The following is a summary of our available-for-sale debt securities recorded within short-term and long-term investments on the consolidated balance sheets (in millions):


	December 31, 2021
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Available-for-sale securities:
Commercial paper	$	528	$	—	$	—	$	528
Corporate notes and bonds	2,418		1		(7)		2,412
Certificates of deposit	28		—		—		28
U.S. government and agency securities	140		—		—		140
Mortgage and asset backed securities	100		—		(2)		98
Total available-for-sale securities	$	3,214	$	1	$	(9)	$	3,206


	December 31, 2020
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Available-for-sale securities:
Commercial paper	$	406	$	—	$	—	$	406
Corporate notes and bonds	2,298		10		—		2,308
Certificates of deposit	23		—		—		23
U.S. government and agency securities	145		1		—		146
Total available-for-sale securities	$	2,872	$	11	$	—	$	2,883

As of December 31, 2021, the contractual maturities of our available-for-sale debt securities, excluding those securities classified within cash and cash equivalents on the consolidated balance sheet and mortgage and asset backed securities that do not have a single maturity, did not exceed 36 months. The fair values of available-for-sale securities, by remaining contractual maturity, are as follows (in millions):


	December 31, 2021
Due within 1 year	$	1,576
Due in 1 year through 5 years	1,532
Instruments not due in single maturity	98
Total	$	3,206

As of December 31, 2021 and December 31, 2020, the gross unrealized losses that have been in a continuous unrealized loss position related to $2,416 million and $637 million available-for-sale debt securities, respectively, were not material.

The decline in fair value below amortized cost basis was not considered other than temporary as it is more likely than not we will hold the securities until maturity or a recovery of the cost basis, and credit-related impairment losses were not deemed material as of December 31, 2021.

As of December 31, 2021 and 2020, the total amount of equity investments in privately-held companies included in other assets on our consolidated balance sheets was $99 million and $28 million, respectively. We classify these assets as Level 3 within the fair value hierarchy as only an impairment or observable adjustment is recognized based on observable transaction price at the transaction date of identical or similar investment of the same issuer and other unobservable inputs such as volatility.