| Document and Entity Information | Feb. 16, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001057352 |
| Document Type | 8-K |
| Document Period End Date | Feb. 16, 2021 |
| Entity Registrant Name | COSTAR GROUP, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 0-24531 |
| Entity Tax Identification Number | 52-2091509 |
| Entity Address, Address Line One | 1331 L Street |
| Entity Address, Address Line Two | 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 | true |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock ($0.01 par value) |
| Trading Symbol | CSGP |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
