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Commitments and Contingencies (Gas Supply Contracts) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2018
USD ($)
Natural Gas Supply and Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
2019 $ 314
2020 287
2021 255
2022 225
2023 148
Thereafter 1,067
Total $ 2,296
Maximum [Member] | Pipeline and Storage Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 16 years
Maximum [Member] | Natural Gas Supply Contract Fixed Payments [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 7 years
Maximum [Member] | Natural Gas Supply Contract Commitment [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 12 years
Duke Energy Ohio [Member] | Natural Gas Supply and Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
2019 $ 38
2020 30
2021 29
2022 11
2023 4
Thereafter 0
Total $ 112
Duke Energy Ohio [Member] | Maximum [Member] | Pipeline and Storage Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 16 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 7 years
Duke Energy Ohio [Member] | Maximum [Member] | Natural Gas Supply Contract Commitment [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 12 years
Piedmont Natural Gas [Member] | Natural Gas Supply and Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
2019 $ 276
2020 257
2021 226
2022 214
2023 144
Thereafter 1,067
Total $ 2,184
Piedmont Natural Gas [Member] | Maximum [Member] | Pipeline and Storage Capacity Contracts [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 16 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 7 years
Piedmont Natural Gas [Member] | Maximum [Member] | Natural Gas Supply Contract Commitment [Member]  
Long-term Purchase Commitment [Line Items]  
Long-term Purchase Commitment, Period 12 years