GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Guarantor and Nonguarantor Financial Statements [Abstract]
Foreign earnings repatriated	$ 276,000
Condensed Financial Statements, Captions [Line Items]
Revenue	995,075	$ 760,833
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	201,962	145,958
Selling and marketing expense	402,832	350,411
General and administrative expense	184,184	143,595
Product development expense	76,937	54,760
Depreciation	19,257	19,888
Amortization of intangibles	19,953	9,161
Total operating costs and expenses	905,125	723,773
Operating income	89,950	37,060
Equity in earnings of unconsolidated affiliates	0	0
Interest expense	(26,505)	(24,792)
Other (expense) income, net (a)	(4,619)	(7,714)
Earnings before income taxes	58,826	4,554
Income tax benefit (provision)	29,013	23,909
Net earnings	87,839	28,463
Net earnings attributable to noncontrolling interests	(16,757)	(2,254)
Net earnings attributable to IAC shareholders	71,082	26,209
Comprehensive income (loss) attributable to IAC shareholders	99,439	44,987
IAC
Condensed Financial Statements, Captions [Line Items]
Revenue	0	0
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	76	113
Selling and marketing expense	213	326
General and administrative expense	31,409	26,136
Product development expense	652	570
Depreciation	266	438
Amortization of intangibles	0	0
Total operating costs and expenses	32,616	27,583
Operating income	(32,616)	(27,583)
Equity in earnings of unconsolidated affiliates	102,750	51,456
Interest expense	(429)	(5,828)
Other (expense) income, net (a)	(16,847)	(5,805)
Earnings before income taxes	52,858	12,240
Income tax benefit (provision)	18,224	13,969
Net earnings	71,082	26,209
Net earnings attributable to noncontrolling interests	0	0
Net earnings attributable to IAC shareholders	71,082	26,209
Comprehensive income (loss) attributable to IAC shareholders	99,439	44,987
Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenue	212,889	181,572
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	56,224	32,708
Selling and marketing expense	90,138	91,950
General and administrative expense	15,381	15,590
Product development expense	14,269	15,387
Depreciation	3,340	7,133
Amortization of intangibles	509	5,085
Total operating costs and expenses	179,861	167,853
Operating income	33,028	13,719
Equity in earnings of unconsolidated affiliates	(327)	(2,683)
Interest expense	0	0
Other (expense) income, net (a)	286,883	6,091
Earnings before income taxes	319,584	17,127
Income tax benefit (provision)	(10,966)	(9,689)
Net earnings	308,618	7,438
Net earnings attributable to noncontrolling interests	0	0
Net earnings attributable to IAC shareholders	308,618	7,438
Comprehensive income (loss) attributable to IAC shareholders	308,961	11,491
Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenue	782,260	579,474
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	145,712	113,310
Selling and marketing expense	312,526	258,189
General and administrative expense	137,373	101,855
Product development expense	62,016	38,803
Depreciation	15,651	12,317
Amortization of intangibles	19,444	4,076
Total operating costs and expenses	692,722	528,550
Operating income	89,538	50,924
Equity in earnings of unconsolidated affiliates	0	0
Interest expense	(26,076)	(18,964)
Other (expense) income, net (a)	2,190	(8,000)
Earnings before income taxes	65,652	23,960
Income tax benefit (provision)	21,755	19,629
Net earnings	87,407	43,589
Net earnings attributable to noncontrolling interests	(16,757)	(2,254)
Net earnings attributable to IAC shareholders	70,650	41,335
Comprehensive income (loss) attributable to IAC shareholders	105,327	63,768
Eliminations
Condensed Financial Statements, Captions [Line Items]
Revenue	(74)	(213)
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	(50)	(173)
Selling and marketing expense	(45)	(54)
General and administrative expense	21	14
Product development expense	0	0
Depreciation	0	0
Amortization of intangibles	0	0
Total operating costs and expenses	(74)	(213)
Operating income	0	0
Equity in earnings of unconsolidated affiliates	(102,423)	(48,773)
Interest expense	0	0
Other (expense) income, net (a)	(276,845)	0
Earnings before income taxes	(379,268)	(48,773)
Income tax benefit (provision)	0	0
Net earnings	(379,268)	(48,773)
Net earnings attributable to noncontrolling interests	0	0
Net earnings attributable to IAC shareholders	(379,268)	(48,773)
Comprehensive income (loss) attributable to IAC shareholders	$ (414,288)	$ (75,259)