Consolidated Statements of Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenues	$ 323,149	$ 277,507	$ 259,838
Costs and operating expenses:
Cost of revenue (exclusive of depreciation and amortization presented separately below)	28,227	25,167	23,657
Sales and marketing (includes related party expenses of $16,531, $13,910, and $20,852, for the years ended December 31, 2017, 2016, and 2015, respectively)	185,397	154,406	151,002
Technology and development	59,070	53,580	48,021
General and administrative	61,646	59,908	83,494
Depreciation and amortization	22,472	23,345	17,646
Total costs and operating expenses	356,812	316,406	323,820
Loss from operations	(33,663)	(38,899)	(63,982)
Interest income	1,260	376	107
Interest expense	(2,610)	(2,530)	(443)
Other income	0	0	13
Loss before income taxes	(35,013)	(41,053)	(64,305)
(Benefit from) / provision for income taxes	(2,164)	655	606
Net loss	$ (32,849)	$ (41,708)	$ (64,911)
Net loss per share, basic and diluted (in dollars per share)	$ (0.35)	$ (0.49)	$ (0.79)
Weighted average common shares outstanding, basic and diluted	94,865	84,483	81,914
Other comprehensive loss:
Comprehensive loss	$ (32,849)	$ (41,708)	$ (64,911)