Net Loss per Share - Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic and diluted net loss per share calculation:
Numerator - net loss	$ (80,010)	$ (13,591)	$ (5,120)	$ (32,863)	$ (93,601)	$ (37,983)
Denominator - weighted average ordinary shares outstanding	192,705,535		185,327,383		191,426,864	178,866,391
Basic and diluted net loss per share	$ (0.42)		$ (0.03)		$ (0.49)	$ (0.21)