Employee Benefit Plans (Plan Contributions) (Details) - Qualified Pension Plan [Member] - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year	$ 19,000,000
Contributions by Employer	8,000,000	$ 0
Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year	11,000,000
Duke Energy Ohio [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year	4,000,000
Contributions by Employer	4,000,000
Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year	0
Piedmont Natural Gas [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Current Fiscal Year	11,000,000
Contributions by Employer	0
Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year	$ 11,000,000