GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Condensed Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Balance Sheet
Cash and cash equivalents	$ 2,131,632	$ 1,630,809	$ 1,329,187	$ 1,481,447
Marketable securities	123,665	4,995
Accounts receivable, net of allowance and reserves	279,189	304,027
Other current assets	228,253	185,374
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	318,800	315,170
Goodwill	2,726,859	2,559,066	$ 1,924,052
Intangible assets, net of accumulated amortization	631,422	663,737
Investment in subsidiaries	0	0
Other non-current assets	434,765	204,632
TOTAL ASSETS	6,874,585	5,867,810
Current portion of long-term debt	13,750	13,750
Accounts payable, trade	74,907	76,571
Other current liabilities	794,901	709,407
Long-term debt, net	2,245,548	1,979,469
Income taxes payable	37,584	25,624
Intercompany liabilities	0	0
Other long-term liabilities	90,407	73,299
Redeemable noncontrolling interests	65,687	42,867
Shareholders' equity (deficit)	2,843,125	2,430,028
Noncontrolling interests	708,676	516,795
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,874,585	5,867,810
Eliminations
Condensed Balance Sheet
Cash and cash equivalents	0	0
Marketable securities	0	0
Accounts receivable, net of allowance and reserves	0	0
Other current assets	0	0
Intercompany receivables	(1,423,456)	(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	(2,112,218)	(2,632,896)
Other non-current assets	(185,629)	(132,435)
TOTAL ASSETS	(3,721,303)	(3,434,034)
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	(1,423,456)	(668,703)
Other long-term liabilities	(185,629)	(132,435)
Redeemable noncontrolling interests	0	0
Shareholders' equity (deficit)	(2,112,218)	(2,632,896)
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	(3,721,303)	(3,434,034)
IAC
Condensed Balance Sheet
Long-term debt, net	484,666	468,461
IAC | Reportable Legal Entities
Condensed Balance Sheet
Cash and cash equivalents	1,018,082	585,639
Marketable securities	98,299	4,995
Accounts receivable, net of allowance and reserves	0	31
Other current assets	27,349	49,159
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	6,526	2,811
Goodwill	0	0
Intangible assets, net of accumulated amortization	0	0
Investment in subsidiaries	1,897,699	2,077,898
Other non-current assets	274,789	170,073
TOTAL ASSETS	3,322,744	2,890,606
Current portion of long-term debt	0	0
Accounts payable, trade	1,304	5,163
Other current liabilities	41,721	29,489
Long-term debt, net	34,262	34,572
Income taxes payable	15	16
Intercompany liabilities	402,056	390,827
Other long-term liabilities	261	511
Redeemable noncontrolling interests	0	0
Shareholders' equity (deficit)	2,843,125	2,430,028
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	3,322,744	2,890,606
Guarantor Subsidiaries | Reportable Legal Entities
Condensed Balance Sheet
Cash and cash equivalents	0	0
Marketable securities	0	0
Accounts receivable, net of allowance and reserves	99,970	109,289
Other current assets	29,222	33,387
Intercompany receivables	1,423,456	668,703
Property and equipment, net of accumulated depreciation and amortization	163,281	174,323
Goodwill	412,009	412,010
Intangible assets, net of accumulated amortization	43,914	74,852
Investment in subsidiaries	214,519	554,998
Other non-current assets	94,290	87,306
TOTAL ASSETS	2,480,661	2,114,868
Current portion of long-term debt	0	0
Accounts payable, trade	36,293	30,469
Other current liabilities	95,405	88,050
Long-term debt, net	0	0
Income taxes payable	1,707	1,605
Intercompany liabilities	0	0
Other long-term liabilities	18,181	18,613
Redeemable noncontrolling interests	0	0
Shareholders' equity (deficit)	2,329,075	1,976,131
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	2,480,661	2,114,868
Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Balance Sheet
Cash and cash equivalents	1,113,550	1,045,170
Marketable securities	25,366	0
Accounts receivable, net of allowance and reserves	179,219	194,707
Other current assets	171,682	102,828
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	148,993	138,036
Goodwill	2,314,850	2,147,056
Intangible assets, net of accumulated amortization	587,508	588,885
Investment in subsidiaries	0	0
Other non-current assets	251,315	79,688
TOTAL ASSETS	4,792,483	4,296,370
Current portion of long-term debt	13,750	13,750
Accounts payable, trade	37,310	40,939
Other current liabilities	657,775	591,868
Long-term debt, net	2,211,286	1,944,897
Income taxes payable	35,862	24,003
Intercompany liabilities	1,021,400	277,876
Other long-term liabilities	257,594	186,610
Redeemable noncontrolling interests	65,687	42,867
Shareholders' equity (deficit)	(216,857)	656,765
Noncontrolling interests	708,676	516,795
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 4,792,483	$ 4,296,370