Stock-Based Compensation - Summary of Stock Option Activity under Plans and Related Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Number of stock options outstanding
Beginning Balance (in shares)		11,900,671
Granted (in shares)		4,286,008
Exercised (in shares)		(380,145)
Forfeited, cancelled or expired (in shares)		(3,099,497)
Ending Balance (in shares)		12,707,037	11,900,671
Options vested and exercisable (in shares)		7,553,273
Options vested and expected to vest (in shares)		11,543,799
Weighted-Average Exercise Price
Beginning Balance (in dollars per share)		$ 4.20
Granted (in dollars per share)		2.15
Exercised (in dollars per share)		1.83
Forfeited, cancelled or expired (in dollars per share)		4.26
Ending Balance (in dollars per share)		3.56	$ 4.20
Options vested and exercisable, weighted average exercise price (in dollars per share)		4.34
Options vested and expected to vest at end of period (in dollars per share)		$ 3.68
Weighted-Average Remaining Contractual Term
Balance			7 years 8 months 12 days
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Outstanding Options, Weighted Average Remaining Contractual Term		7 years 5 months 12 days
Options vested and exercisable, weighted average remaining contractual term		6 years 3 months 12 days
Options vested and expected to vest, weighted average remaining contractual term		7 years 2 months 12 days
Aggregate Intrinsic Value
Balance	[1]	$ 137	$ 999
Options vested and exercisable at end of period	[1]	137
Options vested and expected to vest at end of period	[1]	$ 137

[1] The aggregate intrinsic value represents the value by which our closing stock price on the last trading day of the year ended December 31, 2016 exceeds the exercise price of the stock multiplied by the number of options outstanding or exercisable, excluding any that have a negative intrinsic value.