REVENUE FROM CONTRACTS WITH CUSTOMERS, DISAGGREGATED REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 499,319	$ 425,620	$ 1,437,349	$ 1,214,626
CoStar
Disaggregation of Revenue [Line Items]
Revenues	183,265	165,988	532,428	495,997
Information Services
Disaggregation of Revenue [Line Items]
Revenues	35,926	33,174	105,779	96,092
Multifamily
Disaggregation of Revenue [Line Items]
Revenues	171,125	155,184	508,629	438,185
LoopNet
Disaggregation of Revenue [Line Items]
Revenues	52,527	45,084	152,852	131,604
Residential
Disaggregation of Revenue [Line Items]
Revenues	24,747	0	53,939	0
Other Marketplaces
Disaggregation of Revenue [Line Items]
Revenues	31,729	26,190	83,722	52,748
North America
Disaggregation of Revenue [Line Items]
Revenues	482,952	410,904	1,388,489	1,172,713
North America | CoStar
Disaggregation of Revenue [Line Items]
Revenues	174,071	158,235	505,470	473,363
North America | Information Services
Disaggregation of Revenue [Line Items]
Revenues	29,177	26,357	84,959	77,069
North America | Multifamily
Disaggregation of Revenue [Line Items]
Revenues	171,125	155,184	508,629	438,185
North America | LoopNet
Disaggregation of Revenue [Line Items]
Revenues	52,103	44,938	151,770	131,348
North America | Residential
Disaggregation of Revenue [Line Items]
Revenues	24,747	0	53,939	0
North America | Other Marketplaces
Disaggregation of Revenue [Line Items]
Revenues	31,729	26,190	83,722	52,748
International
Disaggregation of Revenue [Line Items]
Revenues	16,367	14,716	48,860	41,913
International | CoStar
Disaggregation of Revenue [Line Items]
Revenues	9,194	7,753	26,958	22,634
International | Information Services
Disaggregation of Revenue [Line Items]
Revenues	6,749	6,817	20,820	19,023
International | Multifamily
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
International | LoopNet
Disaggregation of Revenue [Line Items]
Revenues	424	146	1,082	256
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