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Fair Value Measurements (Quantitative Level 3 Fair Value Disclosures) (Details) $ in Millions | Sep. 30, 2018 USD ($) $ / MWh $ / MMBTU | Jun. 30, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) $ / MWh $ / MMBTU | Jun. 30, 2017 USD ($) | Dec. 31, 2016 USD ($)
--- | --- | --- | --- | --- | --- | ---
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Net derivative asset (liability) | $ (116) | $ (97) | $ (114) | $ (93) | $ (91) | $ (166)
Duke Energy Ohio [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Net derivative asset (liability) | 8 | 9 | 1 | $ 2 | 3 | 5
Duke Energy Ohio [Member] | Financial Transmission Rights [Member] | Valuation, Market Approach [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Net derivative asset (liability) | $ 8 |  | 1 |  |  |
Duke Energy Ohio [Member] | Financial Transmission Rights [Member] | Minimum [Member] | Valuation, Market Approach [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Input measurements (in usd per MWh and usd per MMBtu) | $ / MWh | 1.03 |  |  | 0.07 |  |
Duke Energy Ohio [Member] | Financial Transmission Rights [Member] | Maximum [Member] | Valuation, Market Approach [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Input measurements (in usd per MWh and usd per MMBtu) | $ / MWh | 4.10 |  |  | 1.41 |  |
Duke Energy Indiana [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Net derivative asset (liability) | $ 29 | 44 | 27 | $ 28 | 51 | 16
Duke Energy Indiana [Member] | Financial Transmission Rights [Member] | Valuation, Market Approach [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Net derivative asset (liability) | $ 29 |  | 27 |  |  |
Duke Energy Indiana [Member] | Financial Transmission Rights [Member] | Minimum [Member] | Valuation, Market Approach [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Input measurements (in usd per MWh and usd per MMBtu) | $ / MWh | (2.32) |  |  | (0.77) |  |
Duke Energy Indiana [Member] | Financial Transmission Rights [Member] | Maximum [Member] | Valuation, Market Approach [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Input measurements (in usd per MWh and usd per MMBtu) | $ / MWh | 8.11 |  |  | 7.44 |  |
Piedmont Natural Gas [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Net derivative asset (liability) | $ (153) | $ (150) | (142) | $ (123) | $ (145) | $ (187)
Piedmont Natural Gas [Member] | Derivative Financial Instruments, Liabilities [Member] | Discounted Cash Flow [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Net derivative asset (liability) | $ (153) |  | $ (142) |  |  |
Piedmont Natural Gas [Member] | Derivative Financial Instruments, Liabilities [Member] | Minimum [Member] | Discounted Cash Flow [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Input measurements (in usd per MWh and usd per MMBtu) | $ / MMBTU | 1.74 |  |  | 2.10 |  |
Piedmont Natural Gas [Member] | Derivative Financial Instruments, Liabilities [Member] | Maximum [Member] | Discounted Cash Flow [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques |  |  |  |  |  |
Input measurements (in usd per MWh and usd per MMBtu) | $ / MMBTU | 3.41 |  |  | 2.88 |  |

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