We classify our cash equivalents and marketable debt securities within Level 1 or Level 2 because we use quoted market prices or alternative pricing sources and models utilizing market observable inputs to determine their fair value. Certain other assets are classified within Level 3 because factors used to develop the estimated fair value are unobservable inputs that are not supported by market activity.

The following tables summarize our assets measured at fair value on a recurring basis and the classification by level of input within the fair value hierarchy (in millions):


			Fair Value Measurement at Reporting Date Using
Description	December 31, 2023		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash	$	6,265
Cash equivalents:
Money market funds	32,910		$	32,910	$	—	$	—
U.S. government and agency securities	2,206		2,206		—		—

Time deposits	261		—		261		—
Corporate debt securities	220		—		220		—
Total cash and cash equivalents	41,862		35,116		481		—
Marketable securities:
U.S. government securities	8,439		8,439		—		—
U.S. government agency securities	3,498		3,498		—		—
Corporate debt securities	11,604		—		11,604		—

Total marketable securities	23,541		11,937		11,604		—
Restricted cash equivalents	857		857		—		—
Other assets	101		—		—		101
Total	$	66,361	$	47,910	$	12,085	$	101


			Fair Value Measurement at Reporting Date Using
Description	December 31, 2022		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash	$	6,176
Cash equivalents:
Money market funds	8,305		$	8,305	$	—	$	—
U.S. government and agency securities	16		16		—		—
Time deposits	156		—		156		—
Corporate debt securities	28		—		28		—
Total cash and cash equivalents	14,681		8,321		184		—
Marketable securities:
U.S. government securities	8,708		8,708		—		—
U.S. government agency securities	4,989		4,989		—		—
Corporate debt securities	12,335		—		12,335		—
Marketable equity securities	25		25		—		—
Total marketable securities	26,057		13,722		12,335		—
Restricted cash equivalents	583		583		—		—
Other assets	157		—		—		157
Total	$	41,478	$	22,626	$	12,519	$	157

The following tables summarize our available-for-sale marketable debt securities and cash equivalents with unrealized losses as of December 31, 2023 and 2022, aggregated by major security type and the length of time that individual securities have been in a continuous loss position (in millions):


	December 31, 2023
	Less than 12 months				12 months or greater				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
U.S. government securities	$	336	$	(1)	$	7,041	$	(275)	$	7,377	$	(276)
U.S. government agency securities	71		—		3,225		(164)		3,296		(164)
Corporate debt securities	647		(3)		10,125		(491)		10,772		(494)
Total	$	1,054	$	(4)	$	20,391	$	(930)	$	21,445	$	(934)


	December 31, 2022
	Less than 12 months				12 months or greater				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
U.S. government securities	$	5,008	$	(234)	$	3,499	$	(247)	$	8,507	$	(481)
U.S. government agency securities	524		(17)		4,415		(308)		4,939		(325)
Corporate debt securities	4,555		(249)		7,256		(634)		11,811		(883)
Total	$	10,087	$	(500)	$	15,170	$	(1,189)	$	25,257	$	(1,689)

The decrease in the gross unrealized losses for the year ended December 31, 2023 is mostly due to a shorter average portfolio duration. The allowance for credit losses and the gross unrealized gains on our marketable debt securities were not material as of December 31, 2023 and 2022.

Our non-marketable equity securities accounted for using the measurement alternative are measured at fair value on a non-recurring basis and are classified within Level 3 of the fair value hierarchy because we use significant unobservable inputs to estimate their fair value. Assets remeasured at fair value on a non-recurring basis within Level 3 during the years ended December 31, 2023 and 2022 were $53 million and $198 million, respectively. For additional information, see Note 6 — Non-marketable Equity Securities.