EARNINGS PER SHARE - Summary (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator: Basic
Net earnings	$ 171,577	$ 225,639	$ 540,270	$ 334,659
Net earnings attributable to noncontrolling interests	(25,803)	(45,996)	(105,061)	(62,539)
Net earnings attributable to IAC shareholders	145,774	179,643	435,209	272,120
Numerator: Diluted
Net earnings	171,577	225,639	540,270	334,659
Net earnings attributable to noncontrolling interests	(25,803)	(45,996)	(105,061)	(62,539)
Impact from public subsidiaries' dilutive securities	(8,336)	(23,749)	(19,490)	(34,104)
Net earnings attributable to IAC shareholders	$ 137,438	$ 155,894	$ 415,719	$ 238,016
Denominator: Basic
Weighted average basic shares outstanding (shares)	83,433,000	80,817,000	83,342,000	79,369,000
Denominator: Diluted
Weighted average basic shares outstanding (shares)	83,433,000	80,817,000	83,342,000	79,369,000
Dilutive securities (shares)	8,542,000	6,379,000	8,076,000	5,133,000
Denominator for earnings per share - weighted average shares (shares)	91,975,000	87,196,000	91,418,000	84,502,000
Earnings per share attributable to IAC shareholders: Basic
Basic earnings per share (USD per share)	$ 1.75	$ 2.22	$ 5.22	$ 3.43
Earnings per share attributable to IAC shareholders: Diluted
Diluted earnings per share (USD per share)	1.49	$ 1.79	$ 4.55	$ 2.82
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Exchange price per share (USD per share)	$ 152.18
Stock options, warrants, and subsidiary denominated equity exchange of Exchangeable Notes and vesting of RSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted earnings per share (less than or equal to) (shares)	3,400,000	0	3,400,000	100,000
Market-based awards and PSUs
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted earnings per share (less than or equal to) (shares)	200,000	300,000	200,000	300,000
Exchangeable Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities excluded from calculation of diluted earnings per share (less than or equal to) (shares)	600,000		200,000