THE COMPANY AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) consumer_review	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) operating_segment consumer_review	Jun. 30, 2017 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Entity Operations Information [Abstract]
Number of consumer reviews | consumer_review	15,000,000		15,000,000
Period of consumer reviews collected			20 years
Number of operating segments | operating_segment			2
Retained earnings (accumulated deficit)	$ (82,101)		$ (82,101)				$ (121,764)
Deferred income taxes	4,410		4,410				$ 5,626
Operating income (loss)	$ 23,262	$ (2,836)	$ 12,506	$ (1,448)
Restricted cash included in Other assets								$ 10,600
Class B Common Stock | IAC
Entity Operations Information [Abstract]
Ownership interest (as a percent)	86.40%		86.40%
Voting interest (as a percent)	98.50%		98.50%
Accounting Standards Update 2014-09
Entity Operations Information [Abstract]
Retained earnings (accumulated deficit)						$ 25,600
Current asset for capitalized sales commissions						29,700
Noncurrent asset for capitalized sales commissions						4,200
Deferred income taxes						$ 8,300
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Entity Operations Information [Abstract]
Operating income (loss)	$ 2,118		$ 8,211
Forecast | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Entity Operations Information [Abstract]
Operating income (loss)					$ 10,000