Introduction - Restricted Cash (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 2,284	$ 2,130
Restricted cash:
Total cash, cash equivalents, and restricted cash	2,290	2,147	$ 1,116	$ 1,992
SOUTHERN Co GAS
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	94	73
Restricted cash:
Other accounts and notes receivable	6	5
Deferred charges and other assets		0
Total cash, cash equivalents, and restricted cash	100	78	33	24
SOUTHERN POWER CO
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	82	129
Restricted cash:
Other accounts and notes receivable		0
Deferred charges and other assets		11
Total cash, cash equivalents, and restricted cash	82	140	$ 110	$ 1,112
Southern Company
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	2,284
Restricted cash:
Other accounts and notes receivable	6	5
Deferred charges and other assets		12
Total cash, cash equivalents, and restricted cash	$ 2,290	$ 2,147