Revenue, Deferred Revenue and Deferred Costs and Other - Impact of Adoption of Topic 606 (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2016
Assets:
Deferred costs and other, current	$ 16,051	$ 6,022
Total current assets	487,507	145,442
Deferred costs and other, noncurrent	5,700	2,447
Total assets	560,561	180,253
Liabilities, redeemable convertible preferred stock and stockholders' deficit:
Accrued expenses and other current liabilities	27,801	16,152
Deferred revenue, current	3,945	1,614
Total current liabilities	41,875	24,436
Accumulated deficit	53,197	35,130
Accumulated deficit	(164,198)	(113,613)
Total stockholders' equity	507,364	(91,806)	$ (66,408)		$ (52,907)
Total liabilities, redeemable convertible preferred stock and stockholders' equity	560,561	180,253
Income Statement [Abstract]
Revenue	170,198	68,431	30,850
Gross profit	124,040	48,162	22,538
Sales and marketing	78,060	36,433	16,502
Total operating expenses	184,421	83,157	39,580
Loss from operations	(60,381)	(34,995)	(17,042)
Loss before provision for income taxes	(56,639)	(33,354)	(16,919)
Net loss	(55,270)	(33,382)	(16,858)
Net loss attributable to common stockholders	$ (55,366)	$ (33,544)	$ (17,001)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (1.09)	$ (2.02)	$ (1.18)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	50,930	16,573	14,442
ASC 605
Assets:
Deferred costs and other, current	$ 14,745
Total current assets	486,201
Deferred costs and other, noncurrent	3,833
Total assets	557,388
Liabilities, redeemable convertible preferred stock and stockholders' deficit:
Accrued expenses and other current liabilities	28,812
Deferred revenue, current	4,087
Total current liabilities	43,028
Accumulated deficit	54,350
Accumulated deficit	(168,524)
Total stockholders' equity	503,038
Total liabilities, redeemable convertible preferred stock and stockholders' equity	557,388
Income Statement [Abstract]
Revenue	169,853
Gross profit	123,695
Sales and marketing	77,357
Total operating expenses	183,718
Loss from operations	(60,023)
Loss before provision for income taxes	(56,281)
Net loss	(54,912)
Net loss attributable to common stockholders	$ (55,008)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (1.08)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (in shares)	50,930
Impact of Adoption | ASC 606
Assets:
Deferred costs and other, current	$ 1,306			$ 3,700
Total current assets	1,306
Deferred costs and other, noncurrent	1,867			800
Total assets	3,173
Liabilities, redeemable convertible preferred stock and stockholders' deficit:
Accrued expenses and other current liabilities	(1,011)			$ (200)
Deferred revenue, current	(142)
Total current liabilities	(1,153)
Accumulated deficit	(1,153)
Accumulated deficit	4,326
Total stockholders' equity	4,326
Total liabilities, redeemable convertible preferred stock and stockholders' equity	3,173
Income Statement [Abstract]
Revenue	345
Gross profit	345
Sales and marketing	703
Total operating expenses	703
Loss from operations	(358)
Loss before provision for income taxes	(358)
Net loss	(358)
Net loss attributable to common stockholders	$ (358)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.01)