LONG-TERM DEBT (Details) (USD $)
0 Months Ended 1 Months Ended 3 Months Ended
Apr. 03, 2014
Jun. 30, 2014
Apr. 04, 2014
Mar. 31, 2015
Dec. 31, 2014
Apr. 01, 2014
Debt Instrument [Line Items]            
Line of credit facility, amount outstanding           $ 150,000,000
Repayments of revolving credit facility   150,000,000        
Repayments of term loan 148,800,000          
LIBOR maturity period (in months) 1 month          
Default interest rate per annum on overdue amounts (in percent)           2.00%
Credit facility, collateral The obligations under the 2014 Credit Agreement are guaranteed by all material subsidiaries of the Company and are secured by a lien on substantially all of the assets of the Company and those of its material subsidiaries, in each case subject to certain exceptions, pursuant to security and guarantee documents entered into on the Closing Date.          
Debt issuance costs     10,100,000      
Debt issuance underwriting fees     9,700,000      
Debt Issuance Legal Fees     400,000      
Capitalized debt issuance costs       12,372,000 13,199,000 10,000,000
Revolving Credit Facility [Member]            
Debt Instrument [Line Items]            
Line of credit facility, amount outstanding       0 0  
CoStar Group [Member] | Term Loan [Member]            
Debt Instrument [Line Items]            
Debt instrument borrowing capacity           400,000,000
Term of loan (in years) 5 years          
CoStar Group [Member] | Revolving Credit Facility [Member]            
Debt Instrument [Line Items]            
Debt instrument borrowing capacity           225,000,000
Term of loan (in years) 5 years          
CoStar Group [Member] | Swingline Loan [Member]            
Debt Instrument [Line Items]            
Revolving credit sub-facility for swing-line loans and issuances of letters of credit           10,000,000
CoStar Group [Member] | Letter of Credit [Member]            
Debt Instrument [Line Items]            
Revolving credit sub-facility for swing-line loans and issuances of letters of credit           $ 10,000,000
CoStar Group [Member] | Notes Payable to Banks [Member]            
Debt Instrument [Line Items]            
Annual amortization, first year after closing (in percent)           5.00%
Annual amortization, second year after closing (in percent)           5.00%
Annual amortization, third year after closing (in percent)           5.00%
Annual amortization, fourth year after closing (in percent)           10.00%
Annual amortization, fifth year after closing (in percent)           15.00%
Federal Funds Rate [Member]            
Debt Instrument [Line Items]            
Basis spread on federal funds rate (in percent) 0.50%          
LIBOR [Member]            
Debt Instrument [Line Items]            
Basis spread on federal funds rate (in percent) 2.00%          
Debt instrument, basis spread on variable rate, one month interest period (in percent)           1.00%
Debt instrument, basis spread on variable rate, per annum (in percent)           1.00%