Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue:
Collaboration revenue - related party	[1]	$ 21,766
Total revenue		21,766
Operating expenses:
Research and development expenses		81,989	$ 38,095	$ 10,718
General and administrative expenses		32,616	16,761	4,364
Total operating expenses		114,605	54,856	15,082
Loss from operations		(92,839)	(54,856)	(15,082)
Other income (expense):
Interest income		2,229	638
Interest expense		(969)	(555)	(209)
Revaluation of preferred stock warrant liability			(7)	(1,418)
Total other income (expense), net		1,260	76	(1,627)
Net loss		(91,579)	(54,780)	(16,709)
Accretion of convertible preferred stock to redemption value				(1,291)
Net loss attributable to common stockholders		$ (91,579)	$ (54,780)	$ (18,000)
Net loss per share attributable to common stockholders, basic and diluted		$ (2.30)	$ (2.33)	$ (2.67)
Weighted average common shares outstanding, basic and diluted		39,846,928	23,532,400	6,748,037
Other comprehensive income (loss):
Unrealized gain (loss) on investments, net of tax of $0		$ (179)	$ 30
Total other comprehensive income (loss)		(179)	30
Comprehensive loss		$ (91,758)	$ (54,750)	$ (18,000)

[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 million, 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 million from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016.