GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
ASSETS
Cash and cash equivalents	$ 1,670,984	$ 1,630,809	$ 1,255,317	$ 1,329,187
Marketable securities	208,555	4,995
Accounts receivable, net of allowance	347,158	304,027
Other current assets	246,197	185,374
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	308,465	315,170
Goodwill	2,572,221	2,559,066
Intangible assets, net of accumulated amortization	624,102	663,737
Investment in subsidiaries	0	0
Other non-current assets	394,665	204,632
TOTAL ASSETS	6,372,347	5,867,810
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	13,750	13,750
Accounts payable, trade	76,193	76,571
Other current liabilities	803,562	709,407
Long-term debt, net	1,983,993	1,979,469
Income taxes payable	25,241	25,624
Intercompany liabilities	0	0
Other long-term liabilities	71,486	73,299
Redeemable noncontrolling interests	69,530	42,867
Shareholders' equity (deficit)	2,600,482	2,430,028
Noncontrolling interests	728,110	516,795
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,372,347	5,867,810
Reportable Legal Entities | IAC
ASSETS
Cash and cash equivalents	739,157	585,639
Marketable securities	173,140	4,995
Accounts receivable, net of allowance	0	31
Other current assets	43,501	49,159
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	5,707	2,811
Goodwill	0	0
Intangible assets, net of accumulated amortization	0	0
Investment in subsidiaries	1,888,607	2,077,898
Other non-current assets	220,674	170,073
TOTAL ASSETS	3,070,786	2,890,606
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	0	0
Accounts payable, trade	884	5,163
Other current liabilities	39,385	29,489
Long-term debt, net	34,248	34,572
Income taxes payable	0	16
Intercompany liabilities	395,463	390,827
Other long-term liabilities	324	511
Redeemable noncontrolling interests	0	0
Shareholders' equity (deficit)	2,600,482	2,430,028
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	3,070,786	2,890,606
Reportable Legal Entities | Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	0	0
Marketable securities	0	0
Accounts receivable, net of allowance	100,837	109,289
Other current assets	32,036	33,387
Intercompany receivables	1,225,467	668,703
Property and equipment, net of accumulated depreciation and amortization	166,151	174,323
Goodwill	412,009	412,010
Intangible assets, net of accumulated amortization	73,524	74,852
Investment in subsidiaries	198,380	554,998
Other non-current assets	83,101	87,306
TOTAL ASSETS	2,291,505	2,114,868
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	0	0
Accounts payable, trade	38,487	30,469
Other current liabilities	91,104	88,050
Long-term debt, net	0	0
Income taxes payable	1,649	1,605
Intercompany liabilities	0	0
Other long-term liabilities	18,059	18,613
Redeemable noncontrolling interests	0	0
Shareholders' equity (deficit)	2,142,206	1,976,131
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	2,291,505	2,114,868
Reportable Legal Entities | Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	931,827	1,045,170
Marketable securities	35,415	0
Accounts receivable, net of allowance	246,321	194,707
Other current assets	170,660	102,828
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	136,607	138,036
Goodwill	2,160,212	2,147,056
Intangible assets, net of accumulated amortization	550,578	588,885
Investment in subsidiaries	0	0
Other non-current assets	223,597	79,688
TOTAL ASSETS	4,455,217	4,296,370
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	13,750	13,750
Accounts payable, trade	36,822	40,939
Other current liabilities	673,073	591,868
Long-term debt, net	1,949,745	1,944,897
Income taxes payable	23,592	24,003
Intercompany liabilities	830,004	277,876
Other long-term liabilities	185,810	186,610
Redeemable noncontrolling interests	69,530	42,867
Shareholders' equity (deficit)	(55,219)	656,765
Noncontrolling interests	728,110	516,795
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	4,455,217	4,296,370
Eliminations
ASSETS
Cash and cash equivalents	0	0
Marketable securities	0	0
Accounts receivable, net of allowance	0	0
Other current assets	0	0
Intercompany receivables	(1,225,467)	(668,703)
Property and equipment, net of accumulated depreciation and amortization	0	0
Goodwill	0	0
Intangible assets, net of accumulated amortization	0	0
Investment in subsidiaries	(2,086,987)	(2,632,896)
Other non-current assets	(132,707)	(132,435)
TOTAL ASSETS	(3,445,161)	(3,434,034)
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	0	0
Accounts payable, trade	0	0
Other current liabilities	0	0
Long-term debt, net	0	0
Income taxes payable	0	0
Intercompany liabilities	(1,225,467)	(668,703)
Other long-term liabilities	(132,707)	(132,435)
Redeemable noncontrolling interests	0	0
Shareholders' equity (deficit)	(2,086,987)	(2,632,896)
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ (3,445,161)	$ (3,434,034)