GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
ASSETS
Cash and cash equivalents	$ 1,657,537	$ 1,630,809	$ 1,397,038	$ 1,329,187
Marketable securities	5,630	4,995
Accounts receivable, net of allowance	325,263	304,027
Other current assets	234,502	185,374
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	301,865	315,170
Goodwill	2,601,210	2,559,066
Intangible assets, net of accumulated amortization	653,205	663,737
Investment in subsidiaries	0	0
Other non-current assets	251,762	204,632
TOTAL ASSETS	6,030,974	5,867,810
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	14,120	13,750
Accounts payable, trade	79,588	76,571
Other current liabilities	735,785	709,407
Long-term debt, net	1,980,579	1,979,469
Income taxes payable	24,076	25,624
Intercompany liabilities	0	0
Other long-term liabilities	67,336	73,299
Redeemable noncontrolling interests	47,099	42,867
IAC shareholders' equity	2,494,527	2,430,028
Noncontrolling interests	587,864	516,795
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,030,974	5,867,810
IAC
ASSETS
Cash and cash equivalents	895,406	585,639
Marketable securities	4,990	4,995
Accounts receivable, net of allowance	0	31
Other current assets	46,576	49,159
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	2,457	2,811
Goodwill	0	0
Intangible assets, net of accumulated amortization	0	0
Investment in subsidiaries	1,789,479	2,076,004
Other non-current assets	226,063	170,073
TOTAL ASSETS	2,964,971	2,888,712
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	370	0
Accounts payable, trade	1,509	5,163
Other current liabilities	22,053	29,489
Long-term debt, net	34,216	34,572
Income taxes payable	12	16
Intercompany liabilities	411,831	388,933
Other long-term liabilities	453	511
Redeemable noncontrolling interests	0	0
IAC shareholders' equity	2,494,527	2,430,028
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	2,964,971	2,888,712
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	0	0
Marketable securities	0	0
Accounts receivable, net of allowance	113,977	109,289
Other current assets	26,567	33,387
Intercompany receivables	994,153	668,703
Property and equipment, net of accumulated depreciation and amortization	171,681	174,323
Goodwill	412,010	412,010
Intangible assets, net of accumulated amortization	74,343	74,852
Investment in subsidiaries	300,030	554,998
Other non-current assets	87,067	87,306
TOTAL ASSETS	2,179,828	2,114,868
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	0	0
Accounts payable, trade	35,916	30,469
Other current liabilities	88,329	88,050
Long-term debt, net	0	0
Income taxes payable	1,528	1,605
Intercompany liabilities	0	0
Other long-term liabilities	17,432	18,613
Redeemable noncontrolling interests	0	0
IAC shareholders' equity	2,036,623	1,976,131
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	2,179,828	2,114,868
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	762,131	1,045,170
Marketable securities	640	0
Accounts receivable, net of allowance	211,286	194,707
Other current assets	161,359	102,828
Intercompany receivables	0	0
Property and equipment, net of accumulated depreciation and amortization	127,727	138,036
Goodwill	2,189,200	2,147,056
Intangible assets, net of accumulated amortization	578,862	588,885
Investment in subsidiaries	0	0
Other non-current assets	84,436	79,688
TOTAL ASSETS	4,115,641	4,296,370
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	13,750	13,750
Accounts payable, trade	42,163	40,939
Other current liabilities	625,403	591,868
Long-term debt, net	1,946,363	1,944,897
Income taxes payable	22,536	24,003
Intercompany liabilities	582,322	279,770
Other long-term liabilities	195,255	186,610
Redeemable noncontrolling interests	47,099	42,867
IAC shareholders' equity	52,886	654,871
Noncontrolling interests	587,864	516,795
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	4,115,641	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	(994,153)	(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	(2,089,509)	(2,631,002)
Other non-current assets	(145,804)	(132,435)
TOTAL ASSETS	(3,229,466)	(3,432,140)
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	(994,153)	(668,703)
Other long-term liabilities	(145,804)	(132,435)
Redeemable noncontrolling interests	0	0
IAC shareholders' equity	(2,089,509)	(2,631,002)
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ (3,229,466)	$ (3,432,140)