SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)		3 Months Ended									12 Months Ended
	Jan. 01, 2018 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2018 USD ($) arrangement	Dec. 31, 2017 USD ($) arrangement	Dec. 31, 2016 USD ($) arrangement	Jan. 01, 2019 USD ($)	Oct. 01, 2018 USD ($)
Intangible Assets and Goodwill
Amortization of capitalized contract costs											$ 355,300,000
Current capitalized contract costs		$ 69,817,000				$ 0					69,817,000	$ 0
Non-current capitalized contract costs		4,500,000									$ 4,500,000
Period of payment due from customers for accounts receivable											30 days
Deferred revenue	$ 332,200,000
Deferred revenue recognized during period											$ 330,200,000
Current deferred revenue		360,015,000				342,483,000					360,015,000	342,483,000
Non-current deferred revenue		1,700,000									$ 1,700,000
Maturity period at purchase (less than)											91 days
Revenue		1,104,103,000	$ 1,104,592,000	$ 1,059,122,000	$ 995,075,000	950,585,000	$ 828,434,000	$ 767,387,000	$ 760,833,000		$ 4,262,892,000	3,307,239,000	$ 3,139,882,000
Accounts receivable, net of allowance and reserves		279,189,000				304,027,000					$ 279,189,000	304,027,000
Impairment charges on indefinite-lived intangible assets		27,700,000
Estimated weighted-average useful life (in years)											18 months
Property and equipment, net of accumulated depreciation and amortization		$ 318,800,000				315,170,000					$ 318,800,000	315,170,000
Excess of goodwill fair value over carrying value (less than) (as a percent)		110.00%									110.00%
Goodwill		$ 265,100,000									$ 265,100,000
Excess of indefinite-lived intangible asset fair value over carrying value (less than) (as a percent)		110.00%									110.00%
Indefinite-lived intangible assets		$ 131,300,000									$ 131,300,000
Advertising expense											1,200,000,000	1,100,000,000	1,000,000,000
Amounts reclassified to earnings											$ 52,000	$ 3,360,000	$ (6,937,000)
Put and call arrangements exercised | arrangement											2	2	1
Retained earnings		1,258,794,000				595,038,000					$ 1,258,794,000	$ 595,038,000
Deferred income taxes		23,600,000				35,070,000					23,600,000	35,070,000
Increase (decrease) in deferred revenue											49,468,000	39,154,000	$ 35,803,000
Cumulative effect of adoption of new accounting pronouncement	40,205,000
Unrealized gains, net, on equity securities held											124,170,000
Accounting Standards Update 2014-09
Intangible Assets and Goodwill
Retained earnings	40,200,000					3,400,000						3,400,000
Accounting Standards Update 2016-01
Intangible Assets and Goodwill
Cumulative effect of adoption of new accounting pronouncement	0
Unrealized gains, net, on equity securities held				$ 126,400,000
Accounting Standards Update 2016-02 | Forecast
Intangible Assets and Goodwill
Right-of-use asset														$ 160,000,000
Lease liability														$ 160,000,000
Foreign Currency Translation Adjustment
Intangible Assets and Goodwill
Amounts reclassified to earnings											52,000	(673,000)	(9,850,000)
Foreign Currency Translation Adjustment | Other (Expense) Income, Net
Intangible Assets and Goodwill
Amounts reclassified to earnings											(100,000)	700,000	9,900,000
Redeemable Noncontrolling Interest
Intangible Assets and Goodwill
Purchase of noncontrolling interests											4,100,000	6,300,000	7,900,000
Trade Names
Intangible Assets and Goodwill
Impairment charges on indefinite-lived intangible assets													$ 9,000,000
Software and software development costs
Intangible Assets and Goodwill
Property and equipment, net of accumulated depreciation and amortization		58,100,000				46,400,000					58,100,000	46,400,000
Google Inc.
Intangible Assets and Goodwill
Accounts receivable, net of allowance and reserves		69,100,000				$ 72,400,000					$ 69,100,000	$ 72,400,000
Google Inc. | Revenue | Customer Concentration Risk
Intangible Assets and Goodwill
Concentration risk (as a percent)											19.00%	22.00%	26.00%
Google Inc. | Revenue | Customer Concentration Risk | Operating Segments
Intangible Assets and Goodwill
Concentration risk (as a percent)											94.00%	96.00%	96.00%
Desktop | Trade Names
Intangible Assets and Goodwill
Impairment charges on indefinite-lived intangible assets											$ 27,700,000
College Humor Media | Trade Names
Intangible Assets and Goodwill
Impairment charges on indefinite-lived intangible assets											$ 1,100,000
Ask Media Group
Intangible Assets and Goodwill
Impairment charges on indefinite-lived intangible assets		$ 77,000,000
Minimum | Discount Rate
Intangible Assets and Goodwill
Measurement input (as a percent)		0.125				0.125					0.125	0.125
Minimum | Discount Rate | Indefinite-lived Intangible Assets
Intangible Assets and Goodwill
Measurement input (as a percent)		0.105				0.11					0.105	0.11
Minimum | Royalty Rate | Trade Names
Intangible Assets and Goodwill
Measurement input (as a percent)		0.02									0.02
Minimum | Royalty Rate | Indefinite-lived Intangible Assets
Intangible Assets and Goodwill
Measurement input (as a percent)		0.0075				0.02					0.0075	0.02
Maximum | Discount Rate
Intangible Assets and Goodwill
Measurement input (as a percent)		0.15				0.175					0.15	0.175
Maximum | Discount Rate | Indefinite-lived Intangible Assets
Intangible Assets and Goodwill
Measurement input (as a percent)		0.35				0.16					0.35	0.16
Maximum | Royalty Rate | Trade Names
Intangible Assets and Goodwill
Measurement input (as a percent)		0.06									0.06
Maximum | Royalty Rate | Indefinite-lived Intangible Assets
Intangible Assets and Goodwill
Measurement input (as a percent)		0.080				0.07					0.080	0.07
Match Group
Intangible Assets and Goodwill
Market capitalization															$ 15,700,000,000
Amount by which market capitalization exceeds carrying value															15,100,000,000
Match Group | Operating Segments
Intangible Assets and Goodwill
Revenue											$ 1,729,850,000	$ 1,330,661,000	$ 1,118,110,000
Property and equipment, net of accumulated depreciation and amortization		$ 58,351,000				$ 61,620,000					$ 58,351,000	61,620,000
Match Group | Minimum
Intangible Assets and Goodwill
Period of membership subscription											1 month
Match Group | Maximum
Intangible Assets and Goodwill
Period of membership subscription											6 months
ANGI Homeservices
Intangible Assets and Goodwill
Period of membership subscription											1 year
Market capitalization															10,700,000,000
Amount by which market capitalization exceeds carrying value															$ 9,600,000,000
ANGI Homeservices | Accounting Standards Update 2014-09
Intangible Assets and Goodwill
Current capitalized contract costs	29,700,000
Non-current capitalized contract costs	4,200,000
Retained earnings	25,900,000
Deferred income taxes	8,000,000
ANGI Homeservices | Operating Segments
Intangible Assets and Goodwill
Revenue											$ 1,132,241,000	736,386,000	498,890,000
Property and equipment, net of accumulated depreciation and amortization		70,859,000				53,292,000					$ 70,859,000	53,292,000
ANGI Homeservices | Angie's List
Intangible Assets and Goodwill
Period of membership subscription											1 year
Vimeo | Operating Segments
Intangible Assets and Goodwill
Revenue											$ 159,641,000	103,332,000	78,805,000
Property and equipment, net of accumulated depreciation and amortization		1,014,000				1,972,000					$ 1,014,000	1,972,000
Vimeo | Minimum
Intangible Assets and Goodwill
Period of membership subscription											1 month
Vimeo | Maximum
Intangible Assets and Goodwill
Period of membership subscription											1 year
Publishing and Applications | Google Inc.
Intangible Assets and Goodwill
Revenue											$ 825,200,000	740,700,000	824,400,000
Applications | Accounting Standards Update 2014-09
Intangible Assets and Goodwill
Retained earnings	15,500,000
Deferred income taxes	4,900,000
Increase (decrease) in deferred revenue	$ (20,300,000)
Applications | Operating Segments
Intangible Assets and Goodwill
Revenue											582,287,000	577,998,000	$ 604,140,000
Property and equipment, net of accumulated depreciation and amortization		4,867,000				7,004,000					$ 4,867,000	$ 7,004,000
Applications | Google Inc. | Revenue | Customer Concentration Risk | Operating Segments
Intangible Assets and Goodwill
Concentration risk (as a percent)											73.00%	83.00%	87.00%
Applications | Mosaic Group | Operating Segments
Intangible Assets and Goodwill
Revenue											$ 123,516,000	$ 55,849,000	$ 40,825,000
Applications | Minimum | Desktop
Intangible Assets and Goodwill
Period of membership subscription											1 year
Applications | Minimum | Mosaic Group
Intangible Assets and Goodwill
Period of membership subscription											1 year
Applications | Maximum | Desktop
Intangible Assets and Goodwill
Period of membership subscription											2 years
Applications | Maximum | Mosaic Group
Intangible Assets and Goodwill
Period of membership subscription											2 years
Publishing
Intangible Assets and Goodwill
Impairment charges on indefinite-lived intangible assets										$ 275,400,000
Emerging & Other | Operating Segments
Intangible Assets and Goodwill
Revenue											$ 528,250,000	468,589,000	$ 762,609,000
Property and equipment, net of accumulated depreciation and amortization		1,638,000				2,377,000					$ 1,638,000	$ 2,377,000
Emerging & Other | Google Inc. | Revenue | Customer Concentration Risk | Operating Segments
Intangible Assets and Goodwill
Concentration risk (as a percent)											68.00%	48.00%	35.00%
Dotdash
Intangible Assets and Goodwill
Impairment charges on indefinite-lived intangible assets		198,300,000
Dotdash | Trade Names
Intangible Assets and Goodwill
Impairment charges on indefinite-lived intangible assets													$ 2,600,000
Dotdash | Operating Segments
Intangible Assets and Goodwill
Revenue											$ 130,991,000	$ 90,890,000	$ 77,913,000
Property and equipment, net of accumulated depreciation and amortization		$ 3,229,000				$ 4,077,000					$ 3,229,000	$ 4,077,000