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Commitments and Contingencies, Fuel Contracts (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Coal  
Fuel Contracts [Abstract]  
2018 $ 172
2019 106
2020 64
2021 20
2022 21
Thereafter 0
Total 383
Natural Gas Supply  
Fuel Contracts [Abstract]  
2018 11
2019 0
2020 0
2021 0
2022 0
Thereafter 0
Total 11
Natural Gas Storage and Transportation  
Fuel Contracts [Abstract]  
2018 29
2019 32
2020 32
2021 27
2022 21
Thereafter 50
Total $ 191
Minimum  
Unrecorded Unconditional Purchase Obligation [Line Items]  
Fuel Contract Expiration Date 2018
Maximum  
Unrecorded Unconditional Purchase Obligation [Line Items]  
Fuel Contract Expiration Date 2029