Cover page - shares	3 Months Ended
	Mar. 31, 2020	Apr. 24, 2020
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Mar. 31, 2020
Entity File Number	0-24531
Entity Registrant Name	COSTAR GROUP, INC.
Entity Central Index Key	0001057352
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Amendment Flag	false
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	52-2091509
Entity Address, Address Line One	1331 L Street, NW
Entity Address, City or Town	Washington,
Entity Address, State or Province	DC
Entity Address, Postal Zip Code	20005
City Area Code	(202)
Local Phone Number	346-6500
Title of 12(b) Security	Common Stock ($0.01 par value)
Trading Symbol	CSGP
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		36,726,443