Selected Quarterly Data (Unaudited) - Quarterly Consolidated Statements of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Unaudited Quarterly Consolidated Statements of Operations
Revenue	$ 54,856	$ 48,270	$ 44,346	$ 37,657	$ 36,426	$ 30,768	$ 26,718	$ 24,002	$ 185,129	$ 117,914	$ 73,022
Cost of revenue	15,488	13,644	13,164	11,252	10,722	9,135	7,890	6,844	53,548	34,591	21,285
Gross profit	39,368	34,626	31,182	26,405	25,704	21,633	18,828	17,158	131,581	83,323	51,737
Operating expenses:
Sales and marketing	31,852	28,601	26,274	23,010	21,280	18,624	17,422	16,237	109,737	73,563	45,773
Research and development	5,893	5,716	5,232	4,419	4,261	4,101	3,431	3,093	21,260	14,886	10,149
General and administrative	9,215	8,261	7,530	8,566	7,566	6,600	5,792	5,954	33,572	25,912	15,122
Amortization of certain acquired intangible assets	251	251	251	251	251	251	237	0	1,004	739	0
Total operating expenses	47,211	42,829	39,287	36,246	33,358	29,576	26,882	25,284	165,573	115,100	71,044
Loss from operations	(7,843)	(8,203)	(8,105)	(9,841)	(7,654)	(7,943)	(8,054)	(8,126)	(33,992)	(31,777)	(19,307)
Other income (expense):
Interest income (expense) and other income (expense), net	(3,009)	(2,747)	(714)	(92)	(2)	18	(514)	96
Loss before income tax benefit (provision)	(10,852)	(10,950)	(8,819)	(9,933)	(7,656)	(7,925)	(8,568)	(8,030)	(40,554)	(32,179)	(63,719)
Income tax benefit (provision)	97	(104)	136	(1)	239	298	334	(82)	128	789	(181)
Net loss	$ (10,755)	$ (11,054)	$ (8,683)	$ (9,934)	$ (7,417)	$ (7,627)	$ (8,234)	$ (8,112)	$ (40,426)	$ (31,390)	$ (63,900)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.21)	$ (0.21)	$ (0.17)	$ (0.20)	$ (0.15)	$ (0.15)	$ (0.17)	$ (0.16)	$ (0.79)	$ (0.63)	$ (1.74)
Weighted average common shares outstanding, basic and diluted	52,185	51,544	51,153	50,798	50,486	50,163	49,763	49,384	51,427	49,929	39,824