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, 2013	Mar. 31, 2012	Dec. 31, 2012
Number of Shares
Beginning Balance	7,147,303
Granted	110,900	63,500
Exercised	(453,168)
Cancelled	(61,200)
Ending Balance	6,743,835		7,147,303
Vested and unvested expected to vest at end of period	6,693,959
Options exercisable, at end of period	3,408,985		3,683,786
Options available for future grant	1,624,587
Weighted Average Exercise Price
Beginning Balance	$ 4.11
Granted	$ 4.72
Exercised	$ 1.88
Cancelled	$ 4.88
Ending Balance	$ 4.26		$ 4.11
Vested and unvested expected to vest at end of period	$ 3.85
Options exercisable, at end of period	$ 3.85		$ 3.56
Weighted Average Contractual Term (Years)
Outstanding, at end of period	7 years 2 months 23 days
Vested and unvested expected to vest at end of period	5 years 3 months 11 days
Options exercisable, at end of period	5 years 3 months 11 days		5 years 3 months 11 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 801
Vested and unvested expected to vest at end of period	795
Options exercisable, at end of period	$ 801		$ 3,972