Marketable Securities (Tables)

6 Months Ended

Jun. 30, 2012

Marketable Securities [Abstract]

Investments in marketable securities

	As of June 30, 2012				As of December 31, 2011
(millions)	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Corporate debt securities	$	66	$	66	$	174	$	174
U.S. government and agency debt securities		13		13		32		32
Asset-backed debt securities		13		13		18		18
Certificates of deposit		13		13		35		35
Municipal debt securities		23		23		27		27
Total marketable securities	$	128	$	128	$	286	$	286

Contractual maturities of marketable securities

	Amortized Cost		Fair Value
(millions)
Due in 1 year or less	$	74	$	74
Due in 1-5 years		54		54
Due in more than 5 years		—		—
Total marketable securities	$	128	$	128