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DISCONTINUED OPERATIONS - Narrative (Details) $ in Thousands					1 Months Ended				3 Months Ended				12 Months Ended
	Jun. 17, 2020 USD ($)	Jun. 11, 2020 USD ($)	Dec. 04, 2019 USD ($)	Jun. 21, 2019 USD ($)	Jul. 31, 2020 USD ($)	Feb. 29, 2016 defendant	Jan. 31, 2016 USD ($)	Dec. 31, 2013 network_lender	Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 24, 2020	Jul. 21, 2019 USD ($)	May 31, 2017	Jun. 06, 2012 USD ($)
Loan Loss Obligations
Assets									$ 1,266,637	$ 1,188,990
2025 Convertible Notes \| Convertible Debt
Loan Loss Obligations
Stated interest rate														0.50%
2022 Convertible Notes \| Convertible Debt
Loan Loss Obligations
Stated interest rate																0.625%
Home Loan Center, Inc.
Loan Loss Obligations
Assets															$ 11,200
Cash															$ 5,900
Dividends							$ 40,000
Discover Bank \| Lending Tree Loans
Loan Loss Obligations
Asset purchase agreement proceeds from sale																	$ 55,900
Residential Funding Co. v Home Loan Center \| Pending litigation or appeal
Loan Loss Obligations
Number of loan originators \| network_lender								80
Residential Funding Co. v Home Loan Center \| Judicial Ruling
Loan Loss Obligations
Settlement offer				$ 68,500
Residential Funding Co. v Home Loan Center \| Settled \| ResCap \| Damages
Loan Loss Obligations
Settlement offer	$ 58,500
Loss contingency settlement payment										$ 6,400	$ 26,500
Loss contingency, refund received									$ 8,600
Lehman Brothers Holdings, Inc. Demand Letter
Loan Loss Obligations
Damages sought			$ 44,700
Lehman Brothers Holdings, Inc. Demand Letter \| Pending litigation or appeal
Loan Loss Obligations
Number of defendants \| defendant						149
Lehman Brothers Holdings Inc. v. LendingTree, LLC
Loan Loss Obligations
Damages sought		$ 13,300
Discontinued Operations, Disposed of by Means Other than Sale \| Home Loan Center, Inc.
Loan Loss Obligations
Loss recognized													$ 5,500
Net gain on disposal													4,500
Recognition of liability													$ 10,000
Discontinued Operations, Disposed of by Means Other than Sale \| Obligation to Repurchase Receivables Sold \| Lending Tree Loans
Loan Loss Obligations
Loss contingency settlement payment					$ 36,000
Discontinued Operations, Disposed of by Means Other than Sale \| Pending litigation or appeal \| Obligation to Repurchase Receivables Sold \| Lending Tree Loans \| Discontinued Operations
Loan Loss Obligations
Settlement offer												$ 36,000