Stock Option Activity Under Stock Option Plan (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Number of Shares
Beginning Balance	6,747,302
Granted	80,500	110,900
Exercised	(479,705)
Cancelled	(188,269)
Ending Balance	6,159,828		6,747,302
Vested and unvested expected to vest at end of period	6,105,689
Options exercisable, at end of period	2,843,928		3,471,935
Options available for future grant	338,631
Weighted Average Exercise Price
Beginning Balance	$ 3.81
Granted	$ 4.36
Exercised	$ 2.02
Cancelled	$ 5.10
Ending Balance	$ 3.92		$ 3.81
Vested and unvested expected to vest at end of period	$ 4.28
Options exercisable, at end of period	$ 4.28		$ 4.01
Weighted Average Contractual Term (Years)
Outstanding, at end of period	7 years 5 months 12 days
Vested and unvested expected to vest at end of period	5 years 4 months 13 days
Options exercisable, at end of period	5 years 4 months 13 days		5 years 11 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 6,459
Vested and unvested expected to vest at end of period	5,845
Options exercisable, at end of period	$ 1,810		$ 2,654