
	June 30, 2024
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Available-for-sale debt securities:
Commercial paper	$	394	$	—	$	—	$	394
Corporate notes and bonds	4,063		3		(15)		4,051
Certificates of deposit	70		—		—		70
U.S. government and agency securities	2,140		—		(11)		2,129
Mortgage-backed and asset-backed securities	101		—		(19)		82

Total available-for-sale debt securities	$	6,768	$	3	$	(45)	$	6,726


	December 31, 2023
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Available-for-sale debt securities:
Commercial paper	$	349	$	—	$	—	$	349
Corporate notes and bonds	3,579		10		(13)		3,576
Certificates of deposit	94		—		—		94
U.S. government and agency securities	2,081		3		(6)		2,078
Mortgage-backed and asset-backed securities	102		—		(16)		86

Total available-for-sale debt securities	$	6,205	$	13	$	(35)	$	6,183

As of June 30, 2024, the contractual maturities of our available-for-sale debt securities, excluding those securities classified within cash and cash equivalents on the condensed consolidated balance sheet and mortgage-backed and asset-backed securities that do not have a single maturity, did not exceed 37 months. The fair values of available-for-sale debt securities, by remaining contractual maturity, are as follows (in millions):

As of June 30, 2024 and December 31, 2023, the fair value of available-for-sale debt securities in a continuous unrealized loss position totaled $5,287 million and $3,731 million, respectively, the majority of which has been in a continuous unrealized loss position for less than 12 months. As of June 30, 2024, unrealized losses of $32 million from available-for-sale debt securities are from securities in a continuous unrealized loss position greater than 12 months.

For all available-for-sale debt securities that were in unrealized loss positions, we have determined that it is more likely than not we will hold the securities until maturity or a recovery of the cost basis. Unrealized losses on available-for-sale debt securities were due primarily to changes in market interest rates, and credit-related impairment losses were immaterial as of June 30, 2024.

As of June 30, 2024 and December 31, 2023, the total amount of non-marketable equity investments in privately held companies included in other assets on our condensed consolidated balance sheets was $346 million and $268 million, respectively. Our non-marketable equity investments are primarily accounted for using the measurement alternative, which measures the investments at cost, minus impairment, if any, plus or minus changes resulting from qualifying observable price changes resulting from the issuance of similar or identical securities in an orderly transaction by the same issuer. Determining whether an observed transaction is similar to a security within our portfolio requires judgment based on the rights and preferences of the securities. Recording upward and downward adjustments to the carrying value of our non-marketable equity investments as a result of observable price changes requires quantitative assessments of the fair value of our non-marketable equity investments using various valuation methodologies and involves the use of estimates. The adjustments made during the three and six months ended June 30, 2024 and 2023 were immaterial. We classify these fair value measurements as Level 3 within the fair value hierarchy.