Consolidated Statements of Operations - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Income Statement [Abstract]      
Revenues $ 184    
Operating expenses:      
Research and development 43,971 $ 29,453 $ 11,558
General and administrative 19,240 15,648 5,820
Total operating expenses 63,211 45,101 17,378
Loss from operations (63,027) (45,101) (17,378)
Other income (expense):      
Preferred stock warrant liability re-measurement   (2,559) 126
Loss on extinguishment of debt   (143) (318)
Interest income 188 63 4
Interest expense   (199) (952)
Total other income (expense) 188 (2,838) (1,140)
Net loss (62,839) (47,939) (18,518)
Less: Accretion and dividends on redeemable convertible preferred stock   204 2,388
Net loss attributable to common stockholders $ (62,839) $ (48,143) $ (20,906)
Net loss per share attributable to common stockholders-basic and diluted $ (3.09) $ (3.00) $ (709.57)
Weighted average shares outstanding-basic and diluted 20,320,628 16,070,054 29,463