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, 2019	Dec. 31, 2018
Derivatives (net)
Balance at beginning of period	$ (113)	$ (114)
Purchases	37	57
Settlements	(44)	(57)
Total gains (losses) included on the Consolidated Balance Sheet	18	1
Ending balance	(102)	(113)
Duke Energy Indiana
Derivatives (net)
Balance at beginning of period	22	27
Purchases	28	50
Settlements	(36)	(53)
Total gains (losses) included on the Consolidated Balance Sheet	(3)	(2)
Ending balance	11	22
Piedmont
Derivatives (net)
Balance at beginning of period	(141)	(142)
Total gains (losses) and settlements	24	1
Ending balance	$ (117)	$ (141)