Consolidated Statements of Comprehensive Loss - 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]
Revenues	$ 93,586	$ 82,440	$ 262,497	$ 240,016
Costs and operating expenses:
Cost of revenue (exclusive of depreciation and amortization presented separately below)	7,737	7,088	22,941	20,610
Sales and marketing (includes related party expenses of $6,514 and $4,207 for the three months ended September 30, 2018 and 2017, and $16,332 and $12,750 for the nine months ended September 30, 2018 and 2017, respectively)	57,031	48,383	157,463	137,498
Technology and development	15,345	15,357	46,633	43,117
General and administrative	14,030	14,993	41,005	44,034
Depreciation and amortization	5,992	5,765	16,808	17,517
Total costs and operating expenses	100,135	91,586	284,850	262,776
Loss from operations	(6,549)	(9,146)	(22,353)	(22,760)
Interest income	888	402	2,242	784
Interest expense	(662)	(654)	(1,985)	(1,955)
Loss before income taxes	(6,323)	(9,398)	(22,096)	(23,931)
(Benefit from) / provision for income taxes	(72)	121	(168)	443
Net loss	$ (6,251)	$ (9,519)	$ (21,928)	$ (24,374)
Net loss per share attributable to common stockholders:
Net loss per share, basic and diluted (in dollars per share)	$ (0.06)	$ (0.10)	$ (0.22)	$ (0.26)
Weighted average common shares outstanding, basic and diluted (in shares)	102,765	98,665	101,503	93,108
Other comprehensive loss:
Comprehensive loss	$ (6,251)	$ (9,519)	$ (21,928)	$ (24,374)