EMPLOYEE BENEFIT PLANS (Details) (USD $)	12 Months Ended																														0 Months Ended				12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Employee 401(k) Plan [Member]	Dec. 31, 2012 Employee 401(k) Plan [Member]	Dec. 31, 2011 Employee 401(k) Plan [Member]	Dec. 31, 2013 Employee Pension Plan, London Office [Member]	Dec. 31, 2012 Employee Pension Plan, London Office [Member]	Dec. 31, 2011 Employee Pension Plan, London Office [Member]	Dec. 31, 2013 Options Outstanding 1 [Member]	Dec. 31, 2013 Options Outstanding 2 [Member]	Dec. 31, 2013 Options Outstanding 3 [Member]	Dec. 31, 2013 Options Outstanding 4 [Member]	Dec. 31, 2013 Options Outstanding 5 [Member]	Dec. 31, 2013 Options Outstanding 6 [Member]	Dec. 31, 2013 Options Outstanding 7 [Member]	Dec. 31, 2013 Options Outstanding 8 [Member]	Dec. 31, 2013 Options Outstanding 9 [Member]	Dec. 31, 2013 Options Outstanding 10 [Member]	Dec. 31, 2013 Options Exercisable 1 [Member]	Dec. 31, 2013 Options Exercisable 2 [Member]	Dec. 31, 2013 Options Exercisable 3 [Member]	Dec. 31, 2013 Options Exercisable 4 [Member]	Dec. 31, 2013 Options Exercisable 5 [Member]	Dec. 31, 2013 Options Exercisable 6 [Member]	Dec. 31, 2013 Options Exercisable 7 [Member]	Dec. 31, 2013 Options Exercisable 8 [Member]	Dec. 31, 2013 Options Exercisable 9 [Member]	Dec. 31, 2013 Options Exercisable 10 [Member]	Dec. 31, 2013 1998 Stock Incentive Plan [Member]	Jun. 05, 2012 CoStar Group, Inc. 2007 Stock Incentive Plan [Member]	Jun. 02, 2010 CoStar Group, Inc. 2007 Stock Incentive Plan [Member]	Dec. 31, 2013 CoStar Group, Inc. 2007 Stock Incentive Plan [Member]	Dec. 31, 2012 CoStar Group, Inc. 2007 Stock Incentive Plan [Member]	Dec. 31, 2013 Employee Stock Purchase Plan [Member]	Dec. 31, 2012 Employee Stock Purchase Plan [Member]	Dec. 31, 2013 Stock Options [Member]	Dec. 31, 2012 Stock Options [Member]	Dec. 31, 2011 Stock Options [Member]	Dec. 31, 2013 Restricted Stock [Member]	Dec. 31, 2013 Performance-Based Restricted Common Stock [Member] quarters	Dec. 31, 2012 Performance-Based Restricted Common Stock [Member]	Dec. 31, 2011 Performance-Based Restricted Common Stock [Member]	Dec. 31, 2013 Minimum [Member] 1998 Stock Incentive Plan [Member]	Dec. 31, 2013 Minimum [Member] CoStar Group, Inc. 2007 Stock Incentive Plan [Member]	Dec. 31, 2013 Minimum [Member] Stock Options [Member]	Dec. 31, 2012 Minimum [Member] Stock Options [Member]	Dec. 31, 2011 Minimum [Member] Stock Options [Member]	Dec. 31, 2013 Minimum [Member] Restricted Stock [Member]	Dec. 31, 2013 Maximum [Member] 1998 Stock Incentive Plan [Member]	Dec. 31, 2013 Maximum [Member] CoStar Group, Inc. 2007 Stock Incentive Plan [Member]	Dec. 31, 2013 Maximum [Member] Stock Options [Member]	Dec. 31, 2012 Maximum [Member] Stock Options [Member]	Dec. 31, 2011 Maximum [Member] Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant under the plan (in shares)																														0			1,200,000	1,400,000
Vesting period of options and restricted stock grants																																												3 years	3 years				1 year	4 years	4 years
Increase in shares of common stock pursuant to amendment to the plan (in shares)																																1,300,000
Increase in shares of common stock issued pursuant to stock plan (in shares)																															900,000
Shares of common stock authorized for issuance under the plan (in shares)																														121,875			3,200,000
Cumulative EBITDA required for award shares to vest																																									$ 90,000,000
Number of consecutive quarters to mantain cumulative EBITDA required for award shares to vest (in quarters)																																									4
Compensation expense	41,549,000	12,282,000	8,103,000																																						21,752,000	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period (in shares)																																					673,719	847,102	945,696
Granted (in shares)																																					126,800	102,000	111,470
Exercised (in shares)																																					(409,799)	(274,842)	(198,132)
Canceled or expired (in shares)																																					(16,380)	(541)	(11,932)
Outstanding at end of period (in shares)																																					374,340	673,719	847,102
Exercisable at end of period (in shares)																																					146,161	432,196	558,849
Outstanding at beginning of period, (in dollars per share)																																														$ 25.00	$ 17.34	$ 17.34				$ 60.23	$ 60.23	$ 55.07
Granted (in dollars per share)																																														$ 102.16	$ 58.95	$ 57.16				$ 102.16	$ 58.95	$ 60.23
Exercised (in dollars per share)																																														$ 25.00	$ 17.34	$ 17.97				$ 58.95	$ 57.16	$ 54.51
Canceled or expired (in dollars per share)																																														$ 36.48	$ 54.51	$ 36.48				$ 58.95	$ 54.51	$ 54.51
Outstanding at end of period, (in dollars per share)																																														$ 36.48	$ 25.00	$ 17.34				$ 102.16	$ 60.23	$ 60.23
Exercisable at end of period (in dollars per share)																																														$ 36.48	$ 25.00	$ 17.34				$ 60.23	$ 60.23	$ 55.07
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Outstanding at beginning of period (in dollars per share)																																					$ 45.20	$ 39.93	$ 36.10
Granted (in dollars per share)																																					$ 102.16	$ 58.95	$ 57.28
Exercised (in dollars per share)																																					$ 41.05	$ 34.04	$ 31.37
Canceled or expired (in dollars per share)																																					$ 47.54	$ 54.51	$ 40.65
Outstanding at end of period (in dollars per share)																																					$ 68.94	$ 45.20	$ 39.93
Exercisable at end of period (in dollars per share)																																					$ 47.72	$ 40.22	$ 37.15
Weighted average remaining contract life of options outstanding at end of period																																					7 years 4 months 1 day
Weighted average remaining contract life of options exercisable at end of period																																					5 years 5 months 7 days
Aggregate intrinsic value of options outstanding at end of period																																					43,289,000
Aggregate intrinsic value of options exercisable at end of period																																					20,004,000
Exercised (in shares)																																					(409,799)	(274,842)	(198,132)
Aggregate intrinsic value of options exercised																																					39,000,000	11,900,000	6,100,000
Unrecognized compensation cost expected to be recognized in future years																																					38,600,000				2,100,000
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)																																					2 years 5 months 2 days
Weighted-average grant date fair value of each option granted during the period (in dollars per share)																																					$ 34.10	$ 20.99	$ 21.57
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)	37.00%	40.00%	40.00%
Risk-free interest rate (in hundredths)	0.90%	0.90%	2.20%
Expected life (in years)	5 years	5 years	5 years
Range of Exercise Price, minimum, (in dollars per share)										$ 36.48	$ 41.22	$ 42.51	$ 53.23	$ 55.84	$ 57.62	$ 58.52	$ 59.60	$ 81.20	$ 36.48
Range of Exercise Price, maximum, (in dollars per share)										$ 41.21	$ 42.50	$ 53.22	$ 55.83	$ 57.61	$ 58.51	$ 59.59	$ 81.19	$ 102.16	$ 102.16
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of Shares (in shares)										16,991	45,900	38,296	4,054	61,198	745	78,036	2,320	126,800	374,340
Weighted-Average Remaining Contractual Life (in years)										4 years 11 months 7 days	6 years 2 months 7 days	2 years 3 months 28 days	6 years 10 months 30 days	7 years 2 months 0 days	7 years 1 month 1 day	8 years 1 month 19 days	7 years 4 months 30 days	9 years 2 months 7 days	7 years 4 months 1 day
Weighted- Average Exercise Price (in dollars per share)										$ 37.84	$ 42.29	$ 46.14	$ 54.51	$ 57.16	$ 58.06	$ 58.95	$ 60.23	$ 102.16	$ 68.94
Options Exercisable [Abstract]
Number of Shares (in shares)																				16,991	45,900	36,037	3,243	28,930	0	13,900	1,160	0	146,161
Weighted- Average Exercise Price (in dollars per share)																				$ 37.84	$ 42.29	$ 46.35	$ 54.51	$ 57.16	$ 0.00	$ 58.95	$ 60.23	$ 0.00	$ 47.72
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)																																								1,020,673
Granted (in shares)																																								238,314	0	399,413
Vested (in shares)																																								(206,248)
Canceled (in shares)																																								(84,469)
Unvested restricted stock at end of period (in shares)																																								968,270
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)																																								$ 66.17
Granted (in dollars per share)																																								$ 119.84
Vested (in dollars per share)																																								$ 58.64
Canceled (in dollars per share)																																								$ 71.51
Unvested restricted stock at end of period (in dollars per share)																																								$ 80.52
Employee Plans [Abstract]
Percentage of company match of employee contribution (in hundredths)				100.00%	50.00%	50.00%		50.00%	50.00%
Maximum percentage of employee total compensation matched by employer (in hundredths)				4.00%	6.00%	6.00%	6.00%	6.00%	6.00%
Company match to employee contributions (401k)				5,100,000	2,700,000	1,900,000
Administrative expense				0	0	0
Company match to employee contributions (pension)							$ 280,000	$ 180,000	$ 160,000
Percentage of purchase price of Company's common stock to the market price (in hundredths)																																			90.00%
Shares available for purchase (in shares)																																			34,895	46,186
Shares of Company's common stock purchased during the period (in shares)																																			11,291	10,153