REVENUE FROM CONTRACTS WITH CUSTOMERS , DISAGGREGATED REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 391,847	$ 328,425
Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Revenues	164,956	147,701
Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Revenues	32,382	18,850
Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Revenues	137,460	114,268
Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Revenues	57,049	47,606
North America [Member]
Disaggregation of Revenue [Line Items]
Revenues	377,447	319,237
North America [Member] | Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Revenues	157,335	140,973
North America [Member] | Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Revenues	25,690	16,591
North America [Member] | Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Revenues	137,460	114,268
North America [Member] | Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Revenues	56,962	47,405
International [Member]
Disaggregation of Revenue [Line Items]
Revenues	14,400	9,188
International [Member] | Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Revenues	7,621	6,728
International [Member] | Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Revenues	6,692	2,259
International [Member] | Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Revenues	0	0
International [Member] | Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 87	$ 201