Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenue	$ 370,036	$ 267,196	$ 187,338
Cost of revenue	258,216	184,452	138,492
Gross profit	111,820	82,744	48,846
Operating expenses:
Technology and development	42,532	34,588	27,842
Marketing	32,251	28,571	19,899
General and administrative	53,009	42,369	31,394
Total operating expenses	127,792	105,528	79,135
Income (loss) from operations	(15,972)	(22,784)	(30,289)
Interest income and other income, net:
Interest income	882	173	46
Other income, net	88	85	7
Total interest income and other income, net	970	258	53
Net income (loss)	(15,002)	(22,526)	(30,236)
Accretion of redeemable convertible preferred stock	(175,915)	(55,502)	(102,224)
Net income (loss) attributable to common stock—basic	(190,917)	(78,028)	(132,460)
Net income (loss) attributable to common stock—diluted	$ (190,917)	$ (78,028)	$ (132,460)
Net income (loss) per share attributable to common stock—basic and diluted (in dollars per share)	$ (4.47)	$ (5.42)	$ (9.87)
Weighted average shares used to compute net income (loss) per share attributable to common stock—basic and diluted (in shares)	42,722,114	14,395,067	13,416,411