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Commitments and Contingencies, Purchased Power Agreements (Details) - Capacity - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Purchased Power Agreements (PPAs) [Abstract]      
Payments for capacity $ 19.9 $ 57.6 $ 58.4
Estimated Future Payments Under PPAs [Abstract]      
2020 12.3    
2021 12.5    
2022 12.7    
2023 13.0    
2024 5.9    
Thereafter 0.0    
Total $ 56.4