Investments in Debt and Equity Securities (Maturities) (Details) $ in Millions	Mar. 31, 2018 USD ($)
Debt Instrument [Line Items]
Due in one year or less	$ 100
Due after one through five years	535
Due after five through 10 years	530
Due after 10 years	1,104
Total	2,269
Duke Energy Carolinas [Member]
Debt Instrument [Line Items]
Due in one year or less	11
Due after one through five years	178
Due after five through 10 years	285
Due after 10 years	544
Total	1,018
Progress Energy [Member]
Debt Instrument [Line Items]
Due in one year or less	75
Due after one through five years	300
Due after five through 10 years	198
Due after 10 years	482
Total	1,055
Duke Energy Progress [Member]
Debt Instrument [Line Items]
Due in one year or less	19
Due after one through five years	216
Due after five through 10 years	144
Due after 10 years	368
Total	747
Duke Energy Florida [Member]
Debt Instrument [Line Items]
Due in one year or less	56
Due after one through five years	84
Due after five through 10 years	54
Due after 10 years	114
Total	308
Duke Energy Indiana [Member]
Debt Instrument [Line Items]
Due in one year or less	3
Due after one through five years	15
Due after five through 10 years	6
Due after 10 years	7
Total	$ 31