REVENUE FROM CONTRACTS WITH CUSTOMERS (Disaggregated Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 444,393	$ 425,620	$ 397,159	$ 391,847	$ 374,726	$ 352,808	$ 343,760	$ 328,425	$ 1,659,019	$ 1,399,719	$ 1,191,832
Information And Analytics | CoStar Suite
Disaggregation of Revenue [Line Items]
Revenues									664,735	617,798
Information And Analytics | Information services
Disaggregation of Revenue [Line Items]
Revenues									130,070	88,446
Online Marketplaces | Multifamily
Disaggregation of Revenue [Line Items]
Revenues									598,555	490,631
Online Marketplaces | Commercial property and land
Disaggregation of Revenue [Line Items]
Revenues									265,659	202,844
North America
Disaggregation of Revenue [Line Items]
Revenues									1,602,102	1,360,067
North America | Information And Analytics | CoStar Suite
Disaggregation of Revenue [Line Items]
Revenues									634,205	590,222
North America | Information And Analytics | Information services
Disaggregation of Revenue [Line Items]
Revenues									104,117	76,950
North America | Online Marketplaces | Multifamily
Disaggregation of Revenue [Line Items]
Revenues									598,555	490,631
North America | Online Marketplaces | Commercial property and land
Disaggregation of Revenue [Line Items]
Revenues									265,225	202,264
International
Disaggregation of Revenue [Line Items]
Revenues									56,917	39,652
International | Information And Analytics | CoStar Suite
Disaggregation of Revenue [Line Items]
Revenues									30,530	27,576
International | Information And Analytics | Information services
Disaggregation of Revenue [Line Items]
Revenues									25,953	11,496
International | Online Marketplaces | Multifamily
Disaggregation of Revenue [Line Items]
Revenues									0	0
International | Online Marketplaces | Commercial property and land
Disaggregation of Revenue [Line Items]
Revenues									$ 434	$ 580