(LOSS) EARNINGS PER SHARE - Summary (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Numerator: Basic
Net (loss) earnings	$ (182,643)	$ 112,985
Net earnings attributable to noncontrolling interests	(28,397)	(24,290)
Net (loss) earnings attributable to IAC shareholders	(211,040)	88,695
Numerator: Diluted
Net (loss) earnings	(182,643)	112,985
Net earnings attributable to noncontrolling interests	(28,397)	(24,290)
Impact from publicly-traded subsidiaries' dilutive securities	0	(6,696)
Net (loss) earnings attributable to IAC shareholders	$ (211,040)	$ 81,999
Denominator: Basic
Weighted average basic shares outstanding (shares)	84,839	83,905
Denominator: Diluted
Weighted average basic shares outstanding (shares)	84,839	83,905
Dilutive securities (shares)	0	6,435
Denominator for earnings per share - weighted average shares (shares)	84,839	90,340
Earnings per share attributable to IAC shareholders: Basic
Earnings per share (USD per share)	$ (2.49)	$ 1.06
Earnings per share attributable to IAC shareholders: Diluted
Earnings per share (USD per share)	$ (2.49)	0.91
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Average share price (USD per share)		$ 207.01
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)	19,700
Restricted Stock Units (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
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
Senior Notes \| 2022 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
Exchange price per share (USD per share)	$ 152.18
Senior Notes \| 2026 Exchangeable Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Exchange price per share (USD per share)	302.77
Senior Notes \| 2030 Exchangeable Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Exchange price per share (USD per share)	$ 291.35

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