REVENUE FROM CONTRACTS WITH CUSTOMERS (Disaggregated Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Revenues	$ 605,906	$ 536,308	$ 1,190,272	$ 1,052,133
CoStar
Disaggregation of Revenue [Line Items]
Revenues	229,169	206,566	454,182	405,215
Information Services
Disaggregation of Revenue [Line Items]
Revenues	41,921	38,502	83,550	75,717
Multifamily
Disaggregation of Revenue [Line Items]
Revenues	224,291	182,359	434,988	357,836
LoopNet
Disaggregation of Revenue [Line Items]
Revenues	65,563	56,297	128,800	110,744
Residential
Disaggregation of Revenue [Line Items]
Revenues	12,708	20,154	25,861	38,214
Other Marketplaces
Disaggregation of Revenue [Line Items]
Revenues	32,254	32,430	62,891	64,407
North America
Disaggregation of Revenue [Line Items]
Revenues	584,307	517,437	1,147,886	1,015,014
North America | CoStar
Disaggregation of Revenue [Line Items]
Revenues	219,573	197,380	435,386	386,484
North America | Information Services
Disaggregation of Revenue [Line Items]
Revenues	32,213	30,511	64,313	60,782
North America | Multifamily
Disaggregation of Revenue [Line Items]
Revenues	224,291	182,359	434,988	357,836
North America | LoopNet
Disaggregation of Revenue [Line Items]
Revenues	63,268	54,603	124,447	107,291
North America | Residential
Disaggregation of Revenue [Line Items]
Revenues	12,708	20,154	25,861	38,214
North America | Other Marketplaces
Disaggregation of Revenue [Line Items]
Revenues	32,254	32,430	62,891	64,407
International
Disaggregation of Revenue [Line Items]
Revenues	21,599	18,871	42,386	37,119
International | CoStar
Disaggregation of Revenue [Line Items]
Revenues	9,596	9,186	18,796	18,731
International | Information Services
Disaggregation of Revenue [Line Items]
Revenues	9,708	7,991	19,237	14,935
International | Multifamily
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
International | LoopNet
Disaggregation of Revenue [Line Items]
Revenues	2,295	1,694	4,353	3,453
International | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
International | Other Marketplaces
Disaggregation of Revenue [Line Items]
Revenues	$ 0	$ 0	$ 0	$ 0