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EMPLOYEE BENEFIT PLANS - (Stock Incentive Plans) (Narrative) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2025 USD ($) shares	Jun. 09, 2016 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion ratio	10
Share repurchase program, remaining authorized, amount \| $	$ 436.2
Compensation cost expected to be recognized in future years \| $	$ 327.4
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)	2 years 8 months 12 days
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. 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	10,000,000.0
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 (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 \| $	$ 1.8