GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
ASSETS
Cash and cash equivalents	$ 1,644,829	$ 1,630,809	$ 1,522,300	$ 1,329,187
Marketable securities	120,410	4,995
Accounts receivable, net of allowance	343,576	304,027
Other current assets	237,957	185,374
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	306,602	315,170
Goodwill	2,578,296	2,559,066
Intangible assets, net of accumulated amortization	636,351	663,737
Investment in subsidiaries	0	0
Other non-current assets	367,263	204,632
TOTAL ASSETS	6,235,284	5,867,810
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	13,750	13,750
Accounts payable, trade	79,107	76,571
Other current liabilities	752,823	709,407
Long-term debt, net	1,982,271	1,979,469
Income taxes payable	23,942	25,624
Intercompany liabilities	0	0
Other long-term liabilities	70,724	73,299
Redeemable noncontrolling interests	75,719	42,867
Shareholders' equity (deficit)	2,576,079	2,430,028
Noncontrolling interests	660,869	516,795
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,235,284	5,867,810
Reportable Legal Entities | IAC
ASSETS
Cash and cash equivalents	813,872	585,639
Marketable securities	119,746	4,995
Accounts receivable, net of allowance	0	31
Other current assets	45,720	49,159
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	4,306	2,811
Goodwill	0	0
Intangible assets, net of accumulated amortization	0	0
Investment in subsidiaries	1,828,822	2,077,898
Other non-current assets	227,459	170,073
TOTAL ASSETS	3,039,925	2,890,606
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	0	0
Accounts payable, trade	642	5,163
Other current liabilities	28,128	29,489
Long-term debt, net	34,233	34,572
Income taxes payable	0	16
Intercompany liabilities	400,448	390,827
Other long-term liabilities	395	511
Redeemable noncontrolling interests	0	0
Shareholders' equity (deficit)	2,576,079	2,430,028
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	3,039,925	2,890,606
Reportable Legal Entities | Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	0	0
Marketable securities	0	0
Accounts receivable, net of allowance	118,672	109,289
Other current assets	26,123	33,387
Intercompany receivables	1,129,828	668,703
Property and equipment, net of accumulated depreciation and amortization	168,685	174,323
Goodwill	412,010	412,010
Intangible assets, net of accumulated amortization	73,933	74,852
Investment in subsidiaries	202,616	554,998
Other non-current assets	83,005	87,306
TOTAL ASSETS	2,214,872	2,114,868
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	0	0
Accounts payable, trade	37,027	30,469
Other current liabilities	89,902	88,050
Long-term debt, net	0	0
Income taxes payable	1,434	1,605
Intercompany liabilities	0	0
Other long-term liabilities	18,779	18,613
Redeemable noncontrolling interests	0	0
Shareholders' equity (deficit)	2,067,730	1,976,131
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	2,214,872	2,114,868
Reportable Legal Entities | Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	830,957	1,045,170
Marketable securities	664	0
Accounts receivable, net of allowance	224,904	194,707
Other current assets	166,114	102,828
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	133,611	138,036
Goodwill	2,166,286	2,147,056
Intangible assets, net of accumulated amortization	562,418	588,885
Investment in subsidiaries	0	0
Other non-current assets	222,638	79,688
TOTAL ASSETS	4,307,592	4,296,370
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	13,750	13,750
Accounts payable, trade	41,438	40,939
Other current liabilities	634,793	591,868
Long-term debt, net	1,948,038	1,944,897
Income taxes payable	22,508	24,003
Intercompany liabilities	729,380	277,876
Other long-term liabilities	217,389	186,610
Redeemable noncontrolling interests	75,719	42,867
Shareholders' equity (deficit)	(36,292)	656,765
Noncontrolling interests	660,869	516,795
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	4,307,592	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	(1,129,828)	(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	(2,031,438)	(2,632,896)
Other non-current assets	(165,839)	(132,435)
TOTAL ASSETS	(3,327,105)	(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	(1,129,828)	(668,703)
Other long-term liabilities	(165,839)	(132,435)
Redeemable noncontrolling interests	0	0
Shareholders' equity (deficit)	(2,031,438)	(2,632,896)
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ (3,327,105)	$ (3,434,034)