Investments in Debt and Equity Securities (Maturities) (Details) $ in Millions	Sep. 30, 2018 USD ($)
Debt Instrument [Line Items]
Due in one year or less	$ 100
Due after one through five years	492
Due after five through 10 years	561
Due after 10 years	1,154
Total	2,307
Duke Energy Carolinas [Member]
Debt Instrument [Line Items]
Due in one year or less	12
Due after one through five years	161
Due after five through 10 years	289
Due after 10 years	566
Total	1,028
Progress Energy [Member]
Debt Instrument [Line Items]
Due in one year or less	83
Due after one through five years	268
Due after five through 10 years	217
Due after 10 years	508
Total	1,076
Duke Energy Progress [Member]
Debt Instrument [Line Items]
Due in one year or less	40
Due after one through five years	200
Due after five through 10 years	156
Due after 10 years	385
Total	781
Duke Energy Florida [Member]
Debt Instrument [Line Items]
Due in one year or less	43
Due after one through five years	68
Due after five through 10 years	61
Due after 10 years	123
Total	295
Duke Energy Indiana [Member]
Debt Instrument [Line Items]
Due in one year or less	2
Due after one through five years	20
Due after five through 10 years	5
Due after 10 years	13
Total	$ 40