Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
OPERATING EXPENSES:
Research and development	$ 107,406	$ 68,280	$ 45,719
General and administrative	77,542	30,797	13,674
Loss from operations	(184,948)	(99,077)	(59,393)
OTHER (EXPENSE) INCOME:
Other (expense), net	(293)	(35)	(510)
Loss on retirement of note payable	0	(1,572)	(203)
Interest income	2,437	1,043	94
Interest expense	0	(1,885)	(2,467)
NET LOSS	(182,804)	(101,526)	(62,479)
OTHER COMPREHENSIVE LOSS, NET OF TAX:
Unrealized gain (loss) from available-for-sale securities	66	26	(21)
COMPREHENSIVE LOSS	(182,738)	(101,500)	(62,500)
LOSS ATTRIBUTABLE TO COMMON STOCKHOLDERS—BASIC AND DILUTED (Note 12)	$ (182,804)	$ (101,526)	$ (71,479)
LOSS PER SHARE:
Basic and diluted (in dollars per share)	$ (4.24)	$ (2.56)	$ (4.04)
WEIGHTED AVERAGE SHARES:
Basic and diluted (in shares)	43,067,952	39,643,099	17,699,487