Stock Option Activity (Detail)	12 Months Ended
	Dec. 31, 2016 $ / shares shares
Options
Outstanding at beginning of period | shares	4,796,005
Granted | shares	1,781,134
Exercised | shares	(38,749)
Forfeited | shares	(1,679,712)
Outstanding at end of period | shares	4,858,678
Vested or expected to vest at December 31, 2016 | shares	3,043,129
Exercisable at December 31, 2016 | shares	2,327,806
Weighted-Average Exercise Price
Outstanding at beginning of period | $ / shares	$ 3.78
Granted | $ / shares	1.01
Exercised | $ / shares	0.84
Forfeited | $ / shares	5.17
Outstanding at end of period | $ / shares	2.31
Vested or expected to vest at December 31, 2016 | $ / shares	3.04
Exercisable at December 31, 2016 | $ / shares	$ 3.63
Weighted-Average Remaining Contractual Term
Outstanding at December 31, 2016	7 years 11 months 23 days
Vested or expected to vest at December 31, 2016	7 years 7 months 17 days
Exercisable at December 31, 2016	7 years 4 months 2 days