Stockholders' Equity and Stock-Based Compensation - Summary of Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Shares Underlying Options, Outstanding, Beginning Balance | shares	1,303
Shares Underlying Options, Granted | shares	41
Shares Underlying Options, Exercised | shares	(436)
Shares Underlying Options, Forfeited or Cancelled | shares	(42)
Shares Underlying Options, Outstanding, Ending Balance | shares	866	1,303
Shares Underlying Options, Vested and Expected to Vest | shares	866
Shares Underlying Options, Exercisable | shares	681
Weighted-Average Exercise Price, Outstanding, Beginning Balance | $ / shares	$ 14.09
Weighted-Average Exercise Price, Granted | $ / shares	3.54
Weighted-Average Exercise Price, Exercised | $ / shares	9.04
Weighted-Average Exercise Price, Forfeited or Cancelled | $ / shares	21.45
Weighted-Average Exercise Price, Outstanding, Ending Balance | $ / shares	15.73	$ 14.09
Weighted-Average Exercise Price, Vested and Expected to Vest | $ / shares	15.73
Weighted-Average Exercise Price, Exercisable | $ / shares	$ 12.08
Weighted-Average Remaining Life, Outstanding	6 years 1 month 6 days	6 years 6 months
Weighted-Average Remaining Life, Vested and Expected to Vest	6 years 1 month 6 days
Weighted-Average Remaining Life, Exercisable	5 years 7 months 6 days
Aggregate Intrinsic Value, Outstanding | $	$ 20,329	$ 30,552
Aggregate Intrinsic Value, Vested and Expected to Vest | $	20,329
Aggregate Intrinsic Value, Exercisable | $	$ 18,286