Business Segments (Major Customer Disclosure) (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Revenue, Major Customer [Line Items]
Revenues	$ 5,799	$ 6,482	$ 5,555	$ 5,729	$ 5,577	$ 6,576	$ 5,213	$ 5,377	$ 23,565	$ 22,743	$ 22,371
Duke Energy Progress [Member]
Revenue, Major Customer [Line Items]
Revenues	$ 1,251	$ 1,460	$ 1,199	$ 1,219	$ 1,174	$ 1,583	$ 1,213	$ 1,307	$ 5,129	$ 5,277	$ 5,290
Number of Reportable Segments | segment									1
Major Customer 1 [Member] | Duke Energy Progress [Member]
Revenue, Major Customer [Line Items]
Revenues									$ 521