The Company's primary financial measure is Adjusted EBITDA, which is defined as operating income excluding: (1) stock-based compensation expense; (2) depreciation; and (3) acquisition-related items consisting of (i) amortization of intangible assets and impairments of goodwill and intangible assets, if applicable, and (ii) gains and losses recognized on changes in the fair value of contingent consideration arrangements. The Company believes this measure is useful for analysts and investors as this measure allows a more meaningful comparison between our performance and that of our competitors. Moreover, our management uses this measure internally to evaluate the performance of our business as a whole and our individual business segments, and this measure is one of the primary metrics by which our internal budgets are based and by which management is compensated. The above items are excluded from our Adjusted EBITDA measure because these items are non-cash in nature, and we believe that by excluding these items, Adjusted EBITDA corresponds more closely to the cash operating income generated from our business, from which capital investments are made and debt is serviced. Adjusted EBITDA has certain limitations in that it does not take into account the impact to IAC's statement of operations of certain expenses.

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SEGMENT INFORMATION - Reconciliation of Adjusted EBITDA to Operating Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Other Significant Reconciling Item
Operating income	$ 133,920	$ 172,832	$ 168,437	$ 89,950	$ 94,360	$ (18,589)	$ 75,635	$ 37,060	$ 565,139	$ 188,466	$ (32,625)
Stock-based compensation expense									238,420	264,618	104,820
Depreciation									75,360	74,265	71,676
Amortization of intangibles									108,399	42,143	79,426
Goodwill Impairment									0	0	275,367
Interest expense									(109,327)	(105,295)	(109,110)
Other expense, net									305,746	(16,213)	60,650
Earnings (loss) before income taxes									761,558	66,958	(81,085)
Income tax benefit						(257,000)			(3,811)	291,050	64,934
Net earnings (loss)	217,477	171,577	280,854	87,839	23,349	225,639	80,557	28,463	757,747	358,008	(16,151)
Net earnings attributable to noncontrolling interests									(130,786)	(53,084)	(25,129)
Net earnings (loss) attributable to IAC shareholders	$ 191,752	$ 145,774	$ 218,353	$ 71,082	$ 32,804	$ 179,643	$ 66,268	$ 26,209	626,961	304,924	(41,280)
Operating Segments \| Match Group
Segment Reporting, Other Significant Reconciling Item
Operating income									553,294	360,517	315,549
Stock-based compensation expense									66,031	69,090	52,370
Depreciation									32,968	32,613	27,726
Amortization of intangibles									1,318	1,468	16,932
Acquisition-related Contingent Consideration Fair Value Adjustments									320	5,253	(9,197)
Adjusted EBITDA									653,931	468,941	403,380
Goodwill Impairment											0
Operating Segments \| ANGI Homeservices
Segment Reporting, Other Significant Reconciling Item
Operating income									63,906	(149,176)	25,363
Stock-based compensation expense									97,078	149,230	8,916
Depreciation									24,310	14,543	8,419
Amortization of intangibles									62,212	23,261	3,153
Acquisition-related Contingent Consideration Fair Value Adjustments									0	0	0
Adjusted EBITDA									247,506	37,858	45,851
Goodwill Impairment											0
Operating Segments \| Vimeo
Segment Reporting, Other Significant Reconciling Item
Operating income									(35,594)	(27,328)	(25,350)
Stock-based compensation expense									0	0	0
Depreciation									1,200	1,408	1,085
Amortization of intangibles									6,349	2,313	4,176
Acquisition-related Contingent Consideration Fair Value Adjustments									0	0	(192)
Adjusted EBITDA									(28,045)	(23,607)	(20,281)
Goodwill Impairment											0
Operating Segments \| Dotdash
Segment Reporting, Other Significant Reconciling Item
Operating income									18,778	(15,694)	(248,705)
Stock-based compensation expense									0	0	0
Depreciation									969	2,255	2,775
Amortization of intangibles									1,637	10,676	30,754
Acquisition-related Contingent Consideration Fair Value Adjustments									0	0	0
Adjusted EBITDA									21,384	(2,763)	(16,846)
Goodwill Impairment											198,330
Operating Segments \| Applications
Segment Reporting, Other Significant Reconciling Item
Operating income									94,834	130,176	109,663
Stock-based compensation expense									0	0	0
Depreciation									2,601	3,863	5,095
Amortization of intangibles									33,266	2,170	5,483
Acquisition-related Contingent Consideration Fair Value Adjustments									1,136	548	12,035
Adjusted EBITDA									131,837	136,757	132,276
Goodwill Impairment											0
Operating Segments \| Emerging & Other
Segment Reporting, Other Significant Reconciling Item
Operating income									29,964	17,412	(99,696)
Stock-based compensation expense									919	2,130	1,258
Depreciation									1,678	4,065	12,675
Amortization of intangibles									3,617	2,255	18,928
Acquisition-related Contingent Consideration Fair Value Adjustments									0	0	(91)
Adjusted EBITDA									36,178	25,862	10,111
Goodwill Impairment											77,037
Corporate
Segment Reporting, Other Significant Reconciling Item
Operating income									(160,043)	(127,441)	(109,449)
Stock-based compensation expense									74,392	44,168	42,276
Depreciation									11,634	15,518	13,901
Amortization of intangibles									0	0	0
Acquisition-related Contingent Consideration Fair Value Adjustments									0	0	0
Adjusted EBITDA									$ (74,017)	$ (67,755)	(53,272)
Goodwill Impairment											$ 0