INTRODUCTION - Restricted Cash (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 2,164	$ 1,975
Restricted cash(a):
Total cash, cash equivalents, and restricted cash	2,168	1,978	$ 1,364	$ 1,519
Other accounts and notes receivable
Restricted cash(a):
Restricted cash	0	3
Other current assets
Restricted cash(a):
Restricted cash	3	0
SOUTHERN Co GAS
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	311	46
Restricted cash(a):
Total cash, cash equivalents, and restricted cash	314	49	$ 61	$ 70
SOUTHERN Co GAS | Other accounts and notes receivable
Restricted cash(a):
Restricted cash	0	3
SOUTHERN Co GAS | Other current assets
Restricted cash(a):
Restricted cash	$ 3	$ 0