Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Number of Shares
Outstanding as of January 1, 2017	21,188,000
Granted	3,827,000
Forfeited	(219,000)
Expired	(331,000)
Exercised	(356,656)	(177,146)
Outstanding as of December 31, 2017	24,108,455	21,188,000
Exercisable as of December 31, 2017	16,115,000
Vested and expected to vest as of December 31, 2017	23,709,000
Available for future grant as of December 31, 2017	16,994,000
Weighted Average Exercise Price
Outstanding as of January 1, 2017	$ 3.37
Granted	3.09
Forfeited	2.52
Expired	10.17
Exercised	1.79
Outstanding as of December 31, 2017	3.26	$ 3.37
Exercisable as of December 31, 2017	3.68
Vested and expected to vest as of December 31, 2017	$ 3.27
Weighted Average Remaining Contractual Term
Outstanding as of December 31, 2017	6 years 9 months 18 days
Exercisable as of December 31, 2017	6 years
Vested and expected to vest as of December 31, 2017	6 years 8 months 12 days
Aggregate Intrinsic Value
Outstanding as of December 31, 2017	$ 34,659
Exercisable as of December 31, 2017	21,976
Vested and expected to vest as of December 31, 2017	$ 34,025