GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Condensed Financial Statements, Captions [Line Items]
Revenue	$ 1,186,658	$ 1,059,122	$ 2,292,501	$ 2,054,197
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	276,389	218,224	536,460	420,186
Selling and marketing expense	411,102	369,660	832,962	772,492
General and administrative expense	225,514	188,363	439,130	372,547
Product development expense	78,614	75,445	167,314	152,382
Depreciation	21,091	18,805	40,062	38,062
Amortization of intangibles	19,638	20,188	42,390	40,141
Total operating costs and expenses	1,032,348	890,685	2,058,318	1,795,810
Operating income	154,310	168,437	234,183	258,387
Equity in earnings of unconsolidated affiliates	0	0	0	0
Interest expense	(37,206)	(27,356)	(68,349)	(53,861)
Other (expense) income, net	45,972	171,141	46,623	166,522
Earnings before income taxes	163,076	312,222	212,457	371,048
Income tax benefit (provision)	(16,285)	(31,368)	47,319	(2,355)
Net earnings	146,791	280,854	259,776	368,693
Net earnings attributable to noncontrolling interests	(33,324)	(62,501)	(57,614)	(79,258)
Net earnings attributable to IAC shareholders	113,467	218,353	202,162	289,435
Comprehensive income (loss) attributable to IAC shareholders	114,558	180,909	204,248	280,348
Foreign earnings repatriated		50,000		326,000
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)	19	75	43	151
Selling and marketing expense	384	189	746	402
General and administrative expense	39,634	31,652	77,893	63,061
Product development expense	469	564	1,116	1,216
Depreciation	432	266	810	532
Amortization of intangibles	0	0	0	0
Total operating costs and expenses	40,938	32,746	80,608	65,362
Operating income	(40,938)	(32,746)	(80,608)	(65,362)
Equity in earnings of unconsolidated affiliates	145,997	252,102	261,437	354,852
Interest expense	(424)	(423)	(848)	(852)
Other (expense) income, net	(7,801)	6,436	(9,698)	(10,411)
Earnings before income taxes	96,834	225,369	170,283	278,227
Income tax benefit (provision)	16,633	(7,016)	31,879	11,208
Net earnings	113,467	218,353	202,162	289,435
Net earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to IAC shareholders	113,467	218,353	202,162	289,435
Comprehensive income (loss) attributable to IAC shareholders	114,558	180,909	204,248	280,348
Reportable Legal Entities | Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenue	200,963	207,964	399,893	420,853
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	90,383	61,215	172,529	117,439
Selling and marketing expense	55,710	80,302	113,676	170,440
General and administrative expense	11,270	14,259	21,804	29,640
Product development expense	13,353	14,151	25,390	28,420
Depreciation	2,914	3,126	5,836	6,466
Amortization of intangibles	1,148	410	5,025	919
Total operating costs and expenses	174,778	173,463	344,260	353,324
Operating income	26,185	34,501	55,633	67,529
Equity in earnings of unconsolidated affiliates	4,542	5,054	5,610	4,727
Interest expense	0	0	0	0
Other (expense) income, net	12,741	62,204	26,592	349,087
Earnings before income taxes	43,468	101,759	87,835	421,343
Income tax benefit (provision)	(4,136)	(12,159)	(9,259)	(23,125)
Net earnings	39,332	89,600	78,576	398,218
Net earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to IAC shareholders	39,332	89,600	78,576	398,218
Comprehensive income (loss) attributable to IAC shareholders	39,117	89,304	78,572	398,265
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenue	985,825	851,240	1,892,803	1,633,500
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	186,065	156,991	364,031	302,703
Selling and marketing expense	355,086	289,205	718,648	601,731
General and administrative expense	174,589	142,441	339,382	279,814
Product development expense	64,787	60,730	140,803	122,746
Depreciation	17,745	15,413	33,416	31,064
Amortization of intangibles	18,490	19,778	37,365	39,222
Total operating costs and expenses	816,762	684,558	1,633,645	1,377,280
Operating income	169,063	166,682	259,158	256,220
Equity in earnings of unconsolidated affiliates	0	0	0	0
Interest expense	(36,782)	(26,933)	(67,501)	(53,009)
Other (expense) income, net	52,755	153,770	53,175	155,960
Earnings before income taxes	185,036	293,519	244,832	359,171
Income tax benefit (provision)	(28,782)	(12,193)	24,699	9,562
Net earnings	156,254	281,326	269,531	368,733
Net earnings attributable to noncontrolling interests	(33,324)	(62,501)	(57,614)	(79,258)
Net earnings attributable to IAC shareholders	122,930	218,825	211,917	289,475
Comprehensive income (loss) attributable to IAC shareholders	124,408	173,337	214,508	278,664
Eliminations
Condensed Financial Statements, Captions [Line Items]
Revenue	(130)	(82)	(195)	(156)
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	(78)	(57)	(143)	(107)
Selling and marketing expense	(78)	(36)	(108)	(81)
General and administrative expense	21	11	51	32
Product development expense	5	0	5	0
Depreciation	0	0	0	0
Amortization of intangibles	0	0	0	0
Total operating costs and expenses	(130)	(82)	(195)	(156)
Operating income	0	0	0	0
Equity in earnings of unconsolidated affiliates	(150,539)	(257,156)	(267,047)	(359,579)
Interest expense	0	0	0	0
Other (expense) income, net	(11,723)	(51,269)	(23,446)	(328,114)
Earnings before income taxes	(162,262)	(308,425)	(290,493)	(687,693)
Income tax benefit (provision)	0	0	0	0
Net earnings	(162,262)	(308,425)	(290,493)	(687,693)
Net earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to IAC shareholders	(162,262)	(308,425)	(290,493)	(687,693)
Comprehensive income (loss) attributable to IAC shareholders	$ (163,525)	$ (262,641)	$ (293,080)	$ (676,929)