EMPLOYEE BENEFIT PLANS - Stock Incentive Plans Narrative (Details) shares in Millions, $ in Millions	3 Months Ended
Mar. 31, 2026 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost expected to be recognized in future years \| $	$ 299.0
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)	3 years
Matterport
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants (in years)	4 years
Minimum \| Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants (in years)	1 year
CoStar Group, Inc. 2016 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock authorized for issuance under the plan (in shares) \| shares	13.2
Shares available for future grant under the plan (in shares) \| shares	7.8
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 (in years)	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 (in years)	4 years
CoStar Group, Inc. 2021 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share repurchase program, remaining authorized, amount \| $	$ 2.1

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Number of shares authorized for issuance under share-based payment arrangement.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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