Net Loss Per Share - Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss	$ (23,112)	$ (17,782)	$ (14,185)	$ (16,815)	$ (13,569)	$ (11,136)	$ (12,437)	$ (9,174)	$ (71,893,959)	$ (46,315,451)	$ (27,313,664)
Net loss:	$ (23,112)	$ (17,782)	$ (14,185)	$ (16,815)	$ (13,569)	$ (11,136)	$ (12,437)	$ (9,174)	$ (71,893,959)	$ (46,315,451)	$ (27,313,664)
Denominator:
Weighted average common shares outstanding, basic and diluted	29,347	27,524	22,666	21,570	21,554	21,507	21,475	21,451	25,296,527	21,497,119	13,893,961
Net loss per share, basic and diluted	$ (0.79)	$ (0.65)	$ (0.63)	$ (0.78)	$ (0.63)	$ (0.52)	$ (0.58)	$ (0.43)	$ (2.84)	$ (2.15)	$ (1.97)