Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenue	$ 140,255	$ 109,479	$ 362,791	$ 274,282
Cost of revenue	97,950	70,166	269,576	191,633
Gross profit	42,305	39,313	93,215	82,649
Operating expenses:
Technology and development	14,310	11,483	40,105	31,245
Marketing	8,236	5,588	36,006	26,179
General and administrative	16,470	11,995	48,532	38,828
Total operating expenses	39,016	29,066	124,643	96,252
Income (loss) from operations	3,289	10,247	(31,428)	(13,603)
Interest income	1,775	311	3,082	387
Interest expense	(1,610)	0	(1,610)	0
Other income, net	21	0	200	13
Net income (loss)	3,475	10,558	(29,756)	(13,203)
Accretion of redeemable convertible preferred stock	0	(40,224)	0	(175,915)
Net income (loss) attributable to common stock—basic	3,475	(29,666)	(29,756)	(189,118)
Net income (loss) attributable to common stock—diluted	$ 3,475	$ (29,666)	$ (29,756)	$ (189,118)
Net income (loss) attributable to common stock—basic (in dollars per share)	$ 0.04	$ (0.50)	$ (0.35)	$ (6.37)
Net income (loss) attributable to common stock—diluted (in dollars per share)	$ 0.04	$ (0.50)	$ (0.35)	$ (6.37)
Weighted average shares—basic (in shares)	87,743,223	58,868,903	84,327,266	29,678,082
Weighted average shares—diluted (in shares)	94,642,463	58,868,903	84,327,266	29,678,082