Summary of Significant Accounting Policies - Short-term Investments and Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accumulated other comprehensive loss	$ 0	$ (52)
Realized gains	0
Short-term investments:
Total short-term investments	7,188	90,922
Liabilities
Preferred stock warrants	157	0
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	6,446	94,700
Commercial paper	1,099
Total cash equivalents	7,545	94,700
Short-term investments:
Commercial paper	5,384	27,898
Corporate bonds	1,804	28,012
U.S. government securities		17,957
Total short-term investments	7,188	90,922
Total assets	14,733	185,622
Liabilities
Preferred stock warrants	157
Level 1 | Recurring basis
Cash equivalents:
Money market funds	6,446	94,700
Commercial paper	0
Total cash equivalents	6,446	94,700
Short-term investments:
Commercial paper	0	0
Corporate bonds	0	0
U.S. government securities		17,957
Total short-term investments	0	17,957
Total assets	6,446	112,657
Liabilities
Preferred stock warrants	0
Level 2 | Recurring basis
Cash equivalents:
Money market funds	0	0
Commercial paper	1,099
Total cash equivalents	1,099	0
Short-term investments:
Commercial paper	5,384	27,898
Corporate bonds	1,804	28,012
U.S. government securities		0
Total short-term investments	7,188	72,965
Total assets	8,287	72,965
Liabilities
Preferred stock warrants	0
Level 3 | Recurring basis
Cash equivalents:
Money market funds	0	0
Commercial paper	0
Total cash equivalents	0	0
Short-term investments:
Commercial paper	0	0
Corporate bonds	0	0
U.S. government securities		0
Total short-term investments	0	0
Total assets	0	$ 0
Liabilities
Preferred stock warrants	$ 157