Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Outstanding as of January 1, 2016	17,818,000
Granted	4,400,000
Forfeited	(688,000)
Expired	(165,000)
Exercised	(177,146)	(18,020)
Outstanding as of December 31, 2016	21,188,014	17,818,000
Exercisable as of December 31, 2016	12,217,000
Vested and expected to vest as of December 31, 2016	20,513,000
Available for future grant as of December 31, 2016	4,304,000
Weighted Average Exercise Price
Outstanding as of January 1, 2016	$ 3.76
Granted	1.62
Forfeited	1.92
Expired	7.24
Exercised	1.62
Outstanding as of December 31, 2016	3.37	$ 3.76
Exercisable as of December 31, 2016	4.38
Vested and expected to vest as of December 31, 2016	$ 3.41
Weighted Average Remaining Contractual Term
Outstanding as of December 31, 2016	7 years 3 months 18 days
Exercisable as of December 31, 2016	6 years 3 months 18 days
Vested and expected to vest as of December 31, 2016	7 years 3 months 18 days
Aggregate Intrinsic Value
Outstanding as of December 31, 2016	$ 17,018
Exercisable as of December 31, 2016	7,159
Vested and expected to vest as of December 31, 2016	$ 16,396