DESCRIPTION OF BUSINESS AND BASIS FOR PRESENTATION (Revenue Recognition) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017		Dec. 31, 2017
Condensed Consolidated Statement of Operations
Net revenue	$ 241,096	$ 223,287	[1]	$ 699,033	$ 631,406	[1]
Costs and Expenses:
Sales and marketing	121,759	112,958	[1]	362,054	326,409	[1]
General and administrative	30,302	27,601	[1]	90,892	81,808	[1]
Net income attributable to common stockholders	$ 14,986	$ 8,030	[1]	$ 23,404	$ 11,874	[1],[2]
Basic net income per share (in USD per share)	$ 0.18	$ 0.10	[1]	$ 0.28	$ 0.15	[1]
Diluted net income per share (in USD per share)	$ 0.17	$ 0.09	[1]	$ 0.26	$ 0.14	[1]
Condensed Consolidated Balance Sheet
Allowance for doubtful accounts	$ 8,885			$ 8,885			$ 8,602	[3]
Other non-current assets	42,773			42,773			40,428	[3]
Retained earnings	$ 30,520			30,520			79,170	[3]
Condensed Consolidated Statement of Cash Flows
Bad debt expense				19,285	$ 15,239	[2]
Change in accounts receivable				(24,956)	(19,810)	[2]
Other adjustments to reconcile net income to net cash provided by operating activities				$ (2,085)	(2,077)	[2]
As Previously Reported
Condensed Consolidated Statement of Operations
Net revenue		$ 222,380			628,567
Costs and Expenses:
Sales and marketing		113,041			327,559
General and administrative		26,694			78,969
Net income attributable to common stockholders		7,947			$ 10,724
Basic net income per share (in USD per share)					$ 0.13
Diluted net income per share (in USD per share)					$ 0.12
Condensed Consolidated Balance Sheet
Allowance for doubtful accounts							7,352
Other non-current assets							31,339
Retained earnings							70,081
Condensed Consolidated Statement of Cash Flows
Bad debt expense					$ 12,400
Change in accounts receivable					(16,971)
Other adjustments to reconcile net income to net cash provided by operating activities					(927)
Accounting Standards Update 2014-09 | Impact of ASC 606 Adoption
Condensed Consolidated Statement of Operations
Net revenue		907			2,839
Costs and Expenses:
Sales and marketing		(83)			(1,150)
General and administrative		907			2,839
Net income attributable to common stockholders		$ 83			$ 1,150
Basic net income per share (in USD per share)					$ 0.02
Diluted net income per share (in USD per share)					$ 0.02
Condensed Consolidated Balance Sheet
Allowance for doubtful accounts							1,250
Other non-current assets							9,089
Retained earnings							$ 9,089
Condensed Consolidated Statement of Cash Flows
Bad debt expense					$ 2,839
Change in accounts receivable					(2,839)
Other adjustments to reconcile net income to net cash provided by operating activities					$ (1,150)

[1] As of January 1, 2018, the Company adopted ASC 606 using the full retrospective method. Accordingly, the Company has recast certain amounts in the prior periods presented. See Note 1 below for additional discussion.

[2] As of January 1, 2018, the Company adopted ASC 606 using the full retrospective method. Accordingly, the Company has recast certain amounts in the prior period presented. Also as of January 1, 2018, the Company adopted Accounting Standards Update No. 2016-18, "Statement of Cash Flows (Subtopic 230): Restricted Cash," and recast the prior period presented. See Note 1 below for additional discussion.

[3] As of January 1, 2018, the Company adopted Accounting Standards Update 2014-09, "Revenue from Contracts with Customers (Topic 606)" ("ASC 606"), using the full retrospective method. Accordingly, the Company has recast certain amounts in the prior period presented. See Note 1 below for additional discussion.