GOODWILL AND INTANGIBLE ASSETS - Goodwill by Reporting Unit (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018 USD ($) operating_segment reportingunit	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Goodwill
Balance at beginning of period	$ 2,559,066	$ 2,559,066	$ 1,924,052
Additions		208,423	661,396
(Deductions)		(22,188)	(74,430)
Foreign Exchange Translation		(18,442)	48,048
Balance at end of period		2,726,859	2,559,066
Match Group
Goodwill
Balance at beginning of period	1,247,899	1,247,899	1,206,538
Additions		11,187	255
(Deductions)		0	0
Foreign Exchange Translation		(14,073)	41,106
Balance at end of period		1,245,013	1,247,899
ANGI Homeservices
Goodwill
Balance at beginning of period	768,317	768,317	170,611
Additions		142,768	590,772
(Deductions)		(14,373)	0
Transfers In/(Out)		0
Foreign Exchange Translation		(3,912)	6,934
Balance at end of period		892,800	768,317
Vimeo
Goodwill
Balance at beginning of period	77,303	77,303	9,649
Additions		0	67,654
(Deductions)		(151)	0
Transfers In/(Out)		0
Foreign Exchange Translation		0	0
Balance at end of period		77,152	77,303
Applications
Goodwill
Balance at beginning of period	447,242	447,242	447,242
Additions		50,784	0
(Deductions)		0	0
Transfers In/(Out)		7,323
Foreign Exchange Translation		(457)	0
Balance at end of period		504,892	447,242
Accumulated goodwill impairment loss		529,100
Applications \| Desktop
Goodwill
Balance at beginning of period	265,146	265,146	265,146
Additions		0	0
(Deductions)		0	0
Transfers In/(Out)		0
Foreign Exchange Translation		0	0
Balance at end of period		265,146	265,146
Applications \| Mosaic Group
Goodwill
Balance at beginning of period	182,096	182,096	182,096
Additions		50,784	0
(Deductions)		0	0
Transfers In/(Out)		7,323
Foreign Exchange Translation		(457)	0
Balance at end of period		239,746	182,096
Emerging & Other
Goodwill
Balance at beginning of period	$ 18,305	18,305	90,012
Additions		3,684	2,715
(Deductions)		(7,664)	(74,430)
Transfers In/(Out)		(7,323)
Foreign Exchange Translation			8
Balance at end of period		7,002	$ 18,305
Emerging & Other \| Ask Media Group
Goodwill
Accumulated goodwill impairment loss		399,700
Emerging & Other \| College Humor Media
Goodwill
Accumulated goodwill impairment loss		11,600
Dotdash
Goodwill
Accumulated goodwill impairment loss		$ 198,300
Publishing
Goodwill
Number of operating segments \| operating_segment	1
Number of reporting units \| reportingunit	1

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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