Stock-Based Compensation - Stock Option Activity (Details) - Stock options - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Shares
Options Outstanding at beginning of the period (in shares)	1,699,202
Granted (in shares)	1,490,925
Exercised (in shares)	(36,960)
Canceled (in shares)	(78,335)
Options Outstanding at the end of the period (in shares)	3,074,832	1,699,202
Options Vested and Expected to Vest at the end of the period (in shares)	2,888,532
Options Exercisable at the end of the period (in shares)	871,007
Shares Available for Grant Under the 2008 Plan	907,673
Weighted Average Exercise Price Per Share
Options Outstanding at beginning of the period (in dollars per share)	$ 44.87
Granted (in dollars per share)	10.42
Exercised (in dollars per share)	3.85
Canceled (in dollars per share)	35.80
Options Outstanding at the end of the period (in dollars per share)	28.89	$ 44.87
Options Vested and Expected to Vest at the end of the period (in dollars per share)	30.19
Options Exercisable at the end of the period (in dollars per share)	$ 80.35
Weighted Average Remaining Contractual Term (In Years)
Options Outstanding at the end of the period	8 years 4 months 24 days	8 years
Options Vested and Expected to Vest at the end of the period	8 years 4 months 24 days
Options Exercisable at the end of the period	6 years 2 months 12 days