CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Revenue	$ 118,825	$ 79,413	$ 316,826	$ 205,569
Type of Revenue [Extensible List]	us-gaap:ServiceMember	us-gaap:ServiceMember	us-gaap:ServiceMember	us-gaap:ServiceMember
Operating expenses:
Platform operations	$ 29,344	$ 17,397	$ 78,842	$ 45,097
Type of Cost, Good or Service [Extensible List]	us-gaap:ServiceMember	us-gaap:ServiceMember	us-gaap:ServiceMember	us-gaap:ServiceMember
Sales and marketing	$ 23,287	$ 16,200	$ 60,007	$ 42,842
Technology and development	22,621	13,181	59,806	35,777
General and administrative	21,310	14,227	59,816	41,815
Total operating expenses	96,562	61,005	258,471	165,531
Income from operations	22,263	18,408	58,355	40,038
Other expense (income):
Interest expense (income), net	(237)	513	(113)	1,290
Foreign currency exchange loss, net	395	1,841	2,035	3,159
Total other expense, net	158	2,354	1,922	4,449
Income before income taxes	22,105	16,054	56,433	35,589
Provision for income taxes	1,813	5,825	7,728	1,602
Net income	$ 20,292	$ 10,229	$ 48,705	$ 33,987
Earnings per share:
Basic	$ 0.47	$ 0.25	$ 1.15	$ 0.85
Diluted	$ 0.44	$ 0.23	$ 1.07	$ 0.77
Weighted average shares outstanding:
Basic	42,721	40,700	42,178	39,977
Diluted	46,576	44,245	45,460	43,919