EARNINGS (LOSS) PER SHARE - Summary (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended								12 Months Ended
	Oct. 02, 2017	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
Earnings Per Share [Abstract]
Net earnings (loss)										$ 757,747	$ 358,008	$ (16,151)
Net earnings attributable to noncontrolling interests										(130,786)	(53,084)	(25,129)
Impact from public subsidiaries' dilutive securities										(25,228)	(33,531)	0
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)
Net earnings (loss) attributable to IAC shareholders										$ 601,733	$ 271,393	$ (41,280)
Weighted average basic shares outstanding (shares)										83,407	80,089	80,045
Dilutive securities including subsidiary denominated equity, stock options and RSUs (shares)										7,915	5,221	0
Weighted average diluted shares outstanding (shares)										91,322	85,310	80,045
Earnings (loss) per share attributable to IAC shareholders: Basic
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)
Earnings (loss) per share attributable to IAC shareholders: Diluted
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)
Anti-dilutive weighted average common shares
Potentially dilutive securities excluded from calculation of diluted earnings per share (shares)												11,300
Exchange price (USD per share)										$ 152.18
RSUs
Anti-dilutive weighted average common shares
Potentially dilutive securities excluded from calculation of diluted earnings per share (shares)										3,500	6,900
PSUs
Anti-dilutive weighted average common shares
Potentially dilutive securities excluded from calculation of diluted earnings per share (shares)										100	100
Exchangeable Notes
Anti-dilutive weighted average common shares
Potentially dilutive securities excluded from calculation of diluted earnings per share (shares)										300
IAC | Senior Notes
Anti-dilutive weighted average common shares
Exchange price (USD per share)	$ 152.18									$ 152.18