EMPLOYEE BENEFIT PLANS (Details for Stock Incentive Plans) $ / shares in Units, $ in Thousands			12 Months Ended
	Jun. 05, 2012 shares	Jun. 02, 2010 shares	Dec. 31, 2015 USD ($) quarters $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2013 USD ($) $ / shares shares	Dec. 31, 2012 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense | $			$ 34,537	$ 28,267	$ 41,549
Performance service period			3 years
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (in hundredths)			0.00%	0.00%	0.00%
Expected volatility (in hundredths)			30.00%	30.00%	37.00%
Risk-free interest rate (in hundredths)			1.60%	1.50%	0.90%
Expected life (in years)			5 years	5 years	5 years
Options Outstanding 1 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			$ 36.48
Range of exercise price, maximum, (in dollars per share)			$ 42.50
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			55,051
Weighted-average remaining contractual life (in years)			4 years 23 days
Weighted- average exercise price (in dollars per share)			$ 41.25
Options Outstanding 2 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			42.51
Range of exercise price, maximum, (in dollars per share)			$ 58.06
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			48,638
Weighted-average remaining contractual life (in years)			4 years 10 months 18 days
Weighted- average exercise price (in dollars per share)			$ 55.94
Options Outstanding 3 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			58.07
Range of exercise price, maximum, (in dollars per share)			$ 59.59
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			45,567
Weighted-average remaining contractual life (in years)			6 years 1 month 20 days
Weighted- average exercise price (in dollars per share)			$ 58.95
Options Outstanding 4 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			59.60
Range of exercise price, maximum, (in dollars per share)			$ 81.19
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			2,320
Weighted-average remaining contractual life (in years)			5 years 5 months 1 day
Weighted- average exercise price (in dollars per share)			$ 60.23
Options Outstanding 5 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			81.20
Range of exercise price, maximum, (in dollars per share)			$ 147.93
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			82,801
Weighted-average remaining contractual life (in years)			7 years 2 months 10 days
Weighted- average exercise price (in dollars per share)			$ 102.16
Options Outstanding 6 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			147.94
Range of exercise price, maximum, (in dollars per share)			$ 197.37
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			89,500
Weighted-average remaining contractual life (in years)			9 years 2 months 1 day
Weighted- average exercise price (in dollars per share)			$ 193.69
Options Outstanding 7 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			197.38
Range of exercise price, maximum, (in dollars per share)			$ 201.04
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			76,200
Weighted-average remaining contractual life (in years)			8 years 1 month 28 days
Weighted- average exercise price (in dollars per share)			$ 201.04
Options Outstanding 8 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			36.48
Range of exercise price, maximum, (in dollars per share)			$ 201.04
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			400,077
Weighted-average remaining contractual life (in years)			6 years 11 months 22 days
Weighted- average exercise price (in dollars per share)			$ 122.30
Options Exercisable 1 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			55,051
Weighted-average exercise price (in dollars per share)			$ 41.25
Options Exercisable 2 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			48,638
Weighted-average exercise price (in dollars per share)			$ 55.94
Options Exercisable 3 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			45,567
Weighted-average exercise price (in dollars per share)			$ 58.95
Options Exercisable 4 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			2,320
Weighted-average exercise price (in dollars per share)			$ 60.23
Options Exercisable 5 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			45,599
Weighted-average exercise price (in dollars per share)			$ 102.16
Options Exercisable 6 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			0
Weighted-average exercise price (in dollars per share)			$ 0.00
Options Exercisable 7 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			22,932
Weighted-average exercise price (in dollars per share)			$ 201.04
Options Exercisable 8 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			220,107
Weighted-average exercise price (in dollars per share)			$ 77.63
1998 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant under the plan (in shares) | shares			0
Shares of common stock authorized for issuance under the plan (in shares) | shares			121,875
CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant under the plan (in shares) | shares			1,000,000	1,200,000
Increase in shares of common stock pursuant to amendment to the plan (in shares) | shares		1,300,000
Increase in shares of common stock issued pursuant to stock plan (in shares) | shares	900,000
Shares of common stock authorized for issuance under the plan (in shares) | shares			3,200,000
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares) | shares			887
Granted (in shares) | shares			543
Vested (in shares) | shares			0
Canceled (in shares) | shares			0
Unvested restricted stock at end of period (in shares) | shares			1,430	887
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Unvested restricted stock at beginning of period (in dollars per share)			$ 169.16
Granted (in dollars per share)			211.98
Vested (in dollars per share)			0.00
Canceled (in dollars per share)			0.00
Unvested restricted stock at end of period (in dollars per share)			$ 185.42	$ 169.16
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period (in shares) | shares			370,179	374,340	673,719
Granted (in shares) | shares			89,500	87,700	126,800
Exercised (in shares) | shares			(59,602)	(68,126)	(409,799)
Canceled or expired (in shares) | shares				(23,735)	(16,380)
Outstanding at end of period (in shares) | shares			400,077	370,179	374,340	673,719
Exercisable at end of period (in shares) | shares			220,107	188,656	146,161
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average exercise price, outstanding at beginning of period (in dollars per share)			$ 99.12	$ 68.94	$ 45.20
Weighted-average exercise price, granted (in dollars per share)			193.69	201.04	102.16
Weighted-average exercise price, exercised (in dollars per share)			85.48	55.81	41.05
Weighted-average exercise price, canceled or expired (in dollars per share)				124.09	47.54
Weighted-average exercise price, outstanding at end of period (in dollars per share)			122.30	99.12	68.94	$ 45.20
Weighted-average exercise price, exercisable at end of period (in dollars per share)			$ 77.63	$ 60.54	$ 47.72
Weighted-average remaining contract life of options outstanding at end of period			6 years 11 months 24 days
Weighted-average remaining contract life of options exercisable at end of period			5 years 9 months 2 days
Aggregate intrinsic value of options outstanding at end of period | $			$ 33,761
Aggregate intrinsic value of options exercisable at end of period | $			28,408
Aggregate intrinsic value of options exercised | $			6,800	$ 8,900	$ 39,000
Compensation cost expected to be recognized in future years | $			$ 61,700
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)			2 years 5 months 11 days
Weighted-average grant date fair value of each option granted during the period (in dollars per share)			$ 56.53	$ 58.12	$ 34.10
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares) | shares			556,793
Granted (in shares) | shares			207,442
Vested (in shares) | shares			(221,546)
Canceled (in shares) | shares			(39,576)
Unvested restricted stock at end of period (in shares) | shares			503,113	556,793
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Unvested restricted stock at beginning of period (in dollars per share)			$ 126.01
Granted (in dollars per share)			201.88
Vested (in dollars per share)			102.02
Canceled (in dollars per share)			144.28
Unvested restricted stock at end of period (in dollars per share)			$ 166.42	$ 126.01
Performance-Based Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative EBITDA required for award shares to vest | $			$ 90,000
Number of consecutive quarters to mantain cumulative EBITDA required for award shares to vest (in quarters) | quarters			4
Compensation expense | $			$ 0	$ 2,200	$ 21,800
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted (in shares) | shares						399,413
Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense | $			2,800	$ 1,100
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Compensation cost expected to be recognized in future years | $			$ 5,300
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (in hundredths)			0.00%	0.00%
Expected volatility (in hundredths)			26.00%	30.00%
Risk-free interest rate (in hundredths)			1.00%	0.60%
Expected life (in years)			3 years	3 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares) | shares			22,560
Granted (in shares) | shares			32,400	24,720
Vested (in shares) | shares			0
Canceled (in shares) | shares			0
Unvested restricted stock at end of period (in shares) | shares			54,960	22,560
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Unvested restricted stock at beginning of period (in dollars per share)			$ 216.20
Granted (in dollars per share)			208.08	$ 216.20
Vested (in dollars per share)			0.00
Canceled (in dollars per share)			0.00
Unvested restricted stock at end of period (in dollars per share)			$ 211.41	216.20
Minimum [Member] | 1998 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants			3 years
Minimum [Member] | CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants			3 years
Minimum [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period, (in dollars per share)			$ 36.48	36.48	$ 25.00
Granted (in dollars per share)			193.69	201.04	102.16
Exercised (in dollars per share)			36.48	39.00	25.00
Canceled or expired (in dollars per share)				58.95	36.48
Outstanding at end of period, (in dollars per share)			36.48	36.48	36.48	$ 25.00
Exercisable at end of period (in dollars per share)			$ 36.48	36.48	36.48
Minimum [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants			1 year
Minimum [Member] | Performance-Based Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage			0.00%
Minimum [Member] | Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage			80.00%
Maximum [Member] | 1998 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants			4 years
Maximum [Member] | CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants			4 years
Maximum [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period, (in dollars per share)			$ 201.04	102.16	60.23
Granted (in dollars per share)			193.69	201.04	102.16
Exercised (in dollars per share)			201.04	102.16	58.95
Canceled or expired (in dollars per share)				201.04	58.95
Outstanding at end of period, (in dollars per share)			201.04	201.04	102.16	$ 60.23
Exercisable at end of period (in dollars per share)			$ 201.04	$ 102.16	$ 60.23
Maximum [Member] | Performance-Based Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage			200.00%
Maximum [Member] | Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage			120.00%