CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenue	$ 159,924	$ 112,333	$ 280,911	$ 198,001
Type of Revenue [Extensible List]	us-gaap:ServiceMember	us-gaap:ServiceMember	us-gaap:ServiceMember	us-gaap:ServiceMember
Operating expenses:
Platform operations	$ 35,330	$ 26,601	$ 68,981	$ 49,498
Type of Cost, Good or Service [Extensible List]	us-gaap:ServiceMember	us-gaap:ServiceMember	us-gaap:ServiceMember	us-gaap:ServiceMember
Sales and marketing	$ 31,072	$ 20,690	$ 53,809	$ 36,720
Technology and development	29,452	19,484	54,764	37,185
General and administrative	32,121	19,396	65,738	38,506
Total operating expenses	127,975	86,171	243,292	161,909
Income from operations	31,949	26,162	37,619	36,092
Other expense (income):
Interest expense (income), net	(1,286)	(32)	(2,283)	124
Foreign currency exchange loss (gain), net	(134)	1,096	1,196	1,640
Total other expense (income), net	(1,420)	1,064	(1,087)	1,764
Income before income taxes	33,369	25,098	38,706	34,328
Provision for income taxes	5,569	5,755	755	5,915
Net income	$ 27,800	$ 19,343	$ 37,951	$ 28,413
Earnings per share:
Basic	$ 0.63	$ 0.46	$ 0.86	$ 0.68
Diluted	$ 0.58	$ 0.43	$ 0.80	$ 0.63
Weighted average shares outstanding:
Basic	44,404	42,174	44,157	41,903
Diluted	47,828	45,242	47,573	44,895