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 | |
|