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Commitments and Contingencies (Gas Supply Contracts) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Natural Gas Supply and Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
2018 $ 314
2019 280
2020 252
2021 249
2022 226
Thereafter 1,121
Total $ 2,442
Maximum [Member] | Pipeline and Storage Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 19 years
Maximum [Member] | Natural Gas Supply Contract Fixed Payments [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 3 years
Maximum [Member] | Natural Gas Supply Contract Commitment [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 15 years
Duke Energy Ohio [Member] | Natural Gas Supply and Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
2018 $ 37
2019 28
2020 25
2021 26
2022 11
Thereafter 3
Total $ 130
Duke Energy Ohio [Member] | Maximum [Member] | Pipeline and Storage Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 19 years
Duke Energy Ohio [Member] | Maximum [Member] | Natural Gas Supply Contract Fixed Payments [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 3 years
Duke Energy Ohio [Member] | Maximum [Member] | Natural Gas Supply Contract Commitment [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 15 years
Piedmont Natural Gas [Member] | Natural Gas Supply and Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
2018 $ 277
2019 252
2020 227
2021 223
2022 215
Thereafter 1,118
Total $ 2,312
Piedmont Natural Gas [Member] | Maximum [Member] | Pipeline and Storage Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 19 years
Piedmont Natural Gas [Member] | Maximum [Member] | Natural Gas Supply Contract Fixed Payments [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 3 years
Piedmont Natural Gas [Member] | Maximum [Member] | Natural Gas Supply Contract Commitment [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 15 years