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Vogtle Units No. 3 and No. 4 Construction Project - Narrative (Details)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2021
USD ($)
unit
Dec. 31, 2020
USD ($)
Dec. 31, 2008
unit
MW
Vogtle Units No. 3 & No. 4      
Public Utility Property Plant and Equipment      
Ownership interest (as a percent) 30.00%    
Total investment in additional Vogtle units $ 6,300.0    
Additional construction contingency, total 106.0 $ 375.0  
Project-Level Contingency 32.0 $ 112.5  
Additional construction contingency, construction productivity and field support 183.0    
Ownership amount of additional construction contingency, construction productivity and field support 55.0    
Monthly delay cost 55.0    
Vogtle Units No. 3 & No. 4 | Minimum      
Public Utility Property Plant and Equipment      
COVID related costs 325.0    
Ownership amount of COVID related costs 98.0    
Vogtle Units No. 3 & No. 4 | Maximum      
Public Utility Property Plant and Equipment      
COVID related costs 415.0    
Ownership amount of COVID related costs $ 125.0    
Vogtle Units No. 3 & No. 4 | Ownership participation agreement      
Public Utility Property Plant and Equipment      
Number of additional nuclear units | unit 2    
Ownership interest (as a percent) 30.00%    
Project budget $ 7,500.0    
Project extension term 1 year    
Vogtle Units No. 3 & No. 4 | EPC Agreement | Westinghouse Electric Company LLC and Stone & Webster, Inc.      
Public Utility Property Plant and Equipment      
Number of nuclear units | unit     2
Generating capacity of each nuclear unit | MW     1,100
Vogtle Units No. 3 & No. 4 | Services Agreement | Westinghouse Electric Company LLC and Stone & Webster, Inc.      
Public Utility Property Plant and Equipment      
Written notice period for termination of agreement 30 days    
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet      
Public Utility Property Plant and Equipment      
Percentage of costs disallowed for recovery 6.00%    
Vogtle Units No. 3 & No. 4 | Global Amendments To Term Sheet | Minimum      
Public Utility Property Plant and Equipment      
Percentage of ownership approval to change primary construction contractor 90.00%    
Vogtle Unit Number 3      
Public Utility Property Plant and Equipment      
Delay period 4 months    
Vogtle Unit Number 4      
Public Utility Property Plant and Equipment      
Delay period 3 months    
Monthly delay cost $ 25.0