GOODWILL AND INTANGIBLE ASSETS - Goodwill by Reporting Unit (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) operating_segment reportingunit	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Goodwill
Balance at beginning of period	$ 2,559,066	$ 2,726,859	$ 2,559,066
Additions		168,866	208,423
(Deductions)		(29,267)	(22,188)
Impairment		(3,318)	0	$ 0
Foreign Exchange Translation		(8,678)	(18,442)
Balance at end of period		2,854,462	2,726,859	2,559,066
Match Group
Goodwill
Balance at beginning of period	1,247,899	1,245,013	1,247,899
Additions		3,553	11,187
(Deductions)		0	0
Foreign Exchange Translation		(8,726)	(14,073)
Balance at end of period		1,239,840	1,245,013	1,247,899
ANGI Homeservices
Goodwill
Balance at beginning of period	768,317	892,800	768,317
Additions		18,326	142,768
(Deductions)		(29,267)	(14,373)
Transfers In/(Out)			0
Impairment		0
Foreign Exchange Translation		192	(3,912)
Balance at end of period		882,051	892,800	768,317
Vimeo
Goodwill
Balance at beginning of period	77,303	77,152	77,303
Additions		142,222	0
(Deductions)		0	(151)
Transfers In/(Out)			0
Impairment		0
Foreign Exchange Translation		0	0
Balance at end of period		219,374	77,152	77,303
Applications
Goodwill
Balance at beginning of period	447,242	504,892	447,242
Additions		0	50,784
(Deductions)		0	0
Transfers In/(Out)			7,323
Impairment		0
Foreign Exchange Translation		(144)	(457)
Balance at end of period		504,748	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
Impairment		0
Foreign Exchange Translation		0	0
Balance at end of period		265,146	265,146	265,146
Applications | Mosaic Group
Goodwill
Balance at beginning of period	182,096	239,746	182,096
Additions		0	50,784
(Deductions)		0	0
Transfers In/(Out)			7,323
Impairment		0
Foreign Exchange Translation		(144)
Balance at end of period		239,602	239,746	182,096
Emerging & Other
Goodwill
Balance at beginning of period	$ 18,305	7,002	18,305
Additions		4,765	3,684
(Deductions)		0	(7,664)
Transfers In/(Out)			(7,323)
Balance at end of period		8,449	7,002	$ 18,305
Emerging & Other | Ask Media Group
Goodwill
Accumulated goodwill impairment loss			399,700
Emerging & Other | College Humor Media
Goodwill
Impairment		(3,318)
Accumulated goodwill impairment loss		$ 14,900	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