Marketable Securities

Marketable securities are classified as available-for-sale securities and reported at fair value, with unrealized gains and losses excluded from earnings and reported in accumulated other comprehensive income on our consolidated balance sheets. Proceeds received from sales and maturities of marketable securities were $99 million for the six months ended June 30, 2014. Our investments in marketable securities consisted of the following:



	As of June 30, 2014				As of December 31, 2013
(millions)	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Corporate debt securities	$	92	$	92	$	87	$	87
U.S. government and agency debt securities	4		4		12		12
Asset-backed debt securities	17		17		20		20
Certificates of deposit	19		19		17		17
Municipal debt securities	6		6		6		6
Total marketable securities	$	138	$	138	$	142	$	142

The realized and unrealized gains and losses for the three and six months ended June 30, 2014 and 2013 were immaterial. Cost basis for securities sold are determined on a first-in-first-out basis.

Contractual maturities of marketable securities as of June 30, 2014 were as follows:



(millions)	Amortized Cost		Fair Value
Due in 1 year or less	$	79	$	79
Due in 1-5 years	59		59
Total marketable securities	$	138	$	138

Actual maturities may differ from the contractual maturities because issuers of the securities may have the right to prepay them.