Stock-based Compensation - Stock Options (Details) - 2009 plan $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Number of Shares
Outstanding at the beginning of the period (in shares) | shares	5,111,690
Granted (in shares) | shares	2,116,425
Exercised (in shares) | shares	(24,714)
Forfeited | shares	(207,632)
Outstanding at the end of the period (in shares) | shares	6,995,769	5,111,690
Vested and expected to vest (in shares) | shares	6,995,769
Options exercisable (in shares) | shares	3,683,751
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 8.96
Granted (in dollars per share) | $ / shares	5.24
Exercised (in dollars per share) | $ / shares	1.70
Forfeited (in dollars per share) | $ / shares	12.64
Outstanding at the end of the period (in dollars per share) | $ / shares	7.76	$ 8.96
Vested and expected to vest (in dollars per share ) | $ / shares	7.76
Options exercisable (in dollars per shares) | $ / shares	$ 6.91
Weighted Average Remaining Contractual Term
Weighted average period	7 years 9 months 18 days	8 years
Vested and expected to vest	7 years 9 months 18 days
Options exercisable	6 years 10 months 24 days
Aggregate Intrinsic Value
Outstanding at the beginning of the period (in dollars) | $	$ 3,771
Outstanding at the end of the period (in dollars) | $	1,429	$ 3,771
Vested and expected to vest (in dollars) | $	1,429
Options exercisable (in dollars) | $	$ 1,365