Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Collaboration revenue	$ 465
Type of Revenue [Extensible List]	us-gaap:CollaborativeArrangementMember	us-gaap:CollaborativeArrangementMember
Operating expenses:
Research and development	$ 86,398	$ 49,270
General and administrative	83,919	28,849
Total operating expenses	170,317	78,119
Loss from operations	(169,852)	(78,119)
Interest income, net	6,442	3,529
Other income (expense), net	4	(8)
Net loss	$ (163,406)	$ (74,598)
Net loss per share—basic and diluted	$ (3.37)	$ (1.68)
Weighted average number of common shares outstanding—basic and diluted	48,491,834	44,325,371
Comprehensive loss:
Net loss	$ (163,406)	$ (74,598)
Other comprehensive items:
Unrealized gain (loss) on marketable securities	409	(156)
Total other comprehensive gain (loss)	409	(156)
Total comprehensive loss	$ (162,997)	$ (74,754)