GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Condensed Statement of Operations (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
Condensed Statements of Operations
Revenue	$ 1,104,103	$ 1,104,592	$ 1,059,122	$ 995,075	$ 950,585	$ 828,434	$ 767,387	$ 760,833	$ 4,262,892	$ 3,307,239	$ 3,139,882
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	253,722	237,238	218,224	201,962	199,727	166,290	139,033	145,958	911,146	651,008	755,730
Selling and marketing expense									1,519,440	1,381,221	1,247,097
General and administrative expense									774,079	719,257	530,446
Product development expense									309,329	250,879	212,765
Depreciation									75,360	74,265	71,676
Amortization of intangibles									108,399	42,143	79,426
Goodwill impairment									0	0	275,367
Total operating costs and expenses									3,697,753	3,118,773	3,172,507
Operating income (loss)	133,920	172,832	168,437	89,950	94,360	(18,589)	75,635	37,060	565,139	188,466	(32,625)
Equity in earnings of unconsolidated affiliates									0	0	0
Interest expense									(109,327)	(105,295)	(109,110)
Other income (expense), net									305,746	(16,213)	60,650
Earnings (loss) before income taxes									761,558	66,958	(81,085)
Income tax (provision) 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)
Comprehensive income attributable to IAC shareholders									601,683	367,370	(76,431)
Foreign cash repatriated									396,200
Eliminations
Condensed Statements of Operations
Revenue									(378)	(617)	(605)
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)									(291)	(505)	(412)
Selling and marketing expense									(150)	(519)	(323)
General and administrative expense									69	407	130
Product development expense									(6)	0	0
Depreciation									0	0	0
Amortization of intangibles									0	0	0
Goodwill impairment											0
Total operating costs and expenses									(378)	(617)	(605)
Operating income (loss)									0	0	0
Equity in earnings of unconsolidated affiliates									(751,917)	(439,904)	(56,319)
Interest expense									0	0	0
Other income (expense), net									(378,438)	0	0
Earnings (loss) before income taxes									(1,130,355)	(439,904)	(56,319)
Income tax (provision) benefit									0	0	0
Net earnings (loss)									(1,130,355)	(439,904)	(56,319)
Net earnings attributable to noncontrolling interests									0	0	0
Net earnings (loss) attributable to IAC shareholders									(1,130,355)	(439,904)	(56,319)
Comprehensive income attributable to IAC shareholders									(1,116,998)	(505,661)	(2,545)
IAC | Reportable Legal Entities
Condensed Statements of Operations
Revenue									0	0	0
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)									195	160	859
Selling and marketing expense									977	1,250	2,353
General and administrative expense									141,727	100,237	89,583
Product development expense									2,003	2,421	4,807
Depreciation									1,203	1,564	1,610
Amortization of intangibles									0	0	0
Goodwill impairment											0
Total operating costs and expenses									146,105	105,632	99,212
Operating income (loss)									(146,105)	(105,632)	(99,212)
Equity in earnings of unconsolidated affiliates									731,834	419,149	49,545
Interest expense									(1,700)	(20,339)	(26,876)
Other income (expense), net									(18,834)	(30,787)	(1,879)
Earnings (loss) before income taxes									565,195	262,391	(78,422)
Income tax (provision) benefit									61,766	42,533	37,142
Net earnings (loss)									626,961	304,924	(41,280)
Net earnings attributable to noncontrolling interests									0	0	0
Net earnings (loss) attributable to IAC shareholders									626,961	304,924	(41,280)
Comprehensive income attributable to IAC shareholders									601,683	367,370	(76,431)
Guarantor Subsidiaries | Reportable Legal Entities
Condensed Statements of Operations
Revenue									850,475	753,858	960,000
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)									262,912	159,488	297,712
Selling and marketing expense									313,769	353,186	417,051
General and administrative expense									49,563	62,340	83,636
Product development expense									56,431	55,232	69,778
Depreciation									12,497	20,668	26,514
Amortization of intangibles									29,437	11,213	41,157
Goodwill impairment											253,245
Total operating costs and expenses									724,609	662,127	1,189,093
Operating income (loss)									125,866	91,731	(229,093)
Equity in earnings of unconsolidated affiliates									20,083	20,755	6,774
Interest expense									0	0	0
Other income (expense), net									503,261	28,434	10,209
Earnings (loss) before income taxes									649,210	140,920	(212,110)
Income tax (provision) benefit									(56,612)	(119,957)	77,851
Net earnings (loss)									592,598	20,963	(134,259)
Net earnings attributable to noncontrolling interests									0	0	0
Net earnings (loss) attributable to IAC shareholders									592,598	20,963	(134,259)
Comprehensive income attributable to IAC shareholders									601,232	7,629	(142,494)
Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Statements of Operations
Revenue									3,412,795	2,553,998	2,180,487
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)									648,330	491,865	457,571
Selling and marketing expense									1,204,844	1,027,304	828,016
General and administrative expense									582,720	556,273	357,097
Product development expense									250,901	193,226	138,180
Depreciation									61,660	52,033	43,552
Amortization of intangibles									78,962	30,930	38,269
Goodwill impairment											22,122
Total operating costs and expenses									2,827,417	2,351,631	1,884,807
Operating income (loss)									585,378	202,367	295,680
Equity in earnings of unconsolidated affiliates									0	0	0
Interest expense									(107,627)	(84,956)	(82,234)
Other income (expense), net									199,757	(13,860)	52,320
Earnings (loss) before income taxes									677,508	103,551	265,766
Income tax (provision) benefit									(8,965)	368,474	(50,059)
Net earnings (loss)									668,543	472,025	215,707
Net earnings attributable to noncontrolling interests									(130,786)	(53,084)	(25,129)
Net earnings (loss) attributable to IAC shareholders									537,757	418,941	190,578
Comprehensive income attributable to IAC shareholders									515,766	$ 498,032	$ 145,039
Foreign cash repatriated									$ 25,200