ALLOWANCE FOR CREDIT LOSSES (Details) $ in Thousands	3 Months Ended
	Mar. 31, 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	6,183
Write-offs charged against the allowance, net of recoveries and other	(2,420)
Ending balance at March 31, 2020	8,311
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	2,634
Write-offs charged against the allowance, net of recoveries and other	(1,372)
Ending balance at March 31, 2020	2,526
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	1,247
Write-offs charged against the allowance, net of recoveries and other	0
Ending balance at March 31, 2020	1,871
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	1,403
Write-offs charged against the allowance, net of recoveries and other	(565)
Ending balance at March 31, 2020	2,033
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	899
Write-offs charged against the allowance, net of recoveries and other	(483)
Ending balance at March 31, 2020	$ 1,881