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REVENUE FROM CONTRACTS WITH CUSTOMERS (Disaggregated Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total revenues	$ 374,726	$ 352,808	$ 343,760	$ 328,425	$ 315,571	$ 305,525	$ 297,018	$ 273,718	$ 1,399,719	$ 1,191,832	$ 965,230
Information And Analytics | CoStar Suite
Disaggregation of Revenue [Line Items]
Total revenues									617,798	545,195
Information And Analytics | Information services
Disaggregation of Revenue [Line Items]
Total revenues									88,446	67,624
Online Marketplaces | Multifamily Online Marketplace
Disaggregation of Revenue [Line Items]
Total revenues									490,631	405,795
Online Marketplaces | Commercial property and land
Disaggregation of Revenue [Line Items]
Total revenues									202,844	173,218
North America
Disaggregation of Revenue [Line Items]
Total revenues									1,360,067	1,157,301
North America | Information And Analytics | CoStar Suite
Disaggregation of Revenue [Line Items]
Total revenues									590,222	519,661
North America | Information And Analytics | Information services
Disaggregation of Revenue [Line Items]
Total revenues									76,950	58,708
North America | Online Marketplaces | Multifamily Online Marketplace
Disaggregation of Revenue [Line Items]
Total revenues									490,631	405,795
North America | Online Marketplaces | Commercial property and land
Disaggregation of Revenue [Line Items]
Total revenues									202,264	173,137
International
Disaggregation of Revenue [Line Items]
Total revenues									39,652	34,531
International | Information And Analytics | CoStar Suite
Disaggregation of Revenue [Line Items]
Total revenues									27,576	25,534
International | Information And Analytics | Information services
Disaggregation of Revenue [Line Items]
Total revenues									11,496	8,916
International | Online Marketplaces | Multifamily Online Marketplace
Disaggregation of Revenue [Line Items]
Total revenues									0	0
International | Online Marketplaces | Commercial property and land
Disaggregation of Revenue [Line Items]
Total revenues									$ 580	$ 81

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

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