SEGMENT REPORTING SEGMENT REPORTING, REVENUES BY SERVICES (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue from External Customer [Line Items]
Revenues		$ 837,630	$ 711,764	$ 575,936
CoStar Suite [Member]
Revenue from External Customer [Line Items]
Revenues	[1]	408,456	360,440	322,696
Information services [Member]
Revenue from External Customer [Line Items]
Revenues	[2]	77,178	75,790	72,525
Multifamily [Member]
Revenue from External Customer [Line Items]
Revenues	[3]	224,835	160,630	76,785
Commercial property and land [Member]
Revenue from External Customer [Line Items]
Revenues	[4]	$ 127,161	$ 114,904	$ 103,930

[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.