Investments in Debt and Equity Securities (Realized Gains and Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Gross Realized Gain (Loss) [Abstract]
Realized gains	$ 202	$ 246	$ 193
Realized losses	160	187	98
Duke Energy Carolinas [Member]
Available-for-sale Securities, Gross Realized Gain (Loss) [Abstract]
Realized gains	135	157	158
Realized losses	103	121	83
Progress Energy [Member]
Available-for-sale Securities, Gross Realized Gain (Loss) [Abstract]
Realized gains	65	84	33
Realized losses	56	64	13
Duke Energy Progress [Member]
Available-for-sale Securities, Gross Realized Gain (Loss) [Abstract]
Realized gains	54	71	26
Realized losses	48	55	11
Duke Energy Florida [Member]
Available-for-sale Securities, Gross Realized Gain (Loss) [Abstract]
Realized gains	11	13	7
Realized losses	$ 8	$ 9	$ 2