Summary of Significant Accounting Policies (Payables) (Details) $ in Millions	Dec. 31, 2020 USD ($)
Payables [Line Items]
Payables sold by suppliers	$ 15
Cumulative payables sold by suppliers	45
Duke Energy Ohio
Payables [Line Items]
Payables sold by suppliers	1
Cumulative payables sold by suppliers	9
Piedmont
Payables [Line Items]
Payables sold by suppliers	14
Cumulative payables sold by suppliers	$ 36