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EMPLOYEE BENEFIT PLANS (Stock Incentive Plans - Narrative) (Details) $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Jun. 07, 2021 | Dec. 31, 2023 USD ($) shares | Dec. 31, 2022 USD ($) shares | Dec. 31, 2021 USD ($) | Jun. 09, 2016 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Conversion ratio | 10 |  |  |  |
Tax benefit from stock-based compensation |  | $ 6.8 | $ 0.0 | $ 2.4 |
Compensation expense |  | $ 85.0 | 75.2 | 63.7 |
CoStar Group, Inc. 2007 Stock Incentive Plan |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Shares available for future grant under the plan (in shares) | shares |  |  |  |  | 0
CoStar Group, Inc. 2016 Stock Incentive Plan |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Shares available for future grant under the plan (in shares) | shares |  | 13,400,000 |  |  |
Shares of common stock authorized for issuance under the plan (in shares) | shares |  | 22,700,000 |  |  |
CoStar Group, Inc. 2016 Stock Incentive Plan | Minimum |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting period of options and restricted stock grants |  | 3 years |  |  |
CoStar Group, Inc. 2016 Stock Incentive Plan | Maximum |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting period of options and restricted stock grants |  | 4 years |  |  |
Restricted Stock | Minimum |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting period of options and restricted stock grants |  | 1 year |  |  |
Stock Options |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Compensation cost expected to be recognized in future years |  | $ 115.3 |  |  |
Weighted-average-period expected to recognize the unrecognized compensation cost (in years) |  | 2 years 4 months 24 days |  |  |
Aggregate intrinsic value of options exercised |  | $ 24.3 | 0.0 | 11.0 |
Performance Shares |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting period of options and restricted stock grants |  | 3 years |  |  |
Performance service period |  | 3 years |  |  |
Performance Shares | Minimum |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Share-based compensation arrangement, award vesting rights, percentage |  | 80.00% |  |  |
Performance Shares | Maximum |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Share-based compensation arrangement, award vesting rights, percentage |  | 120.00% |  |  |
Performance-based RSAs - with Market Condition |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Compensation cost expected to be recognized in future years |  | $ 14.0 |  |  |
Compensation expense |  | $ 13.6 | $ 11.9 | $ 8.0 |
Granted (in shares) | shares |  | 283,440 |  |  |
DSU |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |
Vesting period of options and restricted stock grants |  | 4 years |  |  |
Deferred stock unit, granted percentage |  | 100.00% |  |  |
Granted (in shares) | shares |  | 82,374 | 75,479 |  |

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