CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Revenue	$ 78,786	$ 64,113	$ 221,966	$ 186,012
Cost of revenue	22,494	20,504	65,207	60,578
Gross profit	56,292	43,609	156,759	125,434
Operating expenses
Research and development	16,209	14,377	47,705	40,680
Sales and marketing	25,322	18,967	70,319	55,054
General and administrative	16,519	11,707	46,309	34,102
Provision for transaction losses	1,214	1,892	2,706	4,612
Total operating expenses	59,264	46,943	167,039	134,448
Loss from operations	(2,972)	(3,334)	(10,280)	(9,014)
Interest expense	317	589	1,047	1,674
Other (income) expense, net	(462)	3,423	(1,773)	3,845
Loss before income taxes	(2,827)	(7,346)	(9,554)	(14,533)
Income tax provision	0	0	(28)	(9)
Net loss	$ (2,827)	$ (7,346)	$ (9,582)	$ (14,542)
Net loss per share, basic and diluted (in dollars per share)	$ (0.03)	$ (0.20)	$ (0.09)	$ (0.41)
Weighted-average shares used to compute net loss per share, basic and diluted (in shares)	111,163,396	36,069,502	108,844,495	35,129,250