SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 1,975	$ 1,396
Restricted cash:
Restricted cash		0
Total cash, cash equivalents, and restricted cash	1,978	1,519	$ 2,147	$ 1,992
Georgia Power
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	52	4
Restricted cash:
Restricted cash		108
Total cash, cash equivalents, and restricted cash	52	112	852	3
Southern Company Gas
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	46	64
Restricted cash:
Restricted cash		0
Total cash, cash equivalents, and restricted cash	$ 49	70	$ 78	$ 24
Cash and cash equivalents classified as assets held for sale
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents		9
Cash and cash equivalents classified as assets held for sale | Georgia Power
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents		0
Cash and cash equivalents classified as assets held for sale | Southern Company Gas
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents		$ 0