Stock-Based Compensation - Schedule of Shares Available for Grant and Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares Available for Grant
Balance as of December 31, 2018 (in shares)		1,741,000	3,014,000	208,000
Shares authorized (in shares)		10,504,000	3,196,000	8,661,000
Adjustment to plan (in shares)		59,000
Granted (in shares)			(5,016,000)	(5,996,000)
Forfeited/cancelled (in shares)		(785,000)	(1,619,000)	(1,205,000)
Balance as of June 30, 2019 (in shares)		8,160,000	1,741,000	3,014,000	208,000
Shares Subject to Options Outstanding
Balance as of December 31, 2018 (in shares)		17,571,000	15,628,000	12,209,000
Exercised (in shares)		(2,766,000)	(1,454,000)	(1,372,000)
Forfeited/cancelled (in shares)		(785,000)	(1,619,000)	(1,205,000)
Balance as of June 30, 2019 (in shares)		14,020,000	17,571,000	15,628,000	12,209,000
Options vested and exercisable as of June 30, 2019 (in shares)		9,698,000	8,999,000
Weighted- Average Exercise Price
Balance as of December 31, 2018 (in USD per share)		$ 1.80	$ 1.20	$ 0.79
Granted (in USD per share)			3.62	1.88
Exercised (in USD per share)		1.12	1.19	0.78
Forfeited/cancelled (in USD per share)		3.36	2.25	0.87
Balance as of June 30, 2019 (in USD per share)		1.85	1.80	$ 1.20	$ 0.79
Options vested and exercisable as of June 30, 2019 (in USD per share)		$ 1.44	$ 0.97
Weighted- Average Remaining Contractual Life (Years) and Aggregate Intrinsic Value
Weighted- Average Remaining Contractual Life (Years), Options outstanding		6 years 8 months 12 days	7 years 8 months 12 days	8 years 2 months 12 days	8 years 3 months 18 days
Weighted- Average Remaining Contractual Life (Years), Options vested and exercisable		6 years 2 months 12 days	6 years 8 months 12 days
Aggregate Intrinsic Value, Options outstanding		$ 325,474	$ 89,990	$ 10,559	$ 9,623
Aggregate Intrinsic Value, Options vested and exercisable		$ 229,110	$ 53,566
Restricted Stock Awards
Shares Available for Grant
Granted (in shares)	(982,301)	(982,000)	0	(1,064,000)
Forfeited (in shares)			754,000
Performance-Based RSUs
Shares Available for Grant
Granted (in shares)			(1,830,000)
Forfeited (in shares)			4,000
Restricted Stock Units, Performance RSUs and Performance Stock Units
Shares Available for Grant
Granted (in shares)		(4,103,000)
Forfeited (in shares)		121,000
Returned (in shares)		35,000