CASH, CASH EQUIVALENTS, AND MARKETABLE SECURITIES

As of December 31, 2016 and 2017, our cash, cash equivalents, and marketable securities primarily consisted of cash, U.S. and foreign government and agency securities, AAA-rated money market funds, and other investment grade securities. Cash equivalents and marketable securities are recorded at fair value. The following tables summarize, 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):



	December 31, 2016
	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Total Estimated Fair Value
Cash	$	6,883	$	—	$	—		$	6,883
Level 1 securities:
Money market funds	11,940		—		—			11,940
Equity securities	20		31		—			51
Level 2 securities:
Foreign government and agency securities	337		—		—			337
U.S. government and agency securities	4,821		2		(7		)	4,816
Corporate debt securities	2,105		1		(2		)	2,104
Asset-backed securities	355		—		(2		)	353
Other fixed income securities	97		—		—			97
	$	26,558	$	34	$	(11	)	$	26,581
Less: Restricted cash, cash equivalents, and marketable securities (1)								(600		)
Total cash, cash equivalents, and marketable securities								$	25,981



	December 31, 2017
	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Total Estimated Fair Value
Cash	$	9,982	$	—	$	—		$	9,982
Level 1 securities:
Money market funds	11,343		—		—			11,343
Equity securities	23		30		—			53
Level 2 securities:
Foreign government and agency securities	620		—		—			620
U.S. government and agency securities	4,841		1		(19		)	4,823
Corporate debt securities	4,265		1		(9		)	4,257
Asset-backed securities	910		—		(5		)	905
Other fixed income securities	340		—		(2		)	338
	$	32,324	$	32	$	(35	)	$	32,321
Less: Restricted cash, cash equivalents, and marketable securities (1)								(1,335		)
Total cash, cash equivalents, and marketable securities								$	30,986

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(1)

We are required to pledge or otherwise restrict a portion of our cash, cash equivalents, and marketable securities as collateral for real estate leases, workers’ compensation obligations, amounts due to third-party sellers in certain jurisdictions, debt, and standby and trade letters of credit. We classify cash, cash equivalents, and marketable securities with use restrictions of less than twelve months as “Accounts receivable, net and other” and of twelve months or longer as non-current “Other assets” on our consolidated balance sheets. See “Note 7—Commitments and Contingencies.”

The following table summarizes gross gains and gross losses realized on sales of available-for-sale marketable securities (in millions):



	Year Ended December 31,
	2015		2016		2017
Realized gains	$	2	$	3	$	5
Realized losses	7		11		11

The following table summarizes the contractual maturities of our cash equivalents and marketable fixed-income securities as of December 31, 2017 (in millions):



	Amortized Cost		Estimated Fair Value
Due within one year	$	17,293	$	17,287
Due after one year through five years	4,149		4,129
Due after five years through ten years	302		300
Due after ten years	575		570
Total	$	22,319	$	22,286

Actual maturities may differ from the contractual maturities because borrowers may have certain prepayment conditions.