Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
General and administrative					$ 69,976	$ 24,523	$ 11,080
Research and development expenses					150,391	58,892	27,493
Total operating expenses					220,367	83,415	38,573
Loss from operations	$ (25,547)	$ (21,180)	$ (15,798)	$ (20,889)	(220,367)	(83,415)	(38,573)
Other income	172	99	79	53			84
Interest income (expense)					2,316	403	15
Loss before income taxes					(218,051)	(83,012)	(38,474)
Net loss	$ (25,375)	$ (21,082)	$ (15,719)	$ (20,836)	$ (218,051)	$ (83,012)	$ (38,474)
Basic and diluted net loss per common share (in dollars per share)	$ (0.92)	$ (0.87)	$ (0.68)	$ (1.24)	$ (7.28)	$ (3.67)	$ (5.43)
Weighted-average basic and diluted common shares outstanding (in shares)	27,678	24,166	23,014	16,775	29,935	22,648	7,088