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Variable Interest Entity and Equity Method Investments - Narrative (Details) - USD ($)
$ in Billions
1 Months Ended 3 Months Ended 6 Months Ended
May 20, 2018
May 31, 2018
Jun. 30, 2018
Jun. 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% 33.00% 33.00%  
Wholly Owned Subsidiary Of Southern Power | SP Solar Holdings I, LP | SOUTHERN POWER CO        
Schedule of Equity Method Investments [Line Items]        
Limited liability company (llc) or limited partnership (lp), managing member or general partner, ownership interest 66.00%      
Variable Interest Entity, Primary Beneficiary | SP Solar Holdings I, LP | SOUTHERN POWER CO        
Schedule of Equity Method Investments [Line Items]        
Assets     $ 6.5 $ 6.5
Liabilities     0.1 0.1
Noncontrolling Interest in Variable Interest Entity     $ 1.2 $ 1.2
General Partner | Wholly Owned Subsidiary Of Southern Power | SP Solar Holdings I, LP | SOUTHERN POWER CO        
Schedule of Equity Method Investments [Line Items]        
Limited liability company (llc) or limited partnership (lp), managing member or general partner, ownership interest 1.00%