CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - 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	$ 64,113	$ 52,262	$ 186,012	$ 147,793
Cost of revenue	20,504	16,894	60,578	47,847
Gross profit	43,609	35,368	125,434	99,946
Operating expenses:
Research and development	14,377	11,514	40,680	32,519
Sales and marketing	18,967	13,626	55,054	37,327
General and administrative	11,707	8,952	34,102	25,415
Provision for transaction losses	1,892	1,073	4,612	2,857
Total operating expenses	46,943	35,165	134,448	98,118
Income (loss) from operations	(3,334)	203	(9,014)	1,828
Interest expense	589	199	1,674	629
Other expense, net	3,423	260	3,845	75
Income (loss) before income taxes	(7,346)	(256)	(14,533)	1,124
Income tax provision		(45)	(9)	(56)
Net income (loss)	(7,346)	(301)	(14,542)	1,068
Undistributed earnings allocable to preferred stockholders				$ (1,068)
Net loss attributable to common stockholders	$ (7,346)	$ (301)	$ (14,542)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.20)	$ (0.01)	$ (0.41)
Weighted-average shares used to compute net loss per share attributable to common stockholders, basic and diluted	36,069,502	33,298,773	35,129,250	32,760,339