LONG-TERM DEBT (Details) - USD ($)		1 Months Ended
	Apr. 02, 2014	Jun. 30, 2014	Sep. 30, 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			$ 10,713,000	$ 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%