Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue:
Collaboration revenue - related party	[1]	$ 32,100	$ 21,766
Total revenue		32,100	21,766
Operating expenses:
Research and development expenses		89,455	81,989	$ 38,095
General and administrative expenses		34,040	32,616	16,761
Total operating expenses		123,495	114,605	54,856
Loss from operations		(91,395)	(92,839)	(54,856)
Other income (expense):
Interest income (expense), net		1,590	1,260	83
Other income		425
Revaluation of preferred stock warrant liability				(7)
Total other income (expense), net		2,015	1,260	76
Net loss		$ (89,380)	$ (91,579)	$ (54,780)
Net loss per share attributable to common stockholders, basic and diluted		$ (2.21)	$ (2.30)	$ (2.33)
Weighted average common shares outstanding, basic and diluted		40,449,410	39,846,928	23,532,400
Other comprehensive income (loss):
Unrealized gain (loss) on investments, net of tax of $0		$ 3	$ (179)	$ 30
Total other comprehensive income (loss)		3	(179)	30
Comprehensive loss		$ (89,377)	$ (91,758)	$ (54,750)

[1] In January 2016, the Company entered into a Collaboration and License Agreement with NHS for the development and commercialization of certain of its product candidates. In exchange for the license, NHS paid the Company an upfront cash payment of $120,000, which the Company received in February 2016 and is being recognized as revenue over the ten year performance period. During the quarter ended September 30, 2016, the Company received $10,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016. During the quarter ended September 30, 2017, the Company received $20,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2017.