Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 97,049	$ 91,852	$ 85,384	$ 80,301	$ 75,221	$ 69,440	$ 63,592	$ 59,107	$ 354,586	$ 267,360	$ 187,727
Cost of revenue	18,429	15,245	13,918	13,226	12,399	12,161	9,879	8,728	60,818	43,167	25,588
Gross profit	78,620	76,607	71,466	67,075	62,822	57,279	53,713	50,379	293,768	224,193	162,139
Operating expenses:
Sales and marketing	62,632	56,699	56,015	52,689	47,380	44,550	41,826	39,588	228,035	173,344	116,299
Research and development	22,668	20,763	21,698	21,935	21,169	20,553	17,791	17,185	87,064	76,698	57,673
General and administrative	20,873	17,472	15,987	15,136	13,864	13,272	10,541	9,055	69,468	46,732	28,927
Total operating expenses	106,173	94,934	93,700	89,760	82,413	78,375	70,158	65,828	384,567	296,774	202,899
Loss from operations	(27,553)	(18,327)	(22,234)	(22,685)	(19,591)	(21,096)	(16,445)	(15,449)	(90,799)	(72,581)	(40,760)
Interest income (expense), net	1,153	1,527	1,594	1,556	1,510	894	(23)	(26)	5,830	2,355	(75)
Other expense, net	(104)	(240)	(122)	(214)	(326)	(185)	(438)	18	(680)	(931)	(16)
Total loss before income taxes	(26,504)	(17,040)	(20,762)	(21,343)	(18,407)	(20,387)	(16,906)	(15,457)	(85,649)	(71,157)	(40,851)
Provision for income taxes	11,801	600	866	97	1,207	482	244	431	13,364	2,364	171
Net loss	(38,305)	(17,640)	(21,628)	(21,440)	(19,614)	(20,869)	(17,150)	(15,888)	(99,013)	(73,521)	(41,022)
Accretion of Series A and B redeemable convertible preferred stock	0	0	0	0	0	(55)	(191)	(188)	0	(434)	(763)
Net loss attributable to common stockholders	$ (38,305)	$ (17,640)	$ (21,628)	$ (21,440)	$ (19,614)	$ (20,924)	$ (17,341)	$ (16,076)	$ (99,013)	$ (73,955)	$ (41,785)
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)	$ (0.39)	$ (0.18)	$ (0.23)	$ (0.23)	$ (0.21)	$ (0.28)	$ (0.73)	$ (0.68)	$ (1.03)	$ (1.38)	$ (1.88)