REVENUE FROM CONTRACTS WITH CUSTOMERS (Disaggregated Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Revenues	$ 556,921	$ 499,319	$ 1,609,054	$ 1,437,349
CoStar
Disaggregation of Revenue [Line Items]
Revenues	212,690	183,265	617,905	532,428
Information Services
Disaggregation of Revenue [Line Items]
Revenues	41,013	35,926	116,730	105,779
Multifamily
Disaggregation of Revenue [Line Items]
Revenues	189,536	171,125	547,372	508,629
LoopNet
Disaggregation of Revenue [Line Items]
Revenues	58,901	52,527	169,645	152,852
Residential
Disaggregation of Revenue [Line Items]
Revenues	19,351	24,747	57,565	53,939
Other Marketplaces
Disaggregation of Revenue [Line Items]
Revenues	35,430	31,729	99,837	83,722
North America
Disaggregation of Revenue [Line Items]
Revenues	537,624	482,952	1,552,638	1,388,489
North America | CoStar
Disaggregation of Revenue [Line Items]
Revenues	203,657	174,071	590,141	505,470
North America | Information Services
Disaggregation of Revenue [Line Items]
Revenues	32,524	29,177	93,306	84,959
North America | Multifamily
Disaggregation of Revenue [Line Items]
Revenues	189,536	171,125	547,372	508,629
North America | LoopNet
Disaggregation of Revenue [Line Items]
Revenues	57,126	52,103	164,417	151,770
North America | Residential
Disaggregation of Revenue [Line Items]
Revenues	19,351	24,747	57,565	53,939
North America | Other Marketplaces
Disaggregation of Revenue [Line Items]
Revenues	35,430	31,729	99,837	83,722
International
Disaggregation of Revenue [Line Items]
Revenues	19,297	16,367	56,416	48,860
International | CoStar
Disaggregation of Revenue [Line Items]
Revenues	9,033	9,194	27,764	26,958
International | Information Services
Disaggregation of Revenue [Line Items]
Revenues	8,489	6,749	23,424	20,820
International | Multifamily
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
International | LoopNet
Disaggregation of Revenue [Line Items]
Revenues	1,775	424	5,228	1,082
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