Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenue	$ 14,414	$ 3,893	$ 1,092
Operating expenses:
Research and development	134,428	79,309	40,818
General and administrative	39,509	26,975	15,994
Total operating expenses	173,937	106,284	56,812
Loss from operations	(159,523)	(102,391)	(55,720)
Other income (expense), net:
Dividend income	3,368	1,578	255
Interest income (expense), net	22	6	337
Other income (expense), net	9,549	(331)	(50)
Other income (expense), net	12,939	1,253	542
Loss before income taxes	(146,584)	(101,138)	(55,178)
Income tax benefit (provision)	(69)	(842)	(482)
Net loss	$ (146,653)	$ (101,980)	$ (55,660)
Net loss per share attributable to ordinary shareholders—basic and diluted	$ (5.06)	$ (3.85)	$ (2.44)
Weighted-average ordinary shares used in computing net loss per share attributable to ordinary shareholders—basic and diluted	28,970,404	26,513,382	22,800,628
Other comprehensive income (loss):
Net loss	$ (146,653)	$ (101,980)	$ (55,660)
Foreign currency translation	37	407	(332)
Comprehensive loss	$ (146,616)	$ (101,573)	$ (55,992)