Investments in Debt and Equity Securities (Maturities) (Details) $ in Millions	Sep. 30, 2017 USD ($)
Debt Instrument [Line Items]
Due in one year or less	$ 92
Due after one through five years	584
Due after five through 10 years	514
Due after 10 years	1,076
Total	2,266
Duke Energy Carolinas [Member]
Debt Instrument [Line Items]
Due in one year or less	5
Due after one through five years	218
Due after five through 10 years	264
Due after 10 years	539
Total	1,026
Progress Energy [Member]
Debt Instrument [Line Items]
Due in one year or less	74
Due after one through five years	309
Due after five through 10 years	194
Due after 10 years	482
Total	1,059
Duke Energy Progress [Member]
Debt Instrument [Line Items]
Due in one year or less	16
Due after one through five years	209
Due after five through 10 years	136
Due after 10 years	371
Total	732
Duke Energy Florida [Member]
Debt Instrument [Line Items]
Due in one year or less	58
Due after one through five years	100
Due after five through 10 years	58
Due after 10 years	111
Total	327
Duke Energy Indiana [Member]
Debt Instrument [Line Items]
Due in one year or less	4
Due after one through five years	12
Due after five through 10 years	8
Due after 10 years	7
Total	$ 31