GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Guarantor and Nonguarantor Financial Statements [Abstract]
Foreign earnings repatriated			$ 50,000
Condensed Financial Statements, Captions [Line Items]
Revenue	$ 1,059,122	$ 767,387	2,054,197	$ 1,528,220
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	218,224	139,033	420,186	284,991
Selling and marketing expense	369,660	320,104	772,492	670,515
General and administrative expense	188,363	150,222	372,547	293,817
Product development expense	75,445	55,430	152,382	110,190
Depreciation	18,805	18,339	38,062	38,227
Amortization of intangibles	20,188	8,624	40,141	17,785
Total operating costs and expenses	890,685	691,752	1,795,810	1,415,525
Operating income	168,437	75,635	258,387	112,695
Equity in earnings of unconsolidated affiliates	0	0	0	0
Interest expense	(27,356)	(24,728)	(53,861)	(49,520)
Other income, net	171,141	10,230	166,522	2,516
Earnings before income taxes	312,222	61,137	371,048	65,691
Income tax benefit (provision)	(31,368)	19,420	(2,355)	43,329
Net earnings	280,854	80,557	368,693	109,020
Net earnings attributable to noncontrolling interests	(62,501)	(14,289)	(79,258)	(16,543)
Net earnings attributable to IAC shareholders	218,353	66,268	289,435	92,477
Comprehensive income (loss) attributable to IAC shareholders	180,909	76,875	280,348	121,862
Reportable Legal Entities | IAC
Condensed Financial Statements, Captions [Line Items]
Revenue	0	0	0	0
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	75	47	151	160
Selling and marketing expense	189	613	402	939
General and administrative expense	31,652	35,778	63,061	61,914
Product development expense	564	936	1,216	1,506
Depreciation	266	445	532	883
Amortization of intangibles	0	0	0	0
Total operating costs and expenses	32,746	37,819	65,362	65,402
Operating income	(32,746)	(37,819)	(65,362)	(65,402)
Equity in earnings of unconsolidated affiliates	233,980	91,382	336,730	142,838
Interest expense	(423)	(5,648)	(852)	(11,476)
Other income, net	6,436	(6,821)	(10,411)	(12,626)
Earnings before income taxes	207,247	41,094	260,105	53,334
Income tax benefit (provision)	11,106	25,174	29,330	39,143
Net earnings	218,353	66,268	289,435	92,477
Net earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to IAC shareholders	218,353	66,268	289,435	92,477
Comprehensive income (loss) attributable to IAC shareholders	180,909	76,875	280,348	121,862
Reportable Legal Entities | Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenue	207,964	169,831	420,853	351,403
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	61,215	30,131	117,439	62,839
Selling and marketing expense	80,302	82,268	170,440	174,218
General and administrative expense	14,259	14,785	29,640	30,375
Product development expense	14,151	13,792	28,420	29,179
Depreciation	3,126	5,884	6,466	13,017
Amortization of intangibles	410	4,667	919	9,752
Total operating costs and expenses	173,463	151,527	353,324	319,380
Operating income	34,501	18,304	67,529	32,023
Equity in earnings of unconsolidated affiliates	8,630	4,706	8,303	2,023
Interest expense	0	0	0	0
Other income, net	62,204	6,807	349,087	12,898
Earnings before income taxes	105,335	29,817	424,919	46,944
Income tax benefit (provision)	(27,557)	(1,139)	(38,523)	(10,828)
Net earnings	77,778	28,678	386,396	36,116
Net earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to IAC shareholders	77,778	28,678	386,396	36,116
Comprehensive income (loss) attributable to IAC shareholders	77,482	30,852	386,443	42,343
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenue	851,240	597,724	1,633,500	1,177,198
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	156,991	108,989	302,703	222,299
Selling and marketing expense	289,205	237,269	601,731	495,458
General and administrative expense	142,441	99,647	279,814	201,502
Product development expense	60,730	40,702	122,746	79,505
Depreciation	15,413	12,010	31,064	24,327
Amortization of intangibles	19,778	3,957	39,222	8,033
Total operating costs and expenses	684,558	502,574	1,377,280	1,031,124
Operating income	166,682	95,150	256,220	146,074
Equity in earnings of unconsolidated affiliates	0	0	0	0
Interest expense	(26,933)	(19,080)	(53,009)	(38,044)
Other income, net	153,770	10,244	155,960	2,244
Earnings before income taxes	293,519	86,314	359,171	110,274
Income tax benefit (provision)	(14,917)	(4,615)	6,838	15,014
Net earnings	278,602	81,699	366,009	125,288
Net earnings attributable to noncontrolling interests	(62,501)	(14,289)	(79,258)	(16,543)
Net earnings attributable to IAC shareholders	216,101	67,410	286,751	108,745
Comprehensive income (loss) attributable to IAC shareholders	170,613	85,140	275,940	148,908
Eliminations
Condensed Financial Statements, Captions [Line Items]
Revenue	(82)	(168)	(156)	(381)
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	(57)	(134)	(107)	(307)
Selling and marketing expense	(36)	(46)	(81)	(100)
General and administrative expense	11	12	32	26
Product development expense	0	0	0	0
Depreciation	0	0	0	0
Amortization of intangibles	0	0	0	0
Total operating costs and expenses	(82)	(168)	(156)	(381)
Operating income	0	0	0	0
Equity in earnings of unconsolidated affiliates	(242,610)	(96,088)	(345,033)	(144,861)
Interest expense	0	0	0	0
Other income, net	(51,269)	0	(328,114)	0
Earnings before income taxes	(293,879)	(96,088)	(673,147)	(144,861)
Income tax benefit (provision)	0	0	0	0
Net earnings	(293,879)	(96,088)	(673,147)	(144,861)
Net earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to IAC shareholders	(293,879)	(96,088)	(673,147)	(144,861)
Comprehensive income (loss) attributable to IAC shareholders	$ (248,095)	$ (115,992)	$ (662,383)	$ (191,251)