Summary of Significant Accounting Policies - Short-term Investments and Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accumulated other comprehensive loss	$ 0	$ 0
Short-term investments:
Short-term Investments, Total	2,107	7,188
Liabilities
Preferred stock warrants		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	120,031	6,446
Commercial paper	500	1,099
Total cash equivalents	120,531	7,545
Short-term investments:
Commercial paper	997	5,384
Corporate bonds	1,110	1,804
Short-term Investments, Total	2,107	7,188
Total assets	122,638	14,733
Liabilities
Preferred stock warrants		157
Level 1 | Recurring basis
Cash equivalents:
Money market funds	120,031	6,446
Total cash equivalents	120,031	6,446
Short-term investments:
Total assets	120,031	6,446
Level 2 | Recurring basis
Cash equivalents:
Commercial paper	500	1,099
Total cash equivalents	500	1,099
Short-term investments:
Commercial paper	997	5,384
Corporate bonds	1,110	1,804
Short-term Investments, Total	2,107	7,188
Total assets	$ 2,607	8,287
Level 3 | Recurring basis
Liabilities
Preferred stock warrants		$ 157