Stock-Based Compensation - Summary of Stock-Based Compensation Expense by Award Type Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 153,231	$ 101,963	$ 35,534
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	140,517	100,342	34,525
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	10,992	651	617
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total Stock-based compensation expense	$ 1,722	$ 970	$ 392