Stock-Based Compensation - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Options
Outstanding, beginning balance (in shares)	16,268	12,897
Granted (in shares)	1,529	5,266
Exercised (in shares)	(3,271)	(687)
Cancelled (in shares)	(936)	(1,208)
Outstanding, ending balance (in shares)	13,590	16,268	12,897
Options vested and exercisable (in shares)	7,974
Options vested and expected to vest (in dollars per share)	13,590
Weighted Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 5.70	$ 5.94
Granted (in dollars per share)	8.63	5.17
Exercised (in dollars per share)	5.23	2.96
Cancelled (in dollars per share)	6.63	7.62
Outstanding, ending balance (in dollars per share)	6.07	$ 5.70	$ 5.94
Options vested and exercisable (in dollars per share)	5.64
Options vested and expected to vest (in dollars per share)	$ 6.07
Weighted Average Remaining Contractual Life
Weighted-average remaining contractual life, options outstanding	6 years 7 months 17 days	7 years 4 months 27 days	7 years 5 months 26 days
Weighted-average remaining contractual life, options vested and exercisable	5 years 6 months 7 days
Weighted-average remaining contractual life, options vested and expected to vest	6 years 7 months 17 days
Aggregate intrinsic value, options outstanding	$ 66,462,000	$ 14,832,000	$ 9,625,000
Aggregate intrinsic value, options vested and exercisable	42,504,000
Aggregate intrinsic value, options vested and expected to vest	$ 66,462,000