Quarterly Financial Data (Unaudited) - Schedule of Quarterly Financial Data (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]                      
Revenue             $ 33 $ 47   $ 80 $ 6
Operating expenses:                      
Research and development $ 7,157 $ 7,092 $ 6,678 $ 5,021 $ 4,701 $ 2,928 2,648 1,495 $ 25,948 11,772 9,700
General and administrative 2,872 2,954 2,717 2,681 2,607 2,441 2,029 1,510 11,224 8,587 6,166
Total operating expenses 10,029 10,046 9,395 7,702 7,308 5,369 4,677 3,005 37,172 20,359 15,866
Other income (expense):                      
Interest income 2 4 1 3 4 2 2 1 10 9 2
Interest expense (689) (509) (513) (721) (163) (191) (268) (461) (2,432) (1,083) (1,487)
Loss on extinguishment of debt             (2,493)     (2,493)  
Decrease in value of preferred stock warrant liability               504   504 202
Total other income (expense), net (687) (505) (512) (718) (159) (189) (2,759) 44 (2,422) (3,063) (1,283)
Net loss attributable to common stockholders $ (10,716) $ (10,551) $ (9,907) $ (8,420) $ (7,467) $ (5,558) $ (7,403) $ (2,914) $ (39,594) $ (23,342) $ (17,143)
Net loss per share attributable to common stockholders:                      
Basic and diluted $ (0.39) $ (0.39) $ (0.37) $ (0.41) $ (0.37) $ (0.28) $ (0.44) $ (3.70) $ (1.56) $ (1.60) $ (25.05)
Weighted-average common shares outstanding:                      
Basic and diluted 27,346,780 27,307,103 26,690,673 20,350,557 20,125,009 20,124,574 16,883,716 786,986 25,431,332 14,548,516 684,330