The following table summarizes, by major security type, our cash, cash equivalents, and marketable securities that are measured at fair value on a recurring basis and are categorized using the fair value hierarchy (in millions):


	June 30, 2011										December 31, 2010
	June 30, 2011										December 31, 2010
	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Total Estimated Fair Value			Total Estimated Fair Value
Cash	$	850	$	—	$	—		$	850		$	613
Level 1 securities:
Money market funds		1,039		—		—			1,039			1,882
Equity securities		2		—		(1	)		1			1
Level 2 securities:
Foreign government and agency securities		1,422		4		(4	)		1,422			2,158
U.S. government and agency securities		2,514		11		(1	)		2,524			3,756
Corporate debt securities		616		3		(2	)		617			459
Asset-backed securities		28		—		—			28			33
Other fixed income securities		22		—		—			22			17

	$	6,493	$	18	$	(8	)	$	6,503		$	8,919

Less: Long-term restricted cash, cash equivalents, and marketable securities (1)									(148	)		(157	)

Total cash, cash equivalents, and marketable securities								$	6,355		$	8,762

(1)

We are required to pledge or otherwise restrict a portion of our cash, cash equivalents, and marketable securities as collateral for standby letters of credit, guarantees, debt and real estate lease agreements. We classify cash and marketable securities with use restrictions of twelve months or longer as non-current “Other assets” on our consolidated balance sheets. See “Note 3 — Commitments and Contingencies.“