DESCRIPTION OF BUSINESS AND BASIS FOR PRESENTATION (Revenue Recognition) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017		Dec. 31, 2017
Income Statement
Net revenue	$ 223,074	$ 198,174	[1]
Costs and Expenses:
Sales and marketing	119,641	108,532	[1]
General and administrative	32,007	27,166	[1]
Net loss attributable to common stockholders	$ (2,286)	$ (4,026)	[1],[2],[3]
Basic earnings per share (in USD per share)	$ (0.03)	$ (0.05)	[1]
Diluted earnings per share (in USD per share)	$ (0.03)	$ (0.05)	[1]
Balance Sheet
Allowance for doubtful accounts	$ 10,135			$ 8,602
Other non-current assets	41,357			40,428	[4]
Retained earnings	54,830			79,170	[4]
Statement of Cash Flows
Provision for doubtful accounts and sales returns	8,143	$ 5,901	[3]
Change in accounts receivable	(6,995)	(4,458)	[3]
Change in prepaid expenses and other assets	$ (5,074)	(1,653)	[3]
As Previously Reported
Income Statement
Net revenue		197,323
Costs and Expenses:
Sales and marketing		109,286
General and administrative		26,315
Net loss attributable to common stockholders		$ (4,780)
Basic earnings per share (in USD per share)		$ (0.06)
Diluted earnings per share (in USD per share)		$ (0.06)
Balance Sheet
Allowance for doubtful accounts				7,352
Other non-current assets				31,339
Retained earnings				70,081
Statement of Cash Flows
Provision for doubtful accounts and sales returns		$ 5,050
Change in accounts receivable		(3,607)
Change in prepaid expenses and other assets		(899)
Accounting Standards Update 2014-09 | Impact of ASC 606 Adoption
Income Statement
Net revenue		851
Costs and Expenses:
Sales and marketing		(754)
General and administrative		851
Net loss attributable to common stockholders		$ 754
Basic earnings per share (in USD per share)		$ 0.01
Diluted earnings per share (in USD per share)		$ 0.01
Balance Sheet
Allowance for doubtful accounts				1,250
Other non-current assets				9,089
Retained earnings				$ 9,089
Statement of Cash Flows
Provision for doubtful accounts and sales returns		$ 851
Change in accounts receivable		(851)
Change in prepaid expenses and other assets		$ (754)

[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 period 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. See Note 1 below for additional discussion.

[3] 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. See Note 1 below for additional discussion.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.

[4] 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 prior periods presented. See Note 1 below for additional discussion.