Condensed Consolidated Statements of Operations (Unaudited) - 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	$ 247,225	$ 163,059	$ 719,536	$ 477,987
Costs and expenses:
Operations and support	111,511	65,352	310,239	187,795
Sales and marketing	49,426	35,138	144,413	105,346
Technology (exclusive of amortization)	21,258	14,292	57,306	41,560
General and administrative	22,195	18,617	58,072	46,627
Depreciation and amortization	20,987	12,613	61,787	33,067
Total costs and expenses	225,377	146,012	631,817	414,395
Income from operations	21,848	17,047	87,719	63,592
Interest (income) expense - net	337	(373)	1,367	(908)
Income before provision for income taxes	21,511	17,420	86,352	64,500
Income tax (benefit) expense	(1,234)	4,432	2,721	19,043
Net income attributable to common stockholders	$ 22,745	$ 12,988	$ 83,631	$ 45,457
Net income per share attributable to common stockholders:
Basic	$ 0.25	$ 0.15	$ 0.94	$ 0.53
Diluted	$ 0.24	$ 0.15	$ 0.91	$ 0.52
Weighted-average shares used to compute net income per share attributable to common stockholders:
Basic	90,494	86,449	89,027	86,162
Diluted	93,678	88,543	92,091	87,788