GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Guarantor and Nonguarantor Financial Statements [Abstract]
Foreign earnings repatriated	$ 20,000		$ 346,000
Condensed Financial Statements, Captions [Line Items]
Revenue	1,104,592	$ 828,434	3,158,789	$ 2,356,654
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	237,238	166,290	657,424	451,281
Selling and marketing expense	386,802	352,879	1,159,294	1,023,394
General and administrative expense	190,903	235,580	563,450	529,397
Product development expense	77,740	70,645	230,122	180,835
Depreciation	18,925	17,263	56,987	55,490
Amortization of intangibles	20,152	4,366	60,293	22,151
Total operating costs and expenses	931,760	847,023	2,727,570	2,262,548
Operating income (loss)	172,832	(18,589)	431,219	94,106
Equity in earnings of unconsolidated affiliates	0	0	0	0
Interest expense	(27,610)	(25,036)	(81,471)	(74,556)
Other (expense) income, 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
Comprehensive income (loss) attributable to IAC shareholders	145,245	216,868	425,593	338,730
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	0	170	160
Selling and marketing expense	225	311	627	1,250
General and administrative expense	37,021	32,060	100,082	93,974
Product development expense	155	592	1,371	2,098
Depreciation	271	407	803	1,290
Amortization of intangibles	0	0	0	0
Total operating costs and expenses	37,691	33,370	103,053	98,772
Operating income (loss)	(37,691)	(33,370)	(103,053)	(98,772)
Equity in earnings of unconsolidated affiliates	162,497	203,817	517,349	346,655
Interest expense	(424)	(5,484)	(1,276)	(16,960)
Other (expense) income, net	(3,812)	(8,157)	(14,223)	(20,783)
Earnings (loss) before income taxes	120,570	156,806	398,797	210,140
Income tax benefit (provision)	25,204	22,837	36,412	61,980
Net earnings	145,774	179,643	435,209	272,120
Net earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to IAC shareholders	145,774	179,643	435,209	272,120
Comprehensive income (loss) attributable to IAC shareholders	145,245	216,868	425,593	338,730
Reportable Legal Entities | Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenue	212,892	185,386	633,745	536,789
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	68,748	41,741	186,187	104,580
Selling and marketing expense	79,865	84,416	250,305	258,634
General and administrative expense	7,238	15,404	36,878	45,779
Product development expense	14,395	13,145	42,815	42,324
Depreciation	3,009	4,338	9,475	17,355
Amortization of intangibles	409	350	1,328	10,102
Total operating costs and expenses	173,664	159,394	526,988	478,774
Operating income (loss)	39,228	25,992	106,757	58,015
Equity in earnings of unconsolidated affiliates	14,555	489	19,282	2,512
Interest expense	0	0	0	0
Other (expense) income, net	29,367	8,309	378,454	21,207
Earnings (loss) before income taxes	83,150	34,790	504,493	81,734
Income tax benefit (provision)	310	(11,616)	(22,815)	(22,444)
Net earnings	83,460	23,174	481,678	59,290
Net earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to IAC shareholders	83,460	23,174	481,678	59,290
Comprehensive income (loss) attributable to IAC shareholders	92,277	3,682	490,542	46,025
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenue	891,771	643,175	2,525,271	1,820,373
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	168,524	124,676	471,227	346,975
Selling and marketing expense	306,747	268,182	908,478	763,640
General and administrative expense	146,627	188,086	426,441	389,588
Product development expense	63,190	56,908	185,936	136,413
Depreciation	15,645	12,518	46,709	36,845
Amortization of intangibles	19,743	4,016	58,965	12,049
Total operating costs and expenses	720,476	654,386	2,097,756	1,685,510
Operating income (loss)	171,295	(11,211)	427,515	134,863
Equity in earnings of unconsolidated affiliates	0	0	0	0
Interest expense	(27,186)	(19,552)	(80,195)	(57,596)
Other (expense) income, net	3,826	(10,368)	159,786	(8,124)
Earnings (loss) before income taxes	147,935	(41,131)	507,106	69,143
Income tax benefit (provision)	(7,272)	268,259	2,290	283,273
Net earnings	140,663	227,128	509,396	352,416
Net earnings attributable to noncontrolling interests	(25,803)	(45,996)	(105,061)	(62,539)
Net earnings attributable to IAC shareholders	114,860	181,132	404,335	289,877
Comprehensive income (loss) attributable to IAC shareholders	122,765	225,089	401,429	373,997
Eliminations
Condensed Financial Statements, Captions [Line Items]
Revenue	(71)	(127)	(227)	(508)
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	(53)	(127)	(160)	(434)
Selling and marketing expense	(35)	(30)	(116)	(130)
General and administrative expense	17	30	49	56
Product development expense	0	0	0	0
Depreciation	0	0	0	0
Amortization of intangibles	0	0	0	0
Total operating costs and expenses	(71)	(127)	(227)	(508)
Operating income (loss)	0	0	0	0
Equity in earnings of unconsolidated affiliates	(177,052)	(204,306)	(536,631)	(349,167)
Interest expense	0	0	0	0
Other (expense) income, net	(21,268)	0	(349,382)	0
Earnings (loss) before income taxes	(198,320)	(204,306)	(886,013)	(349,167)
Income tax benefit (provision)	0	0	0	0
Net earnings	(198,320)	(204,306)	(886,013)	(349,167)
Net earnings attributable to noncontrolling interests	0	0	0	0
Net earnings attributable to IAC shareholders	(198,320)	(204,306)	(886,013)	(349,167)
Comprehensive income (loss) attributable to IAC shareholders	$ (215,042)	$ (228,771)	$ (891,971)	$ (420,022)