GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Condensed Financial Statements, Captions [Line Items]
Revenue	$ 1,105,843	$ 995,075
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	260,071	201,962
Selling and marketing expense	421,860	402,832
General and administrative expense	213,616	184,184
Product development expense	88,700	76,937
Depreciation	18,971	19,257
Amortization of intangibles	22,752	19,953
Total operating costs and expenses	1,025,970	905,125
Operating income	79,873	89,950
Equity in earnings of unconsolidated affiliates	0	0
Interest expense	(31,143)	(26,505)
Other (expense) income, net	651	(4,619)
Earnings before income taxes	49,381	58,826
Income tax benefit (provision)	63,604	29,013
Net earnings	112,985	87,839
Net earnings attributable to noncontrolling interests	(24,290)	(16,757)
Net earnings attributable to IAC shareholders	88,695	71,082
Comprehensive income (loss) attributable to IAC shareholders	89,690	99,439
Foreign earnings repatriated		276,000
Reportable Legal Entities | IAC
Condensed Financial Statements, Captions [Line Items]
Revenue	0	0
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	24	76
Selling and marketing expense	362	213
General and administrative expense	38,259	31,409
Product development expense	647	652
Depreciation	378	266
Amortization of intangibles	0	0
Total operating costs and expenses	39,670	32,616
Operating income	(39,670)	(32,616)
Equity in earnings of unconsolidated affiliates	115,440	102,750
Interest expense	(424)	(429)
Other (expense) income, net	(1,897)	(16,847)
Earnings before income taxes	73,449	52,858
Income tax benefit (provision)	15,246	18,224
Net earnings	88,695	71,082
Net earnings attributable to noncontrolling interests	0	0
Net earnings attributable to IAC shareholders	88,695	71,082
Comprehensive income (loss) attributable to IAC shareholders	89,690	99,439
Reportable Legal Entities | Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenue	198,930	212,889
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	82,146	56,224
Selling and marketing expense	57,966	90,138
General and administrative expense	10,534	15,381
Product development expense	12,037	14,269
Depreciation	2,922	3,340
Amortization of intangibles	3,877	509
Total operating costs and expenses	169,482	179,861
Operating income	29,448	33,028
Equity in earnings of unconsolidated affiliates	1,068	(327)
Interest expense	0	0
Other (expense) income, net	13,851	286,883
Earnings before income taxes	44,367	319,584
Income tax benefit (provision)	(5,123)	(10,966)
Net earnings	39,244	308,618
Net earnings attributable to noncontrolling interests	0	0
Net earnings attributable to IAC shareholders	39,244	308,618
Comprehensive income (loss) attributable to IAC shareholders	39,455	308,961
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Revenue	906,978	782,260
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	177,966	145,712
Selling and marketing expense	363,562	312,526
General and administrative expense	164,793	137,373
Product development expense	76,016	62,016
Depreciation	15,671	15,651
Amortization of intangibles	18,875	19,444
Total operating costs and expenses	816,883	692,722
Operating income	90,095	89,538
Equity in earnings of unconsolidated affiliates	0	0
Interest expense	(30,719)	(26,076)
Other (expense) income, net	420	2,190
Earnings before income taxes	59,796	65,652
Income tax benefit (provision)	53,481	21,755
Net earnings	113,277	87,407
Net earnings attributable to noncontrolling interests	(24,290)	(16,757)
Net earnings attributable to IAC shareholders	88,987	70,650
Comprehensive income (loss) attributable to IAC shareholders	90,100	105,327
Eliminations
Condensed Financial Statements, Captions [Line Items]
Revenue	(65)	(74)
Operating costs and expenses:
Cost of revenue (exclusive of depreciation shown separately below)	(65)	(50)
Selling and marketing expense	(30)	(45)
General and administrative expense	30	21
Product development expense	0	0
Depreciation	0	0
Amortization of intangibles	0	0
Total operating costs and expenses	(65)	(74)
Operating income	0	0
Equity in earnings of unconsolidated affiliates	(116,508)	(102,423)
Interest expense	0	0
Other (expense) income, net	(11,723)	(276,845)
Earnings before income taxes	(128,231)	(379,268)
Income tax benefit (provision)	0	0
Net earnings	(128,231)	(379,268)
Net earnings attributable to noncontrolling interests	0	0
Net earnings attributable to IAC shareholders	(128,231)	(379,268)
Comprehensive income (loss) attributable to IAC shareholders	$ (129,555)	$ (414,288)