| Summary of Cash and Available-for-Sale Securities by Significant Investment Category |
The following tables summarize our cash and available-for-sale securities by significant investment category at December 31, 2019 and 2018 (in thousands): | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | December 31, 2019 | | | Amortized Cost | | Unrealized Gains | | Unrealized Losses | | Fair Value | | Cash and Cash Equivalents | | Current Marketable Securities | | Non- Current Marketable Securities | Cash and cash equivalents | | $ | 225,874 |
| | $ | — |
| | $ | — |
| | $ | 225,874 |
| | $ | 225,874 |
| | $ | — |
| | $ | — |
| Available-for-sale: | | | | | | | | | | | | | | | Level 2: | | | | | | | | | | | | | | | Certificates of deposit | | 82,028 |
| | 79 |
| | (6 | ) | | 82,101 |
| | 10,002 |
| | 69,197 |
| | 2,902 |
| U.S. treasury securities | | 117,891 |
| | 260 |
| | (2 | ) | | 118,149 |
| | — |
| | 110,186 |
| | 7,963 |
| Debt securities of U.S. government agencies and corporate entities | | 834,187 |
| | 2,708 |
| | (32 | ) | | 836,863 |
| | — |
| | 687,741 |
| | 149,122 |
| | | $ | 1,259,980 |
| | $ | 3,047 |
| | $ | (40 | ) | | $ | 1,262,987 |
| | $ | 235,876 |
| | $ | 867,124 |
| | $ | 159,987 |
| | | | December 31, 2018 | | | Amortized Cost | | Unrealized Gains | | Unrealized Losses | | Fair Value | | Cash and Cash Equivalents | | Current Marketable Securities | | Non- Current Marketable Securities | Cash and cash equivalents | | $ | 658,365 |
| | $ | 20 |
| | $ | (21 | ) | | $ | 658,364 |
| | $ | 658,364 |
| | $ | — |
| | $ | — |
| Available-for-sale: | | | | | | | | | | | | | | | Level 2: | | | | | | | | | | | | | | | Certificates of deposit | | 173,102 |
| | 42 |
| | (36 | ) | | 173,108 |
| | — |
| | 157,920 |
| | 15,188 |
| U.S. treasury securities | | 152,205 |
| | 18 |
| | (48 | ) | | 152,175 |
| | — |
| | 152,175 |
| | — |
| Debt securities of U.S. government agencies and corporate entities | | 712,065 |
| | 40 |
| | (1,335 | ) | | 710,770 |
| | — |
| | 552,968 |
| | 157,802 |
| | | $ | 1,695,737 |
| | $ | 120 |
| | $ | (1,440 | ) | | $ | 1,694,417 |
| | $ | 658,364 |
| | $ | 863,063 |
| | $ | 172,990 |
|
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| Amortized Cost and Estimated Fair Value of Marketable Securities, by Contractual Maturity |
The amortized cost and estimated fair value of marketable securities, by contractual maturity at December 31, 2019 and 2018 are as follows (in thousands): | | | | | | | | | | | | December 31, 2019 | | | Amortized Cost | | Estimated Fair Value | Due in one year or less | | $ | 864,958 |
| | $ | 867,124 |
| Due after one year through five years | | 159,148 |
| | 159,987 |
| Total | | $ | 1,024,106 |
| | $ | 1,027,111 |
| | | December 31, 2018 | | | Amortized Cost | | Estimated Fair Value | Due in one year or less | | $ | 864,003 |
| | $ | 863,063 |
| Due after one year through five years | | 173,369 |
| | 172,990 |
| Total | | $ | 1,037,372 |
| | $ | 1,036,053 |
|
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