CASH, CASH EQUIVALENTS, RESTRICTED CASH, AND MARKETABLE SECURITIES

As of December 31, 2017 and 2018, our cash, cash equivalents, restricted cash, and marketable securities primarily consisted of cash, AAA-rated money market funds, U.S. and foreign government and agency securities, and other investment grade securities. Cash equivalents and marketable securities are recorded at fair value. The following table summarizes, by major security type, our cash, cash equivalents, restricted cash, 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, 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



	December 31, 2018
	Cost or Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Total Estimated Fair Value
Cash	$	10,406	$	—	$	—		$	10,406
Level 1 securities:
Money market funds	12,515		—		—			12,515
Equity securities	29		143		(2		)	170
Level 2 securities:
Foreign government and agency securities	815		—		—			815
U.S. government and agency securities	11,686		1		(20		)	11,667
Corporate debt securities	5,008		1		(19		)	4,990
Asset-backed securities	896		—		(4		)	892
Other fixed income securities	190		—		(2		)	188
Equity securities	28		5		—			33
	$	41,573	$	150	$	(47	)	$	41,676
Less: Restricted cash, cash equivalents, and marketable securities (1)								(426		)
Total cash, cash equivalents, and marketable securities								$	41,250

___________________

(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, 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 fixed income marketable securities (in millions):



	Year Ended December 31,
	2016		2017		2018
Realized gains	$	3	$	5	$	2
Realized losses	11		11		9

The following table summarizes the remaining contractual maturities of our cash equivalents and marketable fixed income securities as of December 31, 2018 (in millions):



	Amortized Cost		Estimated Fair Value
Due within one year	$	27,520	$	27,508
Due after one year through five years	2,865		2,845
Due after five years through ten years	187		185
Due after ten years	538		529
Total	$	31,110	$	31,067

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

The following table provides a reconciliation of the amount of cash, cash equivalents, and restricted cash reported within the consolidated balance sheets to the total of the same such amounts shown in the consolidated statements of cash flows (in millions):



	December 31, 2017		December 31, 2018
Cash and cash equivalents	$	20,522	$	31,750
Restricted cash included in accounts receivable, net and other	1,329		418
Restricted cash included in other assets	5		5
Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows	$	21,856	$	32,173