ACQUISITIONS (Schedule of Recognized Identified Assets Acquired and Liabilities Assumed) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	May 24, 2021	Dec. 31, 2020	Dec. 22, 2020	Jun. 24, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Goodwill	$ 2,293,514		$ 2,235,999			$ 1,882,020
Deferred tax liabilities	$ (1,000)
Homes.com
Business Acquisition [Line Items]
Cash and cash equivalents		$ 0
Accounts receivable		1,798
Lease right-of-use assets		371
Goodwill		88,132
Intangible assets		53,400
Deferred tax assets		11,171
Lease liabilities		(371)
Deferred revenue		(1,086)
Other assets and liabilities		(1,240)
Fair value of identifiable net assets acquired		152,175
Homes.com | Previously Reported
Business Acquisition [Line Items]
Cash and cash equivalents		0
Accounts receivable		1,798
Lease right-of-use assets		371
Goodwill		86,314
Intangible assets		53,400
Deferred tax assets		11,171
Lease liabilities		(371)
Deferred revenue		(1,086)
Other assets and liabilities		(1,240)
Fair value of identifiable net assets acquired		150,357
Homes.com | Measurement Period Adjustments
Business Acquisition [Line Items]
Goodwill		1,818
Fair value of identifiable net assets acquired		$ 1,818
Homesnap
Business Acquisition [Line Items]
Cash and cash equivalents				$ 10,225
Accounts receivable				662
Lease right-of-use assets				3,437
Goodwill				184,371
Intangible assets				67,000
Deferred tax liabilities				(2,778)
Lease liabilities				(3,375)
Deferred revenue				(4,000)
Other assets and liabilities				(5,188)
Fair value of identifiable net assets acquired				250,354
Homesnap | Previously Reported
Business Acquisition [Line Items]
Cash and cash equivalents				10,225
Accounts receivable				662
Lease right-of-use assets				3,437
Goodwill				183,016
Intangible assets				67,000
Deferred tax liabilities				(1,423)
Lease liabilities				(3,375)
Deferred revenue				(4,000)
Other assets and liabilities				(5,188)
Fair value of identifiable net assets acquired				250,354
Homesnap | Measurement Period Adjustments
Business Acquisition [Line Items]
Goodwill				1,355
Deferred tax liabilities				(1,355)
Fair value of identifiable net assets acquired				$ 0
Ten-X
Business Acquisition [Line Items]
Cash and cash equivalents					$ 3,290
Accounts receivable					131
Lease right-of-use assets					4,945
Goodwill					134,322
Intangible assets					58,000
Deferred tax liabilities					(2,981)
Lease liabilities					(4,945)
Other assets and liabilities					(5,047)
Fair value of identifiable net assets acquired					$ 187,715