Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 177,984	$ 1,535
Operating expenses:
Research and development	230,893	115,785	$ 43,016
General and administrative	27,008	22,210	18,497
Total operating expenses	257,901	137,995	61,513
Operating loss	(79,917)	(136,460)	(61,513)
Other income (expense):
Interest income	2,799	901	510
Other income/(expense)	204	(188)	287
Net loss	$ (76,914)	$ (135,747)	$ (60,716)
Net loss per share - basic and diluted	$ (1.77)	$ (3.60)	$ (2.29)
Weighted-average number of common shares - basic and diluted	43,500,795	37,716,949	26,469,170
Comprehensive loss:
Net loss	$ (76,914)	$ (135,747)	$ (60,716)
Other comprehensive loss - unrealized loss on securities	(400)	(42)	(234)
Total comprehensive loss	$ (77,314)	$ (135,789)	$ (60,950)