SEGMENT REPORTING, REVENUES BY SERVICES (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Revenue from External Customer [Line Items]
Revenues		$ 206,869	$ 170,657	$ 406,608	$ 329,677
CoStar Suite [Member]
Revenue from External Customer [Line Items]
Revenues	[1]	101,074	88,771	198,708	175,581
Information services [Member]
Revenue from External Customer [Line Items]
Revenues	[2]	19,425	18,752	38,850	37,289
Multifamily [Member]
Revenue from External Customer [Line Items]
Revenues	[3]	54,860	34,742	107,098	60,875
Commercial property and land [Member]
Revenue from External Customer [Line Items]
Revenues	[4]	$ 31,510	$ 28,392	$ 61,952	$ 55,932

[1] CoStar Suite is comprised of CoStar Property Professional, CoStar COMPS Professional, CoStar Tenant; CoStar Market Analytics; and CoStar Portfolio Strategy.

[2] Information services is comprised of LoopNet Premium Searcher; CoStar Real Estate Manager; CoStar Risk Analytics COMPASS; CoStar Investment Analysis Portfolio Maximizer; CoStar Investment Analysis Request; CoStar Brokerage Applications; PROPEX; Grecam; Belbex and Thomas Daily.

[3] Multifamily is comprised of Apartments.com, ApartmentFinder.com and ApartmentHomeLiving.com.

[4] Commercial property and land is comprised of LoopNet Premium Lister; LoopLink; CoStar Advertising; BizBuySell and BizQuest; LandsofAmerica and LandAndFarm; and CoStar Private Sale Network.