| MARKETABLE DEBT SECURITIES |
MARKETABLE DEBT SECURITIES The following tables summarize our marketable debt securities as of December 31, 2019 and 2018, in thousands: | | | | | | | | | | | | | | | | | | | | | | | | | As of December 31, 2019 | | | | | | | | Amortized
Cost | | Gross
Unrealized
Gains | | Gross
Unrealized
Losses | | Fair Value | | Certificates of deposit | $ | 4,303 | | | $ | — | | | $ | (2) | | | $ | 4,301 | | | Commercial paper | 39,913 | | | — | | | — | | | 39,913 | | | Corporate notes | 146,016 | | | 58 | | | (34) | | | 146,040 | | | U.S. government-sponsored enterprise securities | 32,487 | | | 3 | | | (2) | | | 32,488 | | | U.S. treasury securities | 1,092,293 | | | 185 | | | (71) | | | 1,092,407 | | | Total | $ | 1,315,012 | | | $ | 246 | | | $ | (109) | | | $ | 1,315,149 | |
| | | | | | | | | | | | | | | | | | | | | | | | | As of December 31, 2018 | | | | | | | | Amortized
Cost | | Gross
Unrealized
Gains | | Gross
Unrealized
Losses | | Fair Value | | Certificates of deposit | $ | 8,951 | | | $ | — | | | $ | — | | | $ | 8,951 | | | Commercial paper | 57,197 | | | — | | | — | | | 57,197 | | | Corporate notes | 232,695 | | | — | | | (285) | | | 232,410 | | | U.S. government-sponsored enterprise securities | 39,031 | | | — | | | (13) | | | 39,018 | | | U.S. treasury securities | 546,631 | | | 1 | | | (124) | | | 546,508 | | | Total | $ | 884,505 | | | $ | 1 | | | $ | (422) | | | $ | 884,084 | |
The fair values of our marketable debt securities by classification in the consolidated balance sheets were as follows, in thousands: | | | | | | | | | | | | | December 31, 2019 | | December 31, 2018 | | Cash and cash equivalents | $ | 340,132 | | | $ | 221,281 | | | Marketable debt securities | 975,017 | | | 662,803 | | | Total | $ | 1,315,149 | | | $ | 884,084 | |
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