CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Net revenue		$ 846,813	$ 713,069	$ 549,711
Costs and expenses:
Cost of revenue (exclusive of depreciation and amortization shown separately below)		70,518	60,363	51,015
Sales and marketing		438,643	382,854	301,764
Product development		175,787	138,549	107,786
General and administrative		105,673	97,481	80,866
Depreciation and amortization		41,198	35,346	29,604
Restructuring and integration		288	3,455	0
Gain on disposal of a business unit		(164,779)	0	0
Total costs and expenses		667,328	718,048	571,035
Income (loss) from operations		179,485	(4,979)	(21,324)
Other income, net		4,864	1,694	386
Income (loss) before income taxes		184,349	(3,285)	(20,938)
Provision for income taxes		(31,491)	(1,385)	(11,962)
Net income (loss) attributable to common stockholders(1)		$ 152,858	$ (4,670)	$ (32,900)
Net income (loss) per share attributable to common stockholders
Basic (in dollars per share)		$ 1.87	$ (0.06)	$ (0.44)
Diluted (in dollars per share)		$ 1.75	$ (0.06)	$ (0.44)
Weighted-average shares used to compute net income (loss) per share attributable to common stockholders
Basic (in shares)	[1]	81,602	77,186	74,683
Diluted (in shares)	[1]	87,170	77,186	74,683

[1] The structure of the Company’s common stock changed in the year ended December 31, 2016. Refer to Note 13 for details.