| Document and Entity Information Document | Sep. 30, 2019 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Sep. 30, 2019 |
| Entity Registrant Name | COSTAR GROUP, INC. |
| Entity Central Index Key | 0001057352 |
| Amendment Flag | false |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 0-24531 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock ($0.01 par value) |
| Trading Symbol | CSGP |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
