CONSOLIDATED STATEMENT OF OPERATIONS - 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
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)
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)
Per share information attributable to IAC shareholders:
Basic earnings (loss) per share (USD per share)	$ 2.29	$ 1.75	$ 2.61	$ 0.86	$ 0.40	$ 2.22	$ 0.84	$ 0.34	$ 7.52	$ 3.81	$ (0.52)
Diluted earnings (loss) per share (USD per share)	$ 2.04	$ 1.49	$ 2.32	$ 0.71	$ 0.37	$ 1.79	$ 0.70	$ 0.29	$ 6.59	$ 3.18	$ (0.52)
Stock-based compensation expense by function:
Stock-based compensation expense									$ 238,420	$ 264,618	$ 104,820
Cost of revenue
Stock-based compensation expense by function:
Stock-based compensation expense									2,482	1,881	2,305
Selling and marketing expense
Stock-based compensation expense by function:
Stock-based compensation expense									7,943	31,318	6,000
General and administrative expense
Stock-based compensation expense by function:
Stock-based compensation expense									188,510	192,957	77,151
Product development expense
Stock-based compensation expense by function:
Stock-based compensation expense									$ 39,485	$ 38,462	$ 19,364