QUARTERLY RESULTS (UNAUDITED) - Summary (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Data [Abstract]
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
Cost of revenue	253,722	237,238	218,224	201,962	199,727	166,290	139,033	145,958	911,146	651,008	755,730
Operating income	133,920	172,832	168,437	89,950	94,360	(18,589)	75,635	37,060	565,139	188,466	(32,625)
Net earnings	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 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 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 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									$ 238,420	$ 264,618	$ 104,820
Gain (loss) on sale of equity investments			$ 133,300
Impairment of intangible assets	$ 21,300
Income tax expense (benefit)						$ 257,000			$ 3,811	(291,050)	$ (64,934)
Dictionary.com, Electus, Felix, and CityGrid | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Per share information attributable to IAC shareholders:
Gain on sale of business	92,500
HomeAdvisor
Per share information attributable to IAC shareholders:
Stock-based compensation expense	$ 14,400	$ 12,300	12,800	$ 14,600	$ 15,800	60,900
Angie's List
Per share information attributable to IAC shareholders:
After-tax costs			2,000	4,100	13,900	$ 17,400				$ 13,900
Write-off due to deferred revenue			$ 1,800	$ 2,800	$ 7,600