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Variable Interest Entity and Equity Method Investments - Narrative (Details) - USD ($)
$ in Millions
1 Months Ended 9 Months Ended
May 31, 2018
Sep. 30, 2018
SOUTHERN POWER CO    
Schedule of Equity Method Investments [Line Items]    
Distribution made to limited partner, cash distributions paid, percentage   67.00%
Global Atlantic    
Schedule of Equity Method Investments [Line Items]    
Distribution made to limited partner, cash distributions paid, percentage   33.00%
SP Solar Holdings I, LP | SOUTHERN POWER CO    
Schedule of Equity Method Investments [Line Items]    
Limited liability company or limited partnership, members or limited partners, ownership interest 33.00%  
SP Solar Holdings I, LP | Wholly Owned Subsidiary Of Southern Power | SOUTHERN POWER CO    
Schedule of Equity Method Investments [Line Items]    
Ownership interest 66.00%  
SP Solar Holdings I, LP | Variable Interest Entity, Primary Beneficiary | SOUTHERN POWER CO    
Schedule of Equity Method Investments [Line Items]    
Assets   $ 6,400
Liabilities   111
Noncontrolling interests related to other partners' interests   $ 1,200
SP Solar Holdings I, LP | General Partner | Wholly Owned Subsidiary Of Southern Power | SOUTHERN POWER CO    
Schedule of Equity Method Investments [Line Items]    
Ownership interest 1.00%