ALLOWANCE FOR CREDIT LOSSES (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($)
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance at December 31, 2019	$ 4,548
Current-period provision for expected credit losses	15,688
Write-offs charged against the allowance, net of recoveries and other	(6,873)
Ending balance at June 30, 2020	13,363
CoStar Suite [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance at December 31, 2019	1,264
Current-period provision for expected credit losses	7,046
Write-offs charged against the allowance, net of recoveries and other	(3,450)
Ending balance at June 30, 2020	4,860
Information services [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance at December 31, 2019	624
Current-period provision for expected credit losses	2,975
Write-offs charged against the allowance, net of recoveries and other	(76)
Ending balance at June 30, 2020	3,523
Multifamily [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance at December 31, 2019	1,195
Current-period provision for expected credit losses	3,677
Write-offs charged against the allowance, net of recoveries and other	(2,152)
Ending balance at June 30, 2020	2,720
Commercial property and land [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance at December 31, 2019	1,465
Current-period provision for expected credit losses	1,990
Write-offs charged against the allowance, net of recoveries and other	(1,195)
Ending balance at June 30, 2020	$ 2,260