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Commitments and Contingencies, Purchased Power Agreements (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Capacity      
Purchased Power Agreements (PPAs) [Abstract]      
Purchased power expense $ 53.8 $ 52.7 $ 84.1
Estimated Future Payments Under PPAs [Abstract]      
2020 54.5    
2021 62.2    
2022 61.3    
2023 62.8    
2024 64.5    
Thereafter 45.4    
Total 350.7    
Energy      
Purchased Power Agreements (PPAs) [Abstract]      
Purchased power expense 102.4 $ 104.7 $ 96.7
Estimated Future Payments Under PPAs [Abstract]      
2020 109.4    
2021 157.3    
2022 172.9    
2023 176.9    
2024 181.8    
Thereafter 146.3    
Total $ 944.6