EARNINGS PER SHARE - Summary (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator: Basic
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
Numerator: Diluted
Net earnings	112,985	87,839
Net earnings attributable to noncontrolling interests	(24,290)	(16,757)
Impact from public subsidiaries' dilutive securities	(6,696)	(7,442)
Net earnings attributable to IAC shareholders	$ 81,999	$ 63,640
Denominator: Basic
Weighted average basic shares outstanding (shares)	83,905	82,983
Denominator: Diluted
Weighted average basic shares outstanding (shares)	83,905	82,983
Dilutive securities (shares)	6,435	6,086
Denominator for earnings per share - weighted average shares (shares)	90,340	89,069
Earnings per share attributable to IAC shareholders: Basic
Basic earnings per share (USD per share)	$ 1.06	$ 0.86
Earnings per share attributable to IAC shareholders: Diluted
Diluted earnings per share (USD per share)	0.91	$ 0.71
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	6,800
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)	300	200
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)	900