GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
ASSETS
Cash and cash equivalents	$ 3,151,959	$ 2,131,632	$ 1,644,829	$ 1,630,809
Marketable securities	161,749	123,665
Accounts receivable, net of allowance and reserves	365,495	279,189
Other current assets	234,884	228,253
Intercompany receivables	0	0
Right-of-use assets	180,290	0
Property and equipment, net of accumulated depreciation and amortization	361,550	318,800
Goodwill	2,892,962	2,726,859
Intangible assets, net of accumulated amortization	628,418	631,422
Investment in subsidiaries	0	0
Other non-current assets	321,798	434,765
TOTAL ASSETS	8,299,105	6,874,585
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	13,750	13,750
Accounts payable, trade	104,392	74,907
Other current liabilities	835,320	794,901
Long-term debt, net	3,138,531	2,245,548
Income taxes payable	36,533	37,584
Intercompany liabilities	0	0
Other long-term liabilities	246,480	90,407
Redeemable noncontrolling interests	80,502	65,687
Shareholders' equity	2,983,461	2,843,125
Noncontrolling interests	860,136	708,676
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	8,299,105	6,874,585
IAC
LIABILITIES AND SHAREHOLDERS' EQUITY
Long-term debt, net	1,297,567	484,666
Reportable Legal Entities | IAC
ASSETS
Cash and cash equivalents	2,133,905	1,018,082
Marketable securities	0	98,299
Accounts receivable, net of allowance and reserves	0	0
Other current assets	24,852	39,449
Intercompany receivables	0	0
Right-of-use assets	10,861
Property and equipment, net of accumulated depreciation and amortization	6,962	6,526
Goodwill	0	0
Intangible assets, net of accumulated amortization	0	0
Investment in subsidiaries	2,132,545	1,897,699
Other non-current assets	294,891	274,789
TOTAL ASSETS	4,604,016	3,334,844
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	0	0
Accounts payable, trade	2,079	1,304
Other current liabilities	25,552	41,721
Long-term debt, net	34,291	34,262
Income taxes payable	0	15
Intercompany liabilities	1,548,070	414,156
Other long-term liabilities	10,563	261
Redeemable noncontrolling interests	0	0
Shareholders' equity	2,983,461	2,843,125
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	4,604,016	3,334,844
Reportable Legal Entities | Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	0	0
Marketable securities	0	0
Accounts receivable, net of allowance and reserves	90,545	99,970
Other current assets	38,163	29,222
Intercompany receivables	1,414,117	1,423,456
Right-of-use assets	40,151
Property and equipment, net of accumulated depreciation and amortization	171,593	163,281
Goodwill	412,009	412,009
Intangible assets, net of accumulated amortization	43,712	43,914
Investment in subsidiaries	427,476	214,519
Other non-current assets	88,523	94,290
TOTAL ASSETS	2,726,289	2,480,661
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	0	0
Accounts payable, trade	43,999	36,293
Other current liabilities	83,694	95,405
Long-term debt, net	0	0
Income taxes payable	1,612	1,707
Intercompany liabilities	0	0
Other long-term liabilities	54,227	18,181
Redeemable noncontrolling interests	0	0
Shareholders' equity	2,542,757	2,329,075
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	2,726,289	2,480,661
Reportable Legal Entities | Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	1,018,054	1,113,550
Marketable securities	161,749	25,366
Accounts receivable, net of allowance and reserves	274,950	179,219
Other current assets	172,650	171,682
Intercompany receivables	133,953	0
Right-of-use assets	155,833
Property and equipment, net of accumulated depreciation and amortization	182,995	148,993
Goodwill	2,480,953	2,314,850
Intangible assets, net of accumulated amortization	584,706	587,508
Investment in subsidiaries	0	0
Other non-current assets	97,554	251,315
TOTAL ASSETS	5,263,397	4,792,483
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	13,750	13,750
Accounts payable, trade	58,314	37,310
Other current liabilities	731,436	669,875
Long-term debt, net	3,104,240	2,211,286
Income taxes payable	34,921	35,862
Intercompany liabilities	0	1,009,300
Other long-term liabilities	362,834	257,594
Redeemable noncontrolling interests	80,502	65,687
Shareholders' equity	17,264	(216,857)
Noncontrolling interests	860,136	708,676
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	5,263,397	4,792,483
Eliminations
ASSETS
Cash and cash equivalents	0	0
Marketable securities	0	0
Accounts receivable, net of allowance and reserves	0	0
Other current assets	(781)	(12,100)
Intercompany receivables	(1,548,070)	(1,423,456)
Right-of-use assets	(26,555)
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	(2,560,021)	(2,112,218)
Other non-current assets	(159,170)	(185,629)
TOTAL ASSETS	(4,294,597)	(3,733,403)
LIABILITIES AND SHAREHOLDERS' EQUITY
Current portion of long-term debt	0	0
Accounts payable, trade	0	0
Other current liabilities	(5,362)	(12,100)
Long-term debt, net	0	0
Income taxes payable	0	0
Intercompany liabilities	(1,548,070)	(1,423,456)
Other long-term liabilities	(181,144)	(185,629)
Redeemable noncontrolling interests	0	0
Shareholders' equity	(2,560,021)	(2,112,218)
Noncontrolling interests	0	0
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ (4,294,597)	$ (3,733,403)