Marketable Securities (Tables)

3 Months Ended

Mar. 31, 2012

Marketable Securities [Abstract]

Investments in marketable securities

	As of March 31, 2012				As of December 31, 2011
(millions)	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Corporate debt securities		$          86		$          86		$        174		$        174
U.S. government and agency debt securities		12		12		32		32
Asset-backed debt securities		9		9		18		18
Certificates of deposit		16		16		35		35
Municipal debt securities		15		15		27		27
Total marketable securities		$        138		$        138		$        286		$        286

Contractual maturities of marketable securities

(millions)	Amortized Cost		Fair Value
Due in 1 year or less		$      73		$      73
Due in 1-5 years		65		65
Due in more than 5 years		-		-
Total marketable securities		$    138		$    138