Restricted cash	4	0
Other deferred charges and assets
Restricted cash:
Restricted cash	1	0
SOUTHERN POWER CO
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	416	279
Restricted cash:
Total cash, cash equivalents, and restricted cash	417	279	368	181
SOUTHERN POWER CO | Other accounts and notes receivable
Restricted cash:
Restricted cash	0
SOUTHERN POWER CO | Other current assets
Restricted cash:
Restricted cash	0
SOUTHERN POWER CO | Other deferred charges and assets
Restricted cash:
Restricted cash	1
SOUTHERN Co GAS
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	157	46
Restricted cash:
Total cash, cash equivalents, and restricted cash	161	49	$ 62	$ 70
SOUTHERN Co GAS | Other accounts and notes receivable
Restricted cash:
Restricted cash	0	3
SOUTHERN Co GAS | Other current assets
Restricted cash:
Restricted cash	4	0
SOUTHERN Co GAS | Other deferred charges and assets
Restricted cash:
Restricted cash	$ 0	$ 0