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Debt: - Department of Energy Loan Guarantee (Details)
12 Months Ended
Mar. 15, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Mar. 22, 2019
USD ($)
Sep. 30, 2017
USD ($)
Sep. 28, 2017
USD ($)
Sep. 22, 2017
USD ($)
Feb. 20, 2014
USD ($)
item
Debt                  
Aggregate borrowings including capitalized interest   $ 9,347,307,000 $ 8,232,703,000            
Long-term debt proceeds   $ 813,028,000 544,503,000 $ 790,385,000          
Vogtle Units No. 3 & No. 4                  
Debt                  
Guarantee Obligations Maximum Exposure Received     $ 1,104,000,000            
Commercial paper | Subsequent Event                  
Debt                  
Repayment of outstanding commercial paper $ 436,600,000                
Long-term debt                  
Debt                  
Maximum borrowing capacity             $ 4,676,749,167    
Ownership interests voting required to continue construction (as a percent)   90.00%              
Long-term debt | Department of Energy guarantee                  
Debt                  
Aggregate borrowings including capitalized interest   $ 1,794,723,000              
Long-term debt | FFB                  
Debt                  
Number of future advance promissory notes | item                 2
Maximum borrowing capacity   $ 4,676,749,167       $ 1,619,679,706     $ 3,057,069,461
Long-term debt | FFB | Subsequent Event                  
Debt                  
Aggregate borrowings including capitalized interest $ 585,000,000                
Long-term debt | FFB | Vogtle Units No. 3 & No. 4                  
Debt                  
Term of debt   5 years              
Period of cessation of construction activities which would result in prepayment of outstanding principal   12 months              
Period of failure to fund operation and maintenance expenses which would result in prepayment of outstanding principal   12 months              
Long-term debt | FFB | Maximum                  
Debt                  
Maximum borrowing capacity designated for capitalized interest             $ 335,471,604    
Aggregate borrowings including capitalized interest               $ 3,057,069,461  
Long-term debt | FFB | Department of Energy guarantee                  
Debt                  
Aggregate borrowings including capitalized interest           $ 1,619,679,706      
Long-term debt | FFB | Department of Energy guarantee | Subsequent Event | Services Agreement                  
Debt                  
Guarantee Obligations Maximum Exposure Received         $ 4,676,749,167        
Long-term debt | FFB | US Treasury Securities, Current Yield                  
Debt                  
Spread on variable rate (as a percent)   0.375%