ALLOWANCE FOR CREDIT LOSSES (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 12.2	$ 13.4	$ 15.2
Current-period provision (release) for expected credit losses	35.0	18.3	10.9
Write-offs charged against the allowance, net of recoveries and other	(24.0)	(19.5)	(12.7)
Ending balance	23.2	12.2	13.4
CoStar
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.5	5.4	5.6
Current-period provision (release) for expected credit losses	22.1	9.2	5.6
Write-offs charged against the allowance, net of recoveries and other	(16.9)	(10.1)	(5.8)
Ending balance	9.7	4.5	5.4
Information services
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.1	1.8	2.7
Current-period provision (release) for expected credit losses	1.4	(0.6)	(0.4)
Write-offs charged against the allowance, net of recoveries and other	0.0	(0.1)	(0.5)
Ending balance	2.5	1.1	1.8
Multifamily
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.3	3.4	4.4
Current-period provision (release) for expected credit losses	4.9	5.8	3.1
Write-offs charged against the allowance, net of recoveries and other	(1.9)	(4.9)	(4.1)
Ending balance	7.3	4.3	3.4
LoopNet
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.4	2.0	1.7
Current-period provision (release) for expected credit losses	6.0	3.8	2.6
Write-offs charged against the allowance, net of recoveries and other	(4.7)	(4.4)	(2.3)
Ending balance	2.7	1.4	2.0
Other Marketplaces
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0.9	0.8	0.8
Current-period provision (release) for expected credit losses	0.6	0.1	0.0
Write-offs charged against the allowance, net of recoveries and other	(0.5)	0.0	0.0
Ending balance	$ 1.0	$ 0.9	$ 0.8