Summary of Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 1,396	$ 2,130
Cash and cash equivalents classified as assets held for sale	9
Restricted cash:
Restricted cash	0
Total cash, cash equivalents, and restricted cash	1,519	2,147	$ 1,992		$ 1,420
Southern Power
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	181	129
Restricted cash:
Total cash, cash equivalents, and restricted cash	181	140	1,112		835
Georgia Power
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	4	852
Cash and cash equivalents classified as assets held for sale	0
Restricted cash:
Restricted cash	108
Total cash, cash equivalents, and restricted cash	112	852	3		$ 67
Southern Company Gas
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	64	73
Cash and cash equivalents classified as assets held for sale	0
Restricted cash:
Restricted cash	0
Total cash, cash equivalents, and restricted cash	70	78	$ 24	$ 19
Other accounts and notes receivable
Restricted cash:
Other accounts and notes receivable	114	5
Other accounts and notes receivable | Southern Power
Restricted cash:
Other accounts and notes receivable		0
Other accounts and notes receivable | Georgia Power
Restricted cash:
Other accounts and notes receivable	0
Other accounts and notes receivable | Southern Company Gas
Restricted cash:
Other accounts and notes receivable	$ 6	5
Deferred charges and other assets
Restricted cash:
Other accounts and notes receivable		12
Deferred charges and other assets | Southern Power
Restricted cash:
Other accounts and notes receivable		11
Deferred charges and other assets | Southern Company Gas
Restricted cash:
Other accounts and notes receivable		$ 0