Fair Value Measurements (Quantitative Level 3 Fair Value Disclosures) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) $ / MWh $ / MMBTU	Jun. 30, 2017 USD ($) $ / MWh $ / MMBTU	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Measurement Inputs and Valuation Techniques
Net derivative asset (liability)	$ (97)	$ (91)	$ (124)	$ (114)	$ (135)	$ (166)
Duke Energy Ohio [Member]
Fair Value Measurement Inputs and Valuation Techniques
Net derivative asset (liability)	9	$ 3	1	1	1	5
Duke Energy Ohio [Member] | Financial Transmission Rights [Member] | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques
Net derivative asset (liability)	$ 9			1
Duke Energy Ohio [Member] | Financial Transmission Rights [Member] | Minimum [Member] | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques
Fair Value Input Price Per Unit | $ / MWh	0.67	0.07
Duke Energy Ohio [Member] | Financial Transmission Rights [Member] | Maximum [Member] | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques
Fair Value Input Price Per Unit | $ / MWh	3.32	1.41
Duke Energy Indiana [Member]
Fair Value Measurement Inputs and Valuation Techniques
Net derivative asset (liability)	$ 44	$ 51	7	27	9	16
Duke Energy Indiana [Member] | Financial Transmission Rights [Member] | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques
Net derivative asset (liability)	$ 44			27
Duke Energy Indiana [Member] | Financial Transmission Rights [Member] | Minimum [Member] | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques
Fair Value Input Price Per Unit | $ / MWh	(2.31)	(0.77)
Duke Energy Indiana [Member] | Financial Transmission Rights [Member] | Maximum [Member] | Valuation, Market Approach [Member]
Fair Value Measurement Inputs and Valuation Techniques
Fair Value Input Price Per Unit | $ / MWh	11.01	7.44
Piedmont Natural Gas [Member]
Fair Value Measurement Inputs and Valuation Techniques
Net derivative asset (liability)	$ (150)	$ (145)	$ (132)	(142)	$ (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)	$ (150)			$ (142)
Piedmont Natural Gas [Member] | Derivative Financial Instruments, Liabilities [Member] | Minimum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques
Fair Value Input Price Per Unit | $ / MMBTU	1.88	2.10
Piedmont Natural Gas [Member] | Derivative Financial Instruments, Liabilities [Member] | Maximum [Member] | Discounted Cash Flow [Member]
Fair Value Measurement Inputs and Valuation Techniques
Fair Value Input Price Per Unit | $ / MMBTU	3.42	2.88