| Marketable securities |
| | | December 31, 2020 | |
| | | Cost
Basis
| | | Gross Unrealized Gains
| | | Gross Unrealized Losses
| | | Fair Value
| |
| | | | | | | | | | | | | |
| Certificates of deposit | | $ | 3,534,000 | | | $ | 7,000 | | | $ | — | | | $ | 3,541,000 | |
| Fixed income mutual funds | | | 11,255,000 | | | | 80,000 | | | | — | | | | 11,335,000 | |
| Corporate bonds and notes | | | 4,500,000 | | | | 18,000 | | | | (28,000 | ) | | | 4,490,000 | |
| Total marketable securities | | $ | 19,289,000 | | | $ | 105,000 | | | $ | (28,000 | ) | | $ | 19,366,000 | |
| | | December 31, 2019 | |
| | | Cost
Basis
| | | Gross Unrealized Gains
| | | Gross Unrealized Losses
| | | Fair Value
| |
| | | | | | | | | | | | | |
| Certificates of deposit | | $ | 8,953,000 | | | $ | 6,000 | | | $ | — | | | $ | 8,959,000 | |
| Fixed income mutual funds | | | 7,878,000 | | | | 1,000 | | | | — | | | | 7,879,000 | |
| Corporate bonds and notes | | | 8,813,000 | | | | 112,000 | | | | (33,000 | ) | | | 8,892,000 | |
| Total marketable securities | | $ | 25,644,000 | | | $ | 119,000 | | | $ | (33,000 | ) | | $ | 25,730,000 | |
|