Fair Value Measurements (Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis Using Unobservable Inputs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Derivatives (net)
Balance at beginning of period	$ (102)	$ (113)
Total pretax realized or unrealized losses included in comprehensive income	(84)	0
Purchases	14	37
Settlements	(19)	(44)
Net transfers Out of Level 3	117	0
Total gains (losses) included on the Consolidated Balance Sheet	(3)	18
Ending balance	(77)	(102)
Duke Energy Indiana
Derivatives (net)
Balance at beginning of period	11	22
Purchases	10	28
Settlements	(13)	(36)
Total gains (losses) included on the Consolidated Balance Sheet	(2)	(3)
Ending balance	6	11
Piedmont
Derivatives (net)
Balance at beginning of period	(117)	(141)
Net transfers Out of Level 3	117	0
Total gains (losses) and settlements	0	24
Ending balance	$ 0	$ (117)