Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenue	$ 486,920	$ 370,036	$ 267,196
Cost of revenue	367,496	258,216	184,452
Gross profit	119,424	111,820	82,744
Operating expenses
Technology and development	53,797	42,532	34,588
Marketing	44,061	32,251	28,571
General and administrative	65,500	53,009	42,369
Total operating expenses	163,358	127,792	105,528
Loss from operations	(43,934)	(15,972)	(22,784)
Interest income	5,416	882	173
Interest expense	(3,681)	0	0
Other income, net	221	88	85
Net loss	(41,978)	(15,002)	(22,526)
Accretion of redeemable convertible preferred stock	0	(175,915)	(55,502)
Net loss attributable to common stock—basic	(41,978)	(190,917)	(78,028)
Net loss attributable to common stock—diluted	$ (41,978)	$ (190,917)	$ (78,028)
Net income loss per share attributable to common stock—basic and diluted (in dollars per share)	$ (0.49)	$ (4.47)	$ (5.42)
Weighted average shares used to compute net loss per share attributable to common stock—basic and diluted (in shares)	85,669,039	42,722,114	14,395,067