Financing - Schedule Of Credit Arrangements With Project Credit Facilities (Details) - USD ($)	Dec. 31, 2017	Apr. 30, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 8,145,000,000
Line of credit facility, remaining borrowing capacity	8,094,000,000
SOUTHERN POWER CO
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	750,000,000	$ 600,000,000
Line of credit facility, remaining borrowing capacity	728,000,000		$ 522,000,000
SOUTHERN POWER CO | RE Roserock Holdings, LLC | Construction Loan Facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	63,000,000
SOUTHERN POWER CO | RE Roserock Holdings, LLC | Bridge Loan
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	180,000,000
SOUTHERN POWER CO | RE Roserock Holdings, LLC | Construction Loan And Bridge Loan
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	243,000,000
Line of credit facility, remaining borrowing capacity	34,000,000
SOUTHERN POWER CO | RE Roserock Holdings, LLC | Letter of Credit
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	23,000,000
Line of credit facility, remaining borrowing capacity	$ 16,000,000