Fair Value Measurements (Quantitative Level 3 Fair Value Disclosure - Assets) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / MWh	Mar. 31, 2017 USD ($) $ / MWh	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net derivative asset (liability)	$ (124)	$ (135)	$ (114)	$ (166)
Duke Energy Ohio [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net derivative asset (liability)	1	$ 1	1	5
Duke Energy Ohio [Member] | Financial Transmission Rights [Member] | Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net derivative asset (liability)	$ 1		1
Duke Energy Ohio [Member] | Financial Transmission Rights [Member] | Minimum [Member] | Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Input Price Per Unit | $ / MWh		0.07
Duke Energy Ohio [Member] | Financial Transmission Rights [Member] | Maximum [Member] | Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Input Price Per Unit | $ / MWh	2.88	1.41
Duke Energy Indiana [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net derivative asset (liability)	$ 7	$ 9	27	$ 16
Duke Energy Indiana [Member] | Financial Transmission Rights [Member] | Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net derivative asset (liability)	$ 7		$ 27
Duke Energy Indiana [Member] | Financial Transmission Rights [Member] | Minimum [Member] | Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Input Price Per Unit | $ / MWh	(5.09)	(0.77)
Duke Energy Indiana [Member] | Financial Transmission Rights [Member] | Maximum [Member] | Market Approach Valuation Technique [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Input Price Per Unit | $ / MWh	7.58	7.44