Selected Quarterly Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Unaudited Quarterly Consolidated Statements of Operations
Revenue	$ 109,029	$ 107,758	$ 107,013	$ 99,324	$ 95,862	$ 87,271	$ 82,563	$ 73,955	$ 423,124	$ 339,651	$ 263,568
Cost of revenue	34,778	33,369	35,955	31,650	30,063	27,246	27,893	24,662	135,752	109,864	77,684
Gross profit	74,251	74,389	71,058	67,674	65,799	60,025	54,670	49,293	287,372	229,787	185,884
Operating expenses:
Sales and marketing	57,405	53,690	57,835	56,701	56,367	53,255	51,446	45,958	225,631	207,026	162,552
Research and development	12,050	12,130	11,782	11,015	10,784	10,457	9,983	9,767	46,977	40,991	30,618
General and administrative	19,345	18,608	16,538	16,465	13,332	13,194	12,260	11,091	70,956	49,877	41,802
Amortization of certain acquired intangible assets	0	0	0	150	150	150	150	150	150	600	828
Total operating expenses	88,800	84,428	86,155	84,331	80,633	77,056	73,839	66,966	343,714	298,494	235,800
Loss from operations	(14,549)	(10,039)	(15,097)	(16,657)	(14,834)	(17,031)	(19,169)	(17,673)	(56,342)	(68,707)	(49,916)
Other income (expense):
Interest income (expense) and other income (expense), net	(3,756)	(2,131)	(2,350)	(1,051)	(3,618)	(2,986)	(3,713)	(5,311)	(9,288)	(15,628)	(14,128)
Loss before income tax provision	(18,305)	(12,170)	(17,447)	(17,708)	(18,452)	(20,017)	(22,882)	(22,984)	(65,630)	(84,335)	(64,044)
Income tax provision	(313)	(218)	(141)	(535)	(391)	(132)	(380)	(278)	(1,207)	(1,181)	(855)
Net loss	$ (18,618)	$ (12,388)	$ (17,588)	$ (18,243)	$ (18,843)	$ (20,149)	$ (23,262)	$ (23,262)	$ (66,837)	$ (85,516)	$ (64,899)
Net loss per share — basic and diluted (usd per share)	$ (0.33)	$ (0.22)	$ (0.32)	$ (0.33)	$ (0.35)	$ (0.37)	$ (0.43)	$ (0.43)	$ (1.20)	$ (1.58)	$ (1.22)
Weighted average common shares outstanding, basic and diluted (in shares)	56,300	55,964	55,278	54,827	54,551	54,260	53,987	53,876	55,595	54,171	53,267