Shareholders' Equity - Reserved Ordinary Shares (Details) - shares |
Sep. 30, 2021 |
Dec. 31, 2020 |
Dec. 31, 2018 |
|---|---|---|---|
| Class of Stock [Line Items] | |||
| Shares reserved for future issuance (in shares) | 4,646,503 | 4,182,196 | |
| Equity Incentive Plan, 2018 | |||
| Class of Stock [Line Items] | |||
| Shares reserved for future issuance (in shares) | 1,861,783 | 1,641,112 | 1,500,000 |
| Options to purchase shares | |||
| Class of Stock [Line Items] | |||
| Shares reserved for future issuance (in shares) | 2,080,161 | 2,012,960 | |
| Restricted Stock | |||
| Class of Stock [Line Items] | |||
| Shares reserved for future issuance (in shares) | 38,059 | 48,624 | |
| ESPP | |||
| Class of Stock [Line Items] | |||
| Shares reserved for future issuance (in shares) | 661,000 | 474,000 | |
| Rockport Warrants | |||
| Class of Stock [Line Items] | |||
| Shares reserved for future issuance (in shares) | 5,500 | 5,500 |