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Commitments and Contingencies, Fuel Contracts (Details)
$ in Millions
12 Months Ended
Dec. 31, 2016
USD ($)
Coal  
Fuel Contracts [Abstract]  
2017 $ 241.2
2018 143.1
2019 68.6
2020 48.5
2021 49.6
Thereafter 293.4
Total 844.4
Natural Gas Supply  
Fuel Contracts [Abstract]  
2017 293.6
2018 185.7
2019 179.7
2020 184.3
2021 191.5
Thereafter 171.2
Total 1,206.0
Natural Gas Storage and Transportation  
Fuel Contracts [Abstract]  
2017 117.8
2018 69.0
2019 37.1
2020 36.6
2021 34.4
Thereafter 609.9
Total $ 904.8
Minimum  
Unrecorded Unconditional Purchase Obligation [Line Items]  
Fuel Contract Expiration Date 2017
Maximum  
Unrecorded Unconditional Purchase Obligation [Line Items]  
Fuel Contract Expiration Date 2060