Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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Number of share options (or share units) exercised during the current period.

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EMPLOYEE BENEFIT PLANS (Stock Incentive Plans - Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 09, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 52,255	$ 42,718	$ 39,030
Performance service period	3 years
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)				0
Shares of common stock authorized for issuance under the plan (in shares)	815,464
CoStar Group, Inc. 2007 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. 2007 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
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)	2,000,000
Shares of common stock authorized for issuance under the plan (in shares)	1,450,000
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
Management Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)	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	$ 82,000
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)	2 years 6 months
Exercise of stock options (in shares)	116,918	177,299	81,815
Aggregate intrinsic value of options exercised	$ 40,000	$ 45,000	$ 13,000
Weighted-average grant date fair value of each option granted during the period (in dollars per share)	$ 115.17	$ 101.02	$ 59.06
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)		26,160	32,160
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	$ 11,000
Compensation expense	$ 8,000	$ 5,000	$ 5,000
Performance Based RSAs Without Market Condition
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	115,722
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	413