SEGMENT INFORMATION - Reconciliation of Operating Income (Loss) to Adjusted EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating income (loss)	$ 172,832	$ (18,589)	$ 431,219	$ 94,106
Stock-based compensation expense	55,363	134,472	172,006	207,362
Depreciation	18,925	17,263	56,987	55,490
Amortization of intangibles	20,152	4,366	60,293	22,151
Interest expense	(27,610)	(25,036)	(81,471)	(74,556)
Other income (expense), net	8,113	(10,216)	174,635	(7,700)
Earnings (loss) before income taxes	153,335	(53,841)	524,383	11,850
Income tax benefit (provision)	18,242	279,480	15,887	322,809
Net earnings	171,577	225,639	540,270	334,659
Net earnings attributable to noncontrolling interests	(25,803)	(45,996)	(105,061)	(62,539)
Net earnings attributable to IAC shareholders	145,774	179,643	435,209	272,120
Operating segments | Match Group
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating income (loss)	139,895	91,008	402,293	232,854
Stock-based compensation expense	16,141	19,949	49,810	53,627
Depreciation	8,513	8,147	25,059	23,619
Amortization of intangibles	435	401	914	1,208
Acquisition-related Contingent Consideration Fair Value Adjustments	55	59	265	4,397
Adjusted EBITDA	165,039	119,564	478,341	315,705
Operating segments | ANGI Homeservices
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating income (loss)	33,515	(112,505)	46,021	(115,258)
Stock-based compensation expense	22,474	103,980	69,433	120,280
Depreciation	6,100	3,491	18,170	9,705
Amortization of intangibles	15,611	2,768	47,695	6,885
Acquisition-related Contingent Consideration Fair Value Adjustments	0	0	0	0
Adjusted EBITDA	77,700	(2,266)	181,319	21,612
Operating segments | Video
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating income (loss)	(10,250)	(1,809)	(41,107)	(25,227)
Stock-based compensation expense	323	134	1,747	267
Depreciation	396	541	1,518	1,637
Amortization of intangibles	2,141	312	6,410	937
Acquisition-related Contingent Consideration Fair Value Adjustments	0	0	0	0
Adjusted EBITDA	(7,390)	(822)	(31,432)	(22,386)
Operating segments | Applications
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating income (loss)	33,041	29,386	91,579	101,288
Stock-based compensation expense	0	0	0	0
Depreciation	617	1,155	2,145	3,087
Amortization of intangibles	1,331	536	3,421	1,633
Acquisition-related Contingent Consideration Fair Value Adjustments	0	0	0	548
Adjusted EBITDA	34,989	31,077	97,145	106,556
Operating segments | Publishing
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating income (loss)	17,398	5,677	46,016	(2,968)
Stock-based compensation expense	0	0	0	0
Depreciation	435	1,062	1,566	4,011
Amortization of intangibles	634	349	1,853	9,964
Acquisition-related Contingent Consideration Fair Value Adjustments	0	0	0	0
Adjusted EBITDA	18,467	7,088	49,435	11,007
Operating segments | Other
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating income (loss)				(5,621)
Stock-based compensation expense				1,729
Depreciation				836
Amortization of intangibles				1,524
Acquisition-related Contingent Consideration Fair Value Adjustments				0
Adjusted EBITDA	0	0	0	(1,532)
Corporate
Segment Reporting, Other Significant Reconciling Item [Line Items]
Operating income (loss)	(40,767)	(30,346)	(113,583)	(90,962)
Stock-based compensation expense	16,425	10,409	51,016	31,459
Depreciation	2,864	2,867	8,529	12,595
Amortization of intangibles	0	0	0	0
Acquisition-related Contingent Consideration Fair Value Adjustments	0	0	0	0
Adjusted EBITDA	$ (21,478)	$ (17,070)	$ (54,038)	$ (46,908)