Quarterly and Twelve-Month Statements of Comprehensive Income (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended					12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Mar. 31, 2013
Schedule Of Quarterly Financial Information [Line Items]
Revenue	$ 37,313	$ 33,616	$ 32,454	$ 29,951	$ 29,970	$ 133,334
Cost of revenue	14,034	12,611	12,760	12,239	12,243	51,644
Gross profit	23,279	21,005	19,694	17,712	17,727	81,690
Operating expenses:
Marketing and sales	5,263	4,658	4,442	4,557	4,441	18,920
Research and development	2,628	2,515	2,561	2,401	1,660	10,105
General and administrative	3,994	3,564	3,118	3,288	3,988	13,964
Total operating expenses	11,885	10,737	10,121	10,246	10,089	42,989
Income from operations	11,394	10,268	9,573	7,466	7,638	38,701
Other income (expense), net	3	114	314	173	(577)	604
Income before income taxes	11,397	10,382	9,887	7,639	7,061	39,305
Provision for income taxes	3,110	2,987	3,185	2,493	2,279	11,775
Net income	$ 8,287	$ 7,395	$ 6,702	$ 5,146	$ 4,782	$ 27,530
Net income per share:
Basic	$ 0.33	$ 0.30	$ 0.28	$ 0.22	$ 0.23	$ 1.13
Diluted	$ 0.32	$ 0.29	$ 0.26	$ 0.20	$ 0.22	$ 1.08