REVENUE FROM CONTRACTS WITH CUSTOMERS , DISAGGREGATED REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 305,525	$ 247,533	$ 876,261	$ 711,239
Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Total revenues	139,084	117,314	403,257	341,087
Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Total revenues	17,535	18,716	48,425	55,364
Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Total revenues	104,778	72,257	297,254	204,324
Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Total revenues	44,128	39,246	127,325	110,464
North America [Member]
Disaggregation of Revenue [Line Items]
Total revenues	296,916	239,537	849,999	688,704
North America [Member] | Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Total revenues	132,700	111,452	383,876	324,713
North America [Member] | Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Total revenues	15,310	16,582	41,544	49,203
North America [Member] | Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Total revenues	104,778	72,257	297,254	204,324
North America [Member] | Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Total revenues	44,128	39,246	127,325	110,464
International [Member]
Disaggregation of Revenue [Line Items]
Total revenues	8,609	7,996	26,262	22,535
International [Member] | Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Total revenues	6,384	5,862	19,381	16,374
International [Member] | Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Total revenues	2,225	2,134	6,881	6,161
International [Member] | Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Total revenues	0	0	0	0
International [Member] | Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Total revenues	$ 0	$ 0	$ 0	$ 0