Introduction - Restricted Cash (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 1,847	$ 2,130
Restricted cash:
Total cash, cash equivalents, and restricted cash	1,891	2,147	$ 1,858	$ 1,992
SOUTHERN POWER CO
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	192	129
Restricted cash:
Other accounts and notes receivable		0
Deferred charges and other assets		11
Total cash, cash equivalents, and restricted cash	192	140	189	1,112
SOUTHERN Co GAS
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	56	73
Cash and cash equivalents classified as assets held for sale	0
Restricted cash:
Other accounts and notes receivable	6	5
Deferred charges and other assets		0
Total cash, cash equivalents, and restricted cash	62	78	$ 26	$ 24
Southern Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	1,847	2,130
Cash and cash equivalents classified as assets held for sale	37
Restricted cash:
Other accounts and notes receivable	6	5
Deferred charges and other assets		12
Total cash, cash equivalents, and restricted cash	$ 1,891	$ 2,147