STOCKHOLDER'S EQUITY - Summary of Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Number of Stock Options Outstanding
Beginning balance (in shares)	33,152,881
Granted (in shares)	1,184,657
Exercised (in shares)	(10,176,170)
Forfeited and canceled (in shares)	(541,564)
Ending balance (in shares)	23,619,804	33,152,881
Weighted Average Exercise Price
Beginning balance (in USD per share)	$ 9.52
Granted (in USD per share)	72.15
Exercised (in USD per share)	8.09
Forfeited and canceled (in USD per share)	39.88
Ending balance (in USD per share)	$ 12.66	$ 9.52
Additional Disclosures
Weighted Average Remaining Contractual Term (in years)	4 years 10 months 20 days	5 years 5 months 12 days
Aggregate Intrinsic Value	$ 1,191,746	$ 1,543,793
Options exercisable, number of stock options outstanding (in shares)	22,107,017
Options exercisable, weighted average exercise price (in USD per share)	$ 9.77
Options exercisable, weighted average remaining contractual term (in years)	4 years 7 months 17 days
Options exercisable, aggregate intrinsic value	$ 1,168,770