Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details) - USD ($)
$ in Thousands |
Mar. 31, 2018 |
Dec. 31, 2017 |
| Short-term investments: |
|
|
| Short-term Investments, Total |
$ 6,756
|
$ 7,188
|
| Liabilities |
|
|
| Preferred stock warrants |
978
|
157
|
| Fair value of assets transferred from Level 1 to Level 2 |
0
|
0
|
| Fair value of assets transferred from Level 2 to Level 1 |
0
|
0
|
| Fair value of liabilities transferred from Level 1 to Level 2 |
0
|
0
|
| Fair value of liabilities transferred from Level 2 to Level 1 |
0
|
0
|
| Recurring basis |
|
|
| Cash equivalents: |
|
|
| Money market funds |
7,132
|
6,446
|
| Commercial paper |
3,901
|
1,099
|
| Total cash equivalents |
11,033
|
7,545
|
| Short-term investments: |
|
|
| Commercial paper |
3,995
|
5,384
|
| Corporate bonds |
2,760
|
1,804
|
| Short-term Investments, Total |
6,755
|
7,188
|
| Total assets |
17,788
|
14,733
|
| Liabilities |
|
|
| Preferred stock warrants |
978
|
157
|
| Level 1 | Recurring basis |
|
|
| Cash equivalents: |
|
|
| Money market funds |
7,132
|
6,446
|
| Total cash equivalents |
7,132
|
6,446
|
| Short-term investments: |
|
|
| Total assets |
7,132
|
6,446
|
| Level 2 | Recurring basis |
|
|
| Cash equivalents: |
|
|
| Commercial paper |
3,901
|
1,099
|
| Total cash equivalents |
3,901
|
1,099
|
| Short-term investments: |
|
|
| Commercial paper |
3,995
|
5,384
|
| Corporate bonds |
2,760
|
1,804
|
| Short-term Investments, Total |
6,755
|
7,188
|
| Total assets |
10,656
|
8,287
|
| Level 3 | Recurring basis |
|
|
| Liabilities |
|
|
| Preferred stock warrants |
$ 978
|
$ 157
|