REVENUE FROM CONTRACTS WITH CUSTOMERS (Disaggregated Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total revenues	$ 315,571	$ 305,525	$ 297,018	$ 273,718	$ 253,991	$ 247,533	$ 237,153	$ 226,553	$ 1,191,832	$ 965,230	$ 837,630
Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Total revenues									545,195	463,185
Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Total revenues									67,624	72,618
Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Total revenues									405,795	279,855
Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Total revenues									173,218	149,572
North America [Member]
Disaggregation of Revenue [Line Items]
Total revenues									1,157,301	934,464
North America [Member] | Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Total revenues									519,661	440,534
North America [Member] | Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Total revenues									58,708	64,503
North America [Member] | Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Total revenues									405,795	279,855
North America [Member] | Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Total revenues									173,137	149,572
International [Member]
Disaggregation of Revenue [Line Items]
Total revenues									34,531	30,766
International [Member] | Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Total revenues									25,534	22,651
International [Member] | Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Total revenues									8,916	8,115
International [Member] | Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Total revenues									0	0
International [Member] | Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Total revenues									$ 81	$ 0