Investments in Debt and Equity Securities (Unrealized Losses of Available-for-Sale Debt and Equity Securities in a Continuous Loss Position) (Details) (USD $) In Millions	Jun. 30, 2011		Dec. 31, 2010
Fair Value	$ 401	[1]	$ 440	[1]
Unrealized Loss Position greater than 12 months	(48)		(53)
Unrealized Loss Position less than 12 months	(9)		(12)
Equity Securities [Member]
Fair Value	104	[1]	85	[1]
Unrealized Loss Position greater than 12 months	(12)		(11)
Unrealized Loss Position less than 12 months	(7)		(5)
Corporate Debt Securities [Member]
Fair Value	55	[1]	73	[1]
Unrealized Loss Position greater than 12 months	(1)		(2)
Unrealized Loss Position less than 12 months	(1)		(2)
Municipal Bonds [Member]
Fair Value	37	[1]	42	[1]
Unrealized Loss Position greater than 12 months	(7)		(8)
Unrealized Loss Position less than 12 months	(1)		(1)
NDTF [Member]
Fair value of investments	232		226
U.S. Government Bonds [Member]
Fair Value	62	[1]	38	[1]
Unrealized Loss Position less than 12 months			(1)
Auction Rate Securities [Member]
Fair Value	90	[1],[2]	118	[1],[2]
Unrealized Loss Position greater than 12 months	(26)	[2]	(31)	[2]
Other Debt Securities [Member]
Fair Value	53	[1]	84	[1]
Unrealized Loss Position greater than 12 months	(2)		(1)
Unrealized Loss Position less than 12 months			(3)
Duke Energy Indiana Grantor Trust [Member]
Fair value of investments	$ 9		$ 5

[1] The table above includes fair values of $232 million and $226 million at June 30, 2011 and December 31, 2010, respectively, associated with investments held in the NDTF. Additionally, the table above includes fair values of $9 million and $5 million at June 30, 2011 and December 31, 2010, respectively, associated with investments held in the Duke Energy Indiana grantor trust.

[2] See Note 9 for information about fair value measurements related to investments in auction rate debt securities.