Fair Value Measurements (Reconciliation of Assets and Liabilities Measured At Fair Value On A Recurring Basis Using Unobservable Inputs) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Measurement Transfers Between Activity Levels	$ 0	$ 0
Derivatives (net)
Beginning balance	(114,000,000)	(166,000,000)
Settlements	(14,000,000)	(9,000,000)
Total gains (losses) included on the Condensed Consolidated Balance Sheet	4,000,000	40,000,000
Ending balance	(124,000,000)	(135,000,000)
Investments and Derivatives, Net
Beginning balance	(114,000,000)	(161,000,000)
Settlements	(14,000,000)	(9,000,000)
Total gains (losses) included on the Condensed Consolidated Balance Sheet	4,000,000	40,000,000
Ending balance	(124,000,000)	(130,000,000)
Investments [Member]
Investments
Beginning balance		5,000,000
Ending balance		5,000,000
Duke Energy Ohio [Member]
Derivatives (net)
Beginning balance	1,000,000	5,000,000
Settlements	0	(1,000,000)
Total gains (losses) included on the Condensed Consolidated Balance Sheet	0	(3,000,000)
Ending balance	1,000,000	1,000,000
Duke Energy Indiana [Member]
Derivatives (net)
Beginning balance	27,000,000	16,000,000
Settlements	(14,000,000)	(7,000,000)
Total gains (losses) included on the Condensed Consolidated Balance Sheet	(6,000,000)	0
Ending balance	7,000,000	9,000,000
Piedmont Natural Gas [Member]
Derivatives (net)
Beginning balance	(142,000,000)	(187,000,000)
Total gains (losses) and settlements	10,000,000	42,000,000
Ending balance	$ (132,000,000)	$ (145,000,000)