DOCUMENT AND ENTITY INFORMATION DOCUMENT AND ENTITY INFORMATION - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2015	Feb. 19, 2016	Jun. 30, 2015
Document and Entity Information [Abstract]
Entity Registrant Name	COSTAR GROUP INC
Entity Central Index Key	0001057352
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 6.4
Entity Common Stock, Shares Outstanding (in shares)		32,513,536
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015