CoStar (CSGP) 10-K 26 Feb 16 2015 FY Annual report Financial data

DOCUMENT AND ENTITY INFORMATION

DOCUMENT AND ENTITY INFORMATION DOCUMENT AND ENTITY INFORMATION - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2015	Feb. 19, 2016	Jun. 30, 2015
Document and Entity Information [Abstract]
Entity Registrant Name	COSTAR GROUP INC
Entity Central Index Key	1,057,352
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 6.4
Entity Common Stock, Shares Outstanding (in shares)		32,513,536
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Revenues	$ 711,764	$ 575,936	$ 440,943
Cost of revenues	188,885	156,979	129,185
Gross margin	522,879	418,957	311,758
Operating expenses:
Selling and marketing	302,226	150,305	98,708
Software development	65,760	55,426	46,757
General and administrative	115,507	103,916	96,956
Purchase amortization	27,931	28,432	15,183
Total operating expenses	511,424	338,079	257,604
Income from operations	11,455	80,878	54,154
Interest and other income	537	516	326
Interest and other expense	(9,411)	(10,481)	(6,943)
Income before income taxes	2,581	70,913	47,537
Income tax expense, net	6,046	26,044	17,803
Net income (loss)	$ (3,465)	$ 44,869	$ 29,734
Net income (loss) per share — basic (in dollars per share)	$ (0.11)	$ 1.48	$ 1.07
Net income (loss) per share — diluted (in dollars per share)	$ (0.11)	$ 1.46	$ 1.05
Weighted average outstanding shares — basic (in shares)	31,950	30,215	27,670
Weighted average outstanding shares — diluted (in shares)	31,950	30,641	28,212

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (3,465)	$ 44,869	$ 29,734
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment	(1,466)	(1,690)	610
Net decrease in unrealized loss on investments	256	836	378
Total other comprehensive income (loss)	(1,210)	(854)	988
Total comprehensive income (loss)	$ (4,675)	$ 44,015	$ 30,722

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 421,818	$ 527,012
Accounts receivable, net of allowance for doubtful accounts of approximately $4,815 and $7,478 as of December 31, 2014 and 2015, respectively	40,276	38,694
Deferred and other income taxes, net	0	20,007
Income tax receivable	430	1,027
Prepaid expenses and other current assets	10,209	9,736
Total current assets	472,733	596,476
Long-term investments	15,507	17,151
Deferred income taxes, net	9,107	0
Property and equipment, net	88,311	73,753
Goodwill	1,252,945	1,138,805
Intangible assets, net	238,318	241,622
Deposits and other assets	2,650	2,676
Total assets	2,079,571	2,070,483
Current liabilities:
Current portion of long-term debt	16,746	16,665
Accounts payable	9,673	8,608
Accrued wages and commissions	31,045	23,155
Accrued expenses	31,469	27,001
Deferred gain on the sale of building	2,523	2,523
Deferred rent	1,687	0
Deferred revenue	42,138	38,003
Total current liabilities	135,281	115,955
Long-term debt, less current portion	338,366	355,136
Deferred gain on the sale of building	21,239	23,762
Deferred rent	29,628	27,032
Deferred income taxes, net	4,585	30,349
Income taxes payable	6,692	4,703
Total liabilities	$ 535,791	$ 556,937
Commitments and contingencies
Stockholders’ equity:
Preferred stock, $0.01 par value; 2,000 shares authorized; none outstanding	$ 0	$ 0
Common stock, $0.01 par value; 60,000 shares authorized; 32,318 and 32,509 issued and outstanding as of December 31, 2014 and 2015, respectively	325	323
Additional paid-in capital	1,440,321	1,405,414
Accumulated other comprehensive loss	(7,594)	(6,384)
Retained earnings	110,728	114,193
Total stockholders’ equity	1,543,780	1,513,546
Total liabilities and stockholders’ equity	$ 2,079,571	$ 2,070,483

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Allowance for doubtful accounts	$ 7,478	$ 4,815
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	2,000,000	2,000,000
Preferred stock, shares outstanding (in shares)	0	0
Common stock par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	60,000,000	60,000,000
Common stock, shares issued, (in shares)	32,509,000	32,318,000
Common stock, shares outstanding (in shares)	32,509,000	32,318,000

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings
Balance (in shares) at Dec. 31, 2012		28,348
Balance at Dec. 31, 2012	$ 826,343	$ 283	$ 792,988	$ (6,518)	$ 39,590
Net income (loss)	29,734				29,734
Foreign currency translation adjustment	610			610
Net decrease in unrealized loss on investments	378			378
Exercise of stock options (in shares)		409
Exercise of stock options	16,823	$ 3	16,820
Restricted stock grants (in shares)		238
Restricted stock grants	0	$ 2
Restricted stock grants adjustments			(2)
Restricted stock grants surrendered (in shares)		(158)
Restricted stock grants surrendered	(8,469)	$ 0	(8,469)
Stock compensation expense, net of forfeitures	41,403		41,403
Employee stock purchase plan (in shares)		11
Employee stock purchase plan	1,455		1,455
Excess tax benefit from stock-based compensation	19,585		19,585
Balance (in shares) at Dec. 31, 2013		28,848
Balance at Dec. 31, 2013	927,862	$ 288	863,780	(5,530)	69,324
Net income (loss)	44,869				44,869
Foreign currency translation adjustment	(1,690)			(1,690)
Net decrease in unrealized loss on investments	836			836
Exercise of stock options (in shares)		68
Exercise of stock options	3,803	$ 1	3,802
Restricted stock grants (in shares)		260
Restricted stock grants	0	$ 2
Restricted stock grants adjustments			(2)
Restricted stock grants surrendered (in shares)		(321)
Restricted stock grants surrendered	(50,555)	$ (2)	(50,553)
Stock compensation expense, net of forfeitures	28,503		28,503
Stock issued for equity offering (in shares)		3,450
Stock issued for equity offering	529,360	$ 34	529,326
Employee stock purchase plan (in shares)		13
Employee stock purchase plan	2,152		2,152
Excess tax benefit from stock-based compensation	28,406		28,406
Balance (in shares) at Dec. 31, 2014		32,318
Balance at Dec. 31, 2014	1,513,546	$ 323	1,405,414	(6,384)	114,193
Net income (loss)	(3,465)				(3,465)
Foreign currency translation adjustment	(1,466)			(1,466)
Net decrease in unrealized loss on investments	256			256
Exercise of stock options (in shares)		60
Exercise of stock options	5,069	$ 1	5,068
Restricted stock grants (in shares)		239
Restricted stock grants	0	$ 2
Restricted stock grants adjustments			(2)
Restricted stock grants surrendered (in shares)		(121)
Restricted stock grants surrendered	(16,436)	$ (1)	(16,435)
Stock compensation expense, net of forfeitures	35,153		35,153
Employee stock purchase plan (in shares)		13
Employee stock purchase plan	2,595		2,595
Excess tax benefit from stock-based compensation	8,528		8,528
Balance (in shares) at Dec. 31, 2015		32,509
Balance at Dec. 31, 2015	$ 1,543,780	$ 325	$ 1,440,321	$ (7,594)	$ 110,728

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities:
Net income (loss)	$ (3,465)	$ 44,869	$ 29,734
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation	19,967	15,111	12,495
Amortization	58,565	55,261	27,563
Amortization of debt issuance costs	3,311	3,312	3,014
Impairment loss	2,778	1,799	0
Property and equipment write-off	681	1,004	104
Excess tax benefit from stock-based compensation	(8,528)	(28,406)	(19,585)
Stock-based compensation expense	34,537	28,267	41,549
Deferred income tax benefit, net	(5,792)	(1,366)	(12,740)
Provision for losses on accounts receivable	7,002	4,822	2,317
Changes in operating assets and liabilities, net of acquisitions:
Accounts receivable	(3,999)	(12,353)	(6,607)
Income taxes payable	11,380	24,542	29,295
Prepaid expenses and other current assets	367	(2,846)	2,934
Deposits and other assets	686	(157)	399
Accounts payable and other liabilities	9,938	6,078	(3,882)
Deferred revenue	3,817	3,972	1,708
Net cash provided by operating activities	131,245	143,909	108,298
Investing activities:
Proceeds from sale and settlement of investments	1,900	5,675	76
Purchases of property and equipment and other assets	(35,061)	(27,444)	(19,042)
Acquisitions, net of cash acquired	(182,341)	(584,218)	0
Net cash used in investing activities	(215,502)	(605,987)	(18,966)
Financing activities:
Proceeds from long-term debt	0	550,000	0
Payments of long-term debt	(20,000)	(318,125)	(17,500)
Payments of debt issuance costs	0	(9,969)	0
Payments of deferred consideration	0	(1,344)	(1,344)
Excess tax benefit from stock-based compensation	8,528	28,406	19,585
Repurchase of restricted stock to satisfy tax withholding obligations	(16,436)	(50,555)	(8,469)
Proceeds from equity offering, net of transaction costs	0	529,360	0
Proceeds from exercise of stock options and employee stock purchase plan	7,404	5,740	18,133
Net cash provided by (used in) financing activities	(20,504)	733,513	10,405
Effect of foreign currency exchange rates on cash and cash equivalents	(433)	(376)	189
Net increase (decrease) in cash and cash equivalents	(105,194)	271,059	99,926
Cash and cash equivalents at beginning of year	527,012	255,953	156,027
Cash and cash equivalents at end of year	$ 421,818	$ 527,012	$ 255,953

ORGANIZATION

ORGANIZATION	12 Months Ended
	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
ORGANIZATION	ORGANIZATION CoStar Group, Inc. (the “Company” or “CoStar”) provides information, analytics and online marketplace services to the commercial real estate and related business community through its comprehensive, proprietary database of commercial real estate information covering the United States (“U.S.”) and parts of the United Kingdom (“U.K.”), Canada, Spain and France. The Company provides online marketplaces for commercial real estate, apartment rentals, lands for sale and businesses for sale. The Company operates within two operating segments, North America and International, and its services are typically distributed to its clients under subscription-based license agreements that renew automatically, a majority of which have a term of one year .

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Accounting policies are consistent for each operating segment. Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company evaluates its estimates and assumptions, including those related to revenue recognition, allowance for doubtful accounts, useful lives of property and equipment and intangible assets, recoverability of long-lived assets and intangible assets with definite lives, goodwill, income taxes, fair value of equity instruments, fair value of auction rate securities, accounting for business combinations and contingencies, among others. The Company bases these estimates on historical and anticipated results, trends, and various other assumptions that it believes are reasonable, including assumptions as to future events. These estimates form the basis for making judgments about the carrying values of assets and liabilities and recorded revenue and expenses. Actual results could differ from these estimates. Reclassifications Certain previously reported amounts in the consolidated balance sheets as of December 31, 2014 and Note 12 have been reclassified to conform to the Company's current presentation as a result of the retrospective application of the authoritative guidance to simplify the presentation of debt issuance costs. Additionally, certain previously reported amounts in Note 10 have been reclassified to conform to the Company's current presentation within the reconciliation of the Company’s provision for income taxes and the amount computed at the statutory federal income tax rate. Revenue Recognition The Company primarily derives revenues by providing access to its proprietary database of commercial real estate information. The Company generally charges a fixed monthly amount for its subscription-based services. The Company and its subsidiaries' subscription-based services consist primarily of information, analytics and online marketplace services offered over the Internet to commercial real estate industry and related professionals. Subscription contract rates are based on the number of sites, number of users, organization size, the client’s business focus, geography, the number and types of services to which a client subscribes, the number of properties a client advertises and the prominence and placement of a client's advertised properties in the search results. A majority of the subscription-based license agreements have a term of one year and renew automatically. Revenue is recognized when (1) there is persuasive evidence of an arrangement, (2) the fee is fixed and determinable, (3) services have been rendered and payment has been contractually earned and (4) collectability is reasonably assured. Revenues from subscription-based services are recognized on a straight-line basis over the term of the agreement. Deferred revenue results from advance cash receipts from customers or amounts billed in advance to customers from the sale of subscription licenses and is recognized over the term of the license agreement. 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Cost of Revenues Cost of revenues principally consists of salaries and related expenses for the Company’s researchers who collect and analyze the commercial real estate data that is the basis for the Company’s information, analytics and online marketplaces. Additionally, cost of revenues includes the cost of data from third-party data sources, credit card and other transaction fees relating to processing customer transactions, which are expensed as incurred, and the amortization of acquired trade names and other intangible assets and database technology. Foreign Currency Translation The Company’s functional currency in its foreign locations is the local currency. Assets and liabilities are translated into U.S. dollars using the exchange rates as of the balance sheet dates. Revenues, expenses, gains and losses are translated at the average exchange rates in effect during each period. Gains and losses resulting from translation are included in accumulated other comprehensive loss. Net gains or losses resulting from foreign currency exchange transactions are included in the consolidated statements of operations. There were no material gains or losses from foreign currency exchange transactions for the years ended December 31, 2013 , 2014 and 2015 . Accumulated Other Comprehensive Loss The components of accumulated other comprehensive loss were as follows (in thousands): As of December 31, 2014 2015 Foreign currency translation adjustment $ (5,693 ) $ (7,159 ) Accumulated net unrealized loss on investments, net of tax (691 ) (435 ) Total accumulated other comprehensive loss $ (6,384 ) $ (7,594 ) There were no amounts reclassified out of accumulated other comprehensive loss to the consolidated statements of operations for the years ended December 31, 2013 , 2014 and 2015 , respectively. Advertising Costs The Company expenses advertising costs as incurred. Advertising costs include e-commerce, television, radio, print and other media advertising. Advertising costs were approximately $7.9 million , $28.7 million and $132.1 million for the years ended December 31, 2013 , 2014 and 2015 , respectively. Income Taxes Deferred income taxes result from temporary differences between the tax basis of assets and liabilities and the basis reported in the Company’s consolidated financial statements. Deferred tax liabilities and assets are determined based on the difference between the financial statement and the tax basis of assets and liabilities using enacted rates in effect during the year in which the Company expects differences to reverse. Valuation allowances are provided against assets, including net operating losses, if it is anticipated that some or all of an asset may not be realized through future taxable earnings or implementation of tax planning strategies. Interest and penalties related to income tax matters are recognized in income tax expense. Net Income (Loss) Per Share Net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding during the period on a basic and diluted basis. The Company’s potentially dilutive securities include stock options and restricted stock. Diluted net income (loss) per share considers the impact of potentially dilutive securities except in periods in which there is a net loss, as the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Stock-Based Compensation Equity instruments issued in exchange for employee services are accounted for using a fair-value based method and the fair value of such equity instruments is recognized as expense in the consolidated statements of operations. Stock-based compensation expense is measured at the grant date of the stock-based awards that vest over set time periods based on their fair values, and is recognized on a straight-line basis as expense over the vesting periods of the awards, net of an estimated forfeiture rate. For equity instruments that vest based on performance, the Company assesses the probability of the achievement of the performance conditions at the end of each reporting period, or more frequently based upon the occurrence of events that may change the probability of whether the performance conditions would be met. If the Company's initial estimates of the achievement of the performance conditions change, the related stock-based compensation expense and timing of recognition may fluctuate from period to period based on those estimates. For equity instruments that vest based on a performance condition and a market condition, the Company estimates the fair value of each equity instrument granted on the date of grant using a Monte-Carlo simulation model. This pricing model uses multiple simulations to evaluate the probability of achieving the market condition to calculate the fair value of the awards. Stock-based compensation expense is updated based on the expected achievement of the related performance conditions at the end of each reporting period. If the performance conditions are not met, no stock-based compensation expense will be recognized, and any previously recognized stock-based compensation expense will be reversed. In 2012, the Company granted performance-based restricted common stock awards that vest upon the Company’s achievement of $90.0 million of cumulative net income before interest, income taxes, depreciation and amortization (“EBITDA”) over a period of four consecutive calendar quarters if such performance is achieved by March 31, 2017, subject to certain approvals under the CoStar Group, Inc. 2007 Stock Incentive Plan. As of March 31, 2014, the Company had satisfied all performance conditions and the award recipients had satisfied all service conditions, and as a result, the restricted common stock granted under these awards vested. The Company recorded approximately $21.8 million , $2.2 million and $0 of stock-based compensation expense related to the 2012 performance-based restricted common stock awards for the years ended December 31, 2013 , 2014 and 2015 , respectively. In February 2014, the Compensation Committee of the Board of Directors of the Company approved grants of restricted common stock to the executive officers that vest based on the Company’s achievement of a three-year cumulative revenue goal established at the grant date, and are subject to forfeiture in the event the foregoing performance condition is not met by December 31, 2016. In March 2015, the Compensation Committee of the Board of Directors of the Company approved grants of restricted common stock to the executive officers that vest based on the Company’s achievement of a three-year cumulative revenue goal established at the grant date, and are subject to forfeiture in the event the foregoing performance condition is not met by December 31, 2017. These grants of restricted common stock are also subject to continuing employment requirements and a market condition based on total shareholder return (“TSR”). The actual number of shares that vest at the end of the respective three-year period is determined based on the Company’s achievement of the three -year performance goals described above, as well as its TSR relative to the Russell 1000 Index over the same three-year performance period. As of December 31, 2015 , the Company reassessed the probability of achieving the performance and market conditions and determined that it was probable that the performance and market conditions for the 2014 and 2015 awards would be met by their respective forfeiture dates. As a result, the Company recorded a total of approximately $1.1 million and $2.8 million of stock-based compensation expense related to the performance-based restricted common stock awards with a market condition for the years ended December 31, 2014 and 2015 , respectively. The Company expects to record estimated stock-based compensation expense related to the performance-based restricted common stock awards of approximately $5.3 million over the periods 2016, 2017 and 2018. Cash flows resulting from excess tax benefits are classified as part of cash flows from operating and financing activities. Excess tax benefits represent tax benefits for stock-based compensation in excess of the associated deferred tax asset for such equity compensation recorded as an increase to stockholders' equity. Net cash proceeds from the exercise of stock options and the purchase of shares under the Employee Stock Purchase Plan (“ESPP”) were approximately $18.1 million , $5.7 million and $7.4 million for the years ended December 31, 2013 , 2014 and 2015 , respectively. The Company realized approximately $19.6 million , $28.4 million and $8.5 million of excess tax benefits from stock options exercised and restricted stock awards vested for the years ended December 31, 2013 , 2014 and 2015 , respectively. 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Stock-Based Compensation — (Continued) Stock-based compensation expense for stock options and restricted stock issued under equity incentive plans and stock purchases under the ESPP included in the Company’s results of operations were as follows (in thousands): Year Ended December 31, 2013 2014 2015 Cost of revenues $ 4,553 $ 4,759 $ 5,815 Selling and marketing 4,954 3,776 5,114 Software development 7,244 5,095 5,712 General and administrative 24,798 14,637 17,896 Total stock-based compensation $ 41,549 $ 28,267 $ 34,537 Cash and Cash Equivalents The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents. Cash equivalents consist of money market fund investments and commercial paper. Investments The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities (“ARS”). Investments are carried at fair value. Concentration of Credit Risk and Financial Instruments The Company performs ongoing credit evaluations of its customers’ financial conditions and generally does not require that its customers’ obligations to the Company be secured. The Company maintains reserves for estimated inherent credit losses, and such losses have been within management’s expectations. The large size and widespread nature of the Company’s customer base and the Company’s lack of dependence on any individual customer mitigates the risk of nonpayment of the Company’s accounts receivable. No single customer accounted for more than 5% of the Company’s revenues for each of the years ended December 31, 2013 , 2014 and 2015 . The carrying amount of the accounts receivable approximates the net realizable value. The carrying value of the accounts receivable, accounts payable, accrued expenses and long-term debt approximates fair value. The Company holds cash at major financial institutions that often exceed Federal Deposit Insurance Corporation insured limits. The Company believes its credit risk is minimal. The Company manages its credit risk associated with cash concentrations by concentrating its cash deposits in high quality financial institutions and by periodically evaluating the credit quality of the primary financial institutions holding such deposits. Accounts Receivable, Net of Allowance for Doubtful Accounts Accounts receivable are recorded at the invoiced amount. Accounts receivable payment terms vary and amounts due from customers are stated in the financial statements net of an allowance for doubtful accounts. The allowance for doubtful accounts is based on the Company’s assessment of the collectability of customer accounts. The Company regularly reviews the allowance by considering factors such as historical experience, the aging of the balances, and current economic conditions that may affect a customer’s ability to pay. 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Property and Equipment, Net Property and equipment are stated at cost, net of accumulated depreciation and amortization. All repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated on a straight-line basis over the following estimated useful lives of the assets: Leasehold improvements Shorter of lease term or useful life Furniture and office equipment Five to ten years Research vehicles Five to ten years Computer hardware and software Two to five years Qualifying internal-use software costs incurred during the application development stage, which consist primarily of internal product development costs, outside services and purchased software license costs are capitalized and amortized over the estimated useful life of the asset. All other costs are expensed as incurred. Goodwill and Intangible Assets Goodwill represents the excess of costs over the fair value of assets of acquired businesses. Goodwill and intangible assets acquired in a business combination and determined to have an indefinite useful life are not amortized, but instead tested for impairment at least annually by each reporting unit. The Company’s operating segments, North America and International, are the reporting units tested for potential impairment. To determine whether it is necessary to perform the two-step goodwill impairment test, the Company may first assess qualitative factors to evaluate whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount. If the Company concludes that it is more likely than not that the fair value of a reporting unit is less than its carrying amount or if the Company elects not to assess qualitative factors, then the Company performs the two-step process. The first step is to determine the fair value of each reporting unit. The estimate of the fair value of each reporting unit is based on a projected discounted cash flow model that includes significant assumptions and estimates including the Company's discount rate, growth rate and future financial performance. Assumptions about the discount rate are based on a weighted average cost of capital for comparable companies. Assumptions about the growth rate and future financial performance of a reporting unit are based on the Company's forecasts, business plans, economic projections and anticipated future cash flows. The fair value of each reporting unit is compared to the carrying amount of the reporting unit. If the carrying value of the reporting unit exceeds the fair value, then the second step of the process is performed to measure the impairment loss. To determine whether it is necessary to perform the quantitative impairment test for indefinite-lived intangible assets, the Company may first assess qualitative factors to evaluate whether it is more likely than not that the fair value of the indefinite-lived intangible assets is less than the carrying amount. If the Company concludes that it is more likely than not that the fair value of the indefinite-lived intangible assets is less than the carrying amount or if the Company elects not to assess qualitative factors, then the Company performs a quantitative impairment test. The Company estimates the fair value of its existing indefinite-lived intangible assets using the relief from royalty method that includes significant assumptions and estimates including the Company's discount rate, revenue growth rate and royalty rate. Assumptions about the discount rate are based on a weighted average cost of capital for comparable companies. Assumptions about the revenue growth rate are based on the Company's forecasts, business plans and economic projections. Assumptions about the royalty rate are based on royalty agreements for comparable companies with similar intangible assets. Intangible assets with estimable useful lives that arose from acquisitions on or after July 1, 2001 are amortized over their respective estimated useful lives using a method of amortization that reflects the pattern in which the economic benefits of the intangible assets are consumed or otherwise used up. Intangible assets are reviewed for impairment at least annually, and more frequently whenever events or changes in circumstances indicate that the carrying value may not be recoverable . 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Goodwill and Intangible Assets — (Continued) Acquired database technology, customer base and trade names and other intangible assets are related to the Company’s acquisitions (see Notes 3, 7 and 8). Acquired database technology is amortized on a straight-line basis over periods ranging from five months to eight years . With the exception of the acquired trade name recorded in connection with the acquisition of LoopNet, acquired trade names and other intangible assets are amortized on a straight-line basis over periods ranging from seven months to fifteen years . The acquired trade name recorded in connection with the LoopNet acquisition has an indefinite estimated useful life and is not amortized, but is subject to annual impairment tests. Acquired intangible assets characterized as customer base consists of acquired customer contracts and the related customer relationships and are amortized over periods ranging from ten years to thirteen years . Acquired customer bases are typically amortized on an accelerated basis related to the expected economic benefit of the intangible asset. The cost of capitalized building photography is amortized on a straight-line basis over periods ranging from five months to five years . Long-Lived Assets Long-lived assets, such as property and equipment, and purchased intangibles subject to amortization, are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to estimated undiscounted future cash flows expected to be generated by the asset or asset group. If the carrying amount of an asset exceeds its estimated future cash flows, an impairment charge is recognized in the amount by which the carrying amount of the asset exceeds the fair value of the asset. Assets to be disposed of would be separately presented in the balance sheet and reported at the lower of the carrying amount or fair value less costs to sell, and would no longer be depreciated. The assets and liabilities of a disposal group classified as held for sale would be presented separately in the appropriate asset and liability sections of the balance sheet. Debt Issuance Costs Costs incurred in connection with the issuance of long-term debt are capitalized and amortized as interest expense over the term of the related debt using the effective interest method. These amounts are reflected in the consolidated balance sheets as a direct deduction from a combination of the current and long-term portion of debt. Upon a refinancing, previously capitalized debt issuance costs are expensed and included in loss on extinguishment of debt if the Company determines that there has been a substantial modification of the related debt. If the Company determines that there has not been a substantial modification of the related debt, any previously capitalized debt issuance costs are amortized as interest expense over the term of the new debt instrument using the effective interest method. The Company had capitalized debt issuance costs, net of amortization, of approximately $13.2 million and $9.9 million as of December 31, 2014 and 2015 , respectively. The debt issuance costs are associated with the financing commitment received from JPMorgan Chase Bank, N.A. (“J.P. Morgan Bank”) on April 27, 2011, the subsequent term loan facility and revolving credit facility established under a credit agreement dated February 16, 2012 (the “2012 Credit Agreement”), the financing commitment received from J.P. Morgan Bank, Bank of America, N.A., SunTrust Bank and Wells Fargo Bank, National Association on February 28, 2014, and the subsequent term loan facility and revolving credit facility established under a credit agreement dated April 1, 2014 (the “2014 Credit Agreement”). See Note 9 for additional information regarding the term loan facility and revolving credit facility. The Company amortized debt issuance costs of approximately $3.0 million , $3.3 million and $3.3 million for the years ended December 31, 2013 , 2014 and 2015 , respectively. Business Combinations The Company allocates the purchase consideration to the tangible assets acquired, liabilities assumed and intangible assets acquired based on their estimated fair values. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is recorded as goodwill. Such valuations require management to make significant estimates and assumptions, especially with respect to intangible assets. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired database technology, and acquired trade names from a market participant's perspective, useful lives and discount rates. During the measurement period, the Company may record adjustments to the assets acquired and liabilities assumed. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings. See Note 3 for additional information regarding the Company's recent business combinations. Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) and International Accounting Standards Board (“IASB”) jointly issued a new revenue recognition standard that is designed to improve financial reporting by creating common recognition guidance for U.S. GAAP and International Financial Reporting Standards (“IFRS”). This guidance provides a more robust framework for addressing revenue issues, improves the comparability of revenue recognition practices across industries, provides more useful information to users of financial statements through improved disclosure requirements and simplifies the presentation of financial statements. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. This guidance permits the use of either a full retrospective method or a modified retrospective approach. The modified retrospective approach would be applied only to the most current period presented along with a cumulative-effect adjustment at the date of adoption. The original effective date of the new standard was for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. In August 2015, the FASB issued an accounting standards update that defers by one year the effective date of this new revenue recognition standard. As a result, the new standard will be effective for annual reporting periods beginning after December 15, 2017, although companies may adopt the standard as early as the original effective date. Early application prior to the original effective date is not permitted. The Company has not yet determined when it will adopt the standard or selected a transition method and is currently evaluating the impact this guidance will have on its financial statements. In April 2015, the FASB issued authoritative guidance to simplify the presentation of debt issuance costs. This guidance requires a company to present debt issuance costs related to a recognized debt liability in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The recognition and measurement guidance for debt issuance costs are not affected by this guidance. In August 2015, the FASB issued an accounting standards update to address the presentation and subsequent measurement of debt issuance costs related to line-of-credit arrangements. The guidance allows an entity to present debt issuance costs related to a line-of-credit arrangement as an asset and subsequently amortize the deferred debt issuance costs ratably over the term of the line-of-credit arrangement, regardless of whether there are any outstanding borrowings under the line-of-credit arrangement. The April 2015 guidance and the August 2015 guidance are effective on a retrospective basis for annual reporting periods beginning after December 15, 2015, including interim periods within that reporting period. Early application is permitted. The Company has elected to early adopt this guidance on a retrospective basis. This guidance did not have a material impact on the Company's results of operations for the years ended December 31, 2013, 2014 and 2015, but it did require changes to the presentation of the 2014 and 2015 consolidated balance sheets and the notes to the consolidated financial statements. The Company had capitalized debt issuance costs, net of amortization, of approximately $13.2 million and $9.9 million as of December 31, 2014 and December 31, 2015 , respectively. These amounts are reflected in the consolidated balance sheets as a direct deduction from a combination of the current and long-term portion of debt, rather than as an asset, in accordance with the authoritative guidance. See Note 9 for further details on the presentation of debt issuance costs. In November 2015, the FASB issued authoritative guidance to simplify the presentation of deferred income taxes. This guidance requires a company to present deferred income tax assets and liabilities as non-current in the balance sheets. The guidance requiring a tax-paying component of a company to offset deferred tax liabilities and assets and present the deferred taxes as a single amount is not affected by this guidance. This guidance is effective on either a prospective or retrospective basis for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. Early application is permitted. The Company has elected to early adopt this guidance on a prospective basis and as a result, prior periods were not retrospectively adjusted. This guidance did not have a material impact on the Company's results of operations for the year ended December 31, 2015.

ACQUISITIONS

ACQUISITIONS	12 Months Ended
	Dec. 31, 2015
Business Combinations [Abstract]
ACQUISITIONS	ACQUISITIONS Apartments.com On February 28, 2014, the Company and Classified Ventures, LLC (“CV”) entered into an Asset Purchase Agreement (the “Asset Purchase Agreement”). Pursuant to the Asset Purchase Agreement, on April 1, 2014 (the “Closing Date”), the Company purchased from CV certain assets and assumed certain liabilities, in each case, related to the Apartments.com TM business (collectively referred to as “Apartments.com”). Apartments.com is a national online apartment rentals resource for renters, property managers and owners. Apartments.com offers renters a searchable database of apartment listings and provides professional property management companies and landlords with an advertising destination. Renters can conduct personalized searches of apartment listings and view video demonstrations and community reviews through the Apartments.com website and mobile applications. The Apartments.com network of rental websites also includes ApartmentHomeLiving.com, another national online apartment rentals resource. The acquisition increased the Company's presence in the multifamily vertical. In consideration for the purchase of Apartments.com, on April 1, 2014, the Company paid $587.1 million in cash, including an estimated $2.1 million in connection with a preliminary net working capital adjustment as of the Closing Date. Pursuant to the terms of the Asset Purchase Agreement, the purchase price was reduced by approximately $2.9 million following the final determination of the net working capital of Apartments.com as of the Closing Date, and CV paid the Company $2.9 million on July 9, 2014. The Company applied the acquisition method to account for the Apartments.com transaction, which requires that, among other things, assets acquired and liabilities assumed be recorded at their fair values as of the acquisition date. The following table summarizes the amounts for acquired assets and liabilities recorded at their fair values as of the acquisition date (in thousands): Accounts receivable $ 11,402 Goodwill 421,724 Acquired trade names and other intangible assets 71,779 Acquired customer base 69,684 Acquired database technology 11,489 Acquired building photography 1,006 Other assets and liabilities (2,866 ) Fair value of identifiable net assets acquired $ 584,218 The net assets of Apartments.com were recorded at their estimated fair value. In valuing acquired assets and liabilities, fair value estimates were based on, but were not limited to, future expected cash flows, market rate assumptions for contractual obligations, and appropriate discount rates. The acquired customer base for the acquisition consists of one distinct intangible asset, is composed of acquired customer contracts and the related customer relationships, and has an estimated useful life of ten years . The acquired database technology had an estimated useful life of one year due to the Company's intent to replace the acquired database technology, which it did in February of 2015. The acquired trade names and other intangible assets have a weighted average estimated useful life of thirteen years . The acquired building photography has an estimated useful life of three years . Amortization of the acquired customer base is recognized on an accelerated basis related to the expected economic benefit of the intangible asset, while amortization of the acquired database technology, acquired building photography and acquired trade names and other intangible assets is recognized on a straight-line basis over their respective estimated useful lives. Goodwill recorded in connection with this acquisition is not amortized, but is subject to annual impairment tests. The $421.7 million of goodwill recorded as part of the acquisition is associated with the Company's North America operating segment and the Company expects the entire amount of goodwill to be deductible for income tax purposes in future periods. Goodwill is calculated as the excess of the consideration transferred over the net assets recognized and represents the future economic benefits arising from other assets acquired that could not be individually identified and separately recognized. Specifically, the goodwill recorded as part of the Apartments.com acquisition includes but is not limited to: (i) the expected synergies and other benefits that the Company believes will result from combining its operations with Apartments.com's operations; and (ii) any intangible assets that do not qualify for separate recognition, such as the assembled workforce. 3. ACQUISITIONS — (CONTINUED) As a result of the acquisition of Apartments.com, the Company recorded approximately $1.4 million in acquisition-related costs for the year ended December 31, 2014 . These costs include expenses directly related to acquiring Apartments.com, are expensed as incurred and are recorded in general and administrative expense. The Company did not record any acquisition-related costs as a result of the acquisition of Apartments.com for the years ended December 31, 2013 and 2015 . Apartment Finder Pursuant to the definitive agreement and plan of merger with Network Communications, Inc. (“NCI”) dated April 27, 2015 (the “Merger Agreement”), on June 1, 2015, the Company acquired 100% of the outstanding stock of NCI and the related Apartment Finder business (collectively referred to as “Apartment Finder”) from the former stockholders of NCI. Apartment Finder provides lead generation, advertising and Internet marketing solutions to property managers and owners through its main service, ApartmentFinder.com TM . The acquisition furthered the Company's expansion into the multifamily vertical. In consideration for the purchase of Apartment Finder, on June 1, 2015, the Company paid $172.7 million in cash, including an estimated $2.7 million in connection with a preliminary adjustment for net working capital as of the closing date. Pursuant to the terms of the Merger Agreement, the purchase price was increased by approximately $21,000 following the final determination of the net working capital of NCI as of the closing date, and this amount was paid to NCI in the third quarter of 2015. The Company applied the acquisition method to account for the Apartment Finder transaction, which requires that, among other things, assets acquired and liabilities assumed be recorded at their fair values as of the acquisition date. The following table summarizes the preliminary amounts for acquired assets and liabilities recorded at their fair values as of the acquisition date (in thousands): Cash and cash equivalents $ 39 Accounts receivable 4,556 Goodwill 107,692 Acquired trade names and other intangible assets 23,642 Acquired customer base 21,856 Acquired database technology 4,076 Acquired building photography 2,425 Deferred income taxes, net 9,290 Other assets and liabilities (849 ) Fair value of identifiable net assets acquired $ 172,727 The net assets of Apartment Finder were recorded at their estimated fair value. In valuing acquired assets and liabilities, fair value estimates were based on, but were not limited to, future expected cash flows, market rate assumptions for contractual obligations, and appropriate discount rates. The acquired customer base for the acquisition consisted of three distinct intangible assets, is composed of acquired customer contracts and the related customer relationships, and has a weighted average estimated useful life of ten years . The acquired database technology had an estimated useful life of five months due to the Company's intent to replace the acquired database technology, which it did in December of 2015. The acquired trade names and other intangible assets have a weighted average estimated useful life of nine years . The acquired building photography had an estimated useful life of five months . Amortization of the acquired customer base is recognized on an accelerated basis related to the expected economic benefit of the intangible asset, while amortization of the acquired database technology, acquired building photography and acquired trade names and other intangible assets is recognized on a straight-line basis over their respective estimated useful lives. Goodwill recorded in connection with this acquisition is not amortized, but is subject to annual impairment tests. The $107.7 million of goodwill recorded as part of the acquisition is associated with the Company's North America operating segment. None of the goodwill recognized is expected to be deductible for income tax purposes in future periods. 3. ACQUISITIONS — (CONTINUED) Apartment Finder — (Continued) Goodwill is calculated as the excess of the consideration transferred over the net assets recognized and represents the future economic benefits arising from other assets acquired that could not be individually identified and separately recognized. Specifically, the goodwill recorded as part of the Apartment Finder acquisition includes but is not limited to: (i) the expected synergies and other benefits that the Company believes will result from combining its operations with Apartment Finder's operations; and (ii) any intangible assets that do not qualify for separate recognition, such as the assembled workforce.

INVESTMENTS

INVESTMENTS	12 Months Ended
	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
INVESTMENTS	INVESTMENTS The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as ARS. Investments are carried at fair value. Scheduled maturities of investments classified as available-for-sale as of December 31, 2015 are as follows (in thousands): Maturity Fair Value Due in: 2016 $ — 2017 — 2020 1,052 2021 — 2025 — 2026 and thereafter 14,455 Available-for-sale investments $ 15,507 The Company had no realized gains on its investments for the years ended December 31, 2013 , 2014 and 2015 , respectively. The Company had no realized losses on its investments for the years ended December 31, 2013 , 2014 and 2015 , respectively. Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis. Changes in unrealized holding gains and losses, net of the related tax effect, on available-for-sale securities are excluded from earnings and are reported as a separate component of accumulated other comprehensive loss in stockholders’ equity until realized. A decline in market value of any available-for-sale security below cost that is deemed to be other-than-temporary results in a reduction in carrying amount to fair value. The impairment is charged to earnings and a new cost basis for the security is established. Dividend and interest income are recognized when earned. As of December 31, 2015 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 15,942 $ 610 $ (1,045 ) $ 15,507 Available-for-sale investments $ 15,942 $ 610 $ (1,045 ) $ 15,507 As of December 31, 2014 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 17,842 $ 380 $ (1,071 ) $ 17,151 Available-for-sale investments $ 17,842 $ 380 $ (1,071 ) $ 17,151 4. INVESTMENTS — (CONTINUED) The unrealized losses on the Company’s investments as of December 31, 2014 and 2015 were generated primarily from changes in interest rates and ARS that failed to settle at auction, due to adverse conditions in the global credit markets. The losses are considered temporary, as the contractual terms of these investments do not permit the issuer to settle the security at a price less than the amortized cost of the investment. Because the Company does not intend to sell these instruments and it is not more likely than not that the Company will be required to sell these instruments prior to anticipated recovery, which may be at maturity, the Company does not consider these investments to be other-than-temporarily impaired as of December 31, 2014 and 2015 . See Note 5 for further discussion of the fair value of the Company’s financial assets. The components of the Company’s investments in an unrealized loss position for twelve months or longer were as follows (in thousands): December 31, 2014 2015 Aggregate Fair Value Gross Unrealized Losses Aggregate Fair Value Gross Unrealized Losses Auction rate securities $ 16,329 $ (1,071 ) $ 14,455 $ (1,045 ) Investments in an unrealized loss position $ 16,329 $ (1,071 ) $ 14,455 $ (1,045 ) The Company did not have any investments in an unrealized loss position for less than twelve months as of December 31, 2014 and 2015 , respectively.

FAIR VALUE

FAIR VALUE	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
FAIR VALUE	FAIR VALUE Fair value is defined as the price that would be received in the sale of an asset or paid to transfer a liability in an orderly transaction between market participants. There is a three-tier fair value hierarchy, which categorizes the inputs used in measuring fair value. These tiers include: Level 1, defined as observable inputs such as quoted prices in active markets for identical assets or liabilities; Level 2, defined as inputs other than quoted prices in active markets that are either directly or indirectly observable; and Level 3, defined as unobservable inputs for which little or no market data exists, therefore requiring an entity to develop its own assumptions. The following table represents the Company's fair value hierarchy for its financial assets (cash, cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2015 (in thousands): Level 1 Level 2 Level 3 Total Assets: Cash $ 405,597 $ — $ — $ 405,597 Money market funds 5,043 — — 5,043 Commercial paper 11,178 — — 11,178 Auction rate securities — — 15,507 15,507 Total assets measured at fair value $ 421,818 $ — $ 15,507 $ 437,325 5. FAIR VALUE — (CONTINUED) The following table represents the Company's fair value hierarchy for its financial assets (cash, cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2014 (in thousands): Level 1 Level 2 Level 3 Total Assets: Cash $ 160,275 $ — $ — $ 160,275 Money market funds 310,482 — — 310,482 Commercial paper 56,255 — — 56,255 Auction rate securities — — 17,151 17,151 Total assets measured at fair value $ 527,012 $ — $ 17,151 $ 544,163 The Company’s Level 3 assets consist of ARS, whose underlying assets are primarily student loan securities supported by guarantees from the Federal Family Education Loan Program (“FFELP”) of the U.S. Department of Education. The following table summarizes changes in fair value of the Company’s Level 3 assets from December 31, 2013 to December 31, 2015 (in thousands): Auction Rate Securities Balance at December 31, 2013 $ 21,990 Decrease in unrealized loss included in accumulated other comprehensive loss 836 Settlements (5,675 ) Balance at December 31, 2014 $ 17,151 Decrease in unrealized loss included in accumulated other comprehensive loss 256 Settlements (1,900 ) Balance at December 31, 2015 $ 15,507 ARS are variable rate debt instruments whose interest rates are reset approximately every 28 days . The majority of the underlying securities have contractual maturities greater than twenty years . The ARS are recorded at fair value. As of December 31, 2015 , the Company held ARS with $16.8 million par value, all of which failed to settle at auction. The majority of these investments are of high credit quality with AAA credit ratings and are primarily student loan securities supported by guarantees from the FFELP of the U.S. Department of Education. The Company may not be able to liquidate and fully recover the carrying value of the ARS in the near term. As a result, these securities are classified as long-term investments in the Company’s consolidated balance sheet as of December 31, 2015 . While the Company continues to earn interest on its ARS investments at the contractual rate, these investments are not currently actively trading and therefore do not currently have a readily determinable market value. The estimated fair value of the ARS no longer approximates par value. The Company used a discounted cash flow model to determine the estimated fair value of its investment in ARS as of December 31, 2015 . The assumptions used in preparing the discounted cash flow model include estimates for interest rates, credit spreads, timing and amount of contractual cash flows, liquidity risk premiums, expected holding periods and default risk. The Company updates the discounted cash flow model on a quarterly basis to reflect any changes in the assumptions used in the model and settlements of ARS investments that occurred during the period. The only significant unobservable input in the discounted cash flow model is the discount rate. The discount rate used represents the Company's estimate of the yield expected by a market participant from the ARS investments. The weighted average discount rate used in the discounted cash flow model as of December 31, 2014 and 2015 was approximately 4.1% and 4.7% , respectively. Selecting another discount rate within the range used in the discounted cash flow model would not result in a significant change to the fair value of the ARS. 5. FAIR VALUE — (CONTINUED) Based on this assessment of fair value, as of December 31, 2015 , the Company determined there was a net decline in the fair value of its ARS investments of approximately $435,000 . The decline was deemed to be a temporary impairment and recorded as an unrealized loss in accumulated other comprehensive loss in stockholders’ equity. In addition, while a majority of the ARS are currently rated AAA, if the issuers are unable to successfully close future auctions and/or their credit ratings deteriorate, the Company may be required to record additional unrealized losses in accumulated other comprehensive loss or an other-than-temporary impairment charge to earnings on these investments.

PROPERTY AND EQUIPMENT

PROPERTY AND EQUIPMENT	12 Months Ended
	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
PROPERTY AND EQUIPMENT	PROPERTY AND EQUIPMENT Property and equipment consists of the following (in thousands): December 31, 2014 2015 Leasehold improvements $ 42,026 $ 49,752 Furniture, office equipment and research vehicles 31,016 38,129 Computer hardware and software 49,655 56,656 Property and equipment, gross 122,697 144,537 Accumulated depreciation and amortization (48,944 ) (56,226 ) Property and equipment, net $ 73,753 $ 88,311 Depreciation expense for property and equipment was approximately $12.5 million , $15.1 million and $20.0 million for the years ended December 31, 2013 , 2014 and 2015 , respectively.

GOODWILL

GOODWILL	12 Months Ended
	Dec. 31, 2015
Goodwill [Abstract]
GOODWILL	GOODWILL The changes in the carrying amount of goodwill by operating segment consist of the following (in thousands): North America International Total Goodwill, December 31, 2013 $ 692,639 $ 25,948 $ 718,587 Acquisition 421,724 — 421,724 Effect of foreign currency translation — (1,506 ) (1,506 ) Goodwill, December 31, 2014 1,114,363 24,442 1,138,805 Acquisitions 112,947 2,400 115,347 Effect of foreign currency translation — (1,207 ) (1,207 ) Goodwill, December 31, 2015 $ 1,227,310 $ 25,635 $ 1,252,945 The Company recorded goodwill of approximately $421.7 million in connection with the April 1, 2014 acquisition of the Apartments.com Business. The Company recorded goodwill of approximately $107.7 million in connection with the June 1, 2015 acquisition of Apartment Finder and recorded goodwill of approximately $2.4 million in connection with the July 1, 2015 acquisition of the assets of Belbex Corporate, S.L., a small commercial real estate information provider operating in Madrid, Spain. Additionally, the Company recorded goodwill of approximately $5.3 million during the year ended December 31, 2015 in connection with the acquisitions of certain assets related to the business operations of Apartment Finder's independent distributors within various markets. During the fourth quarters of 2013 , 2014 and 2015 , the Company completed the annual impairment test of goodwill and concluded that goodwill was not impaired.

INTANGIBLE ASSETS

INTANGIBLE ASSETS	12 Months Ended
	Dec. 31, 2015
Intangible Assets, Net (Excluding Goodwill) [Abstract]
INTANGIBLE ASSETS	INTANGIBLE ASSETS Intangible assets consist of the following (in thousands, except amortization period data): December 31, Weighted- Average Amortization Period (in years) 2014 2015 Capitalized product development cost $ 2,140 $ 2,243 4 Accumulated amortization (2,140 ) (2,172 ) Capitalized product development cost, net — 71 Building photography 14,943 17,677 4 Accumulated amortization (12,665 ) (15,875 ) Building photography, net 2,278 1,802 Acquired database technology 88,739 77,905 5 Accumulated amortization (60,498 ) (62,818 ) Acquired database technology, net 28,241 15,087 Acquired customer base 199,826 221,409 10 Accumulated amortization (102,443 ) (129,782 ) Acquired customer base, net 97,383 91,627 Acquired trade names and other intangible assets (1) 128,171 153,910 12 Accumulated amortization (14,451 ) (24,179 ) Acquired trade names and other intangible assets, net 113,720 129,731 Intangible assets, net $ 241,622 $ 238,318 (1) The weighted-average amortization period for acquired trade names excludes $48.7 million for acquired trade names recorded in connection with the LoopNet acquisition on April 30, 2012, which amount is not amortized, but is subject to annual impairment tests. Amortization expense for intangible assets was approximately $27.6 million , $55.3 million and $58.6 million for the years ended December 31, 2013 , 2014 and 2015 , respectively. In the aggregate, the Company expects amortization for intangible assets existing as of December 31, 2015 for future periods to be approximately $43.0 million , $28.5 million , $20.3 million , $17.4 million and $15.8 million for the years ending December 31, 2016, 2017, 2018, 2019 and 2020, respectively. During the fourth quarter of 2015 , the Company completed the annual impairment test of the acquired trade name recorded in connection with the LoopNet acquisition and concluded that this indefinite-lived intangible asset was not impaired. During the first quarter of 2014, the Company finalized a branding initiative plan that included, among other things, re-branding some of the services provided by its wholly owned subsidiaries, in order to better organize, update, streamline and optimize the Company’s branding strategy. The Company launched the branding initiative externally in the second quarter of 2014. Following the external launch of the branding initiative, the Company ceased using certain of its trade names. The Company evaluated these assets for impairment during the first quarter of 2014 and determined that the carrying value of trade names that the Company ceased using exceeded the fair value. The adjusted carrying value of the Company's trade name intangible assets associated with the branding initiative was amortized through the date of the external launch of the branding initiative and the fully amortized gross carrying amount was written off during the three months ended June 30, 2014. 8. INTANGIBLE ASSETS — (CONTINUED) During the third quarter of 2014, the Company finalized and launched a separate marketing plan that included the re-branding of a service provided by another one of its wholly owned subsidiaries, in order to provide its customers with a more enhanced experience. Following the external launch of the marketing plan, the Company ceased using one of its trade names. The Company evaluated the asset for impairment during the third quarter of 2014 and determined that the carrying value of the trade name that the Company ceased using exceeded the fair value. As a result of these branding and marketing plans, during 2014, the Company recorded impairment charges of approximately $1.8 million in cost of revenues in the consolidated statements of operations within its North America operating segment for the year ended December 31, 2014. In February 2015, as a result of the Company's product development efforts, it launched an improved Apartments.com website with a cleaner look, information about actual rental availabilities, rents and other fees, and better search functionality. In conjunction with the launch, the Company ceased using the database technology acquired in the acquisition of Apartments.com. The Company evaluated the acquired database technology for impairment during the first quarter of 2015 and determined that the carrying value of the acquired database technology was impaired as the Company had ceased using the asset. The Company recorded an impairment charge of approximately $1.4 million in cost of revenues in the consolidated statements of operations within the Company's North America operating segment for the year ended December 31, 2015. In June 2015, following the June 1, 2015 acquisition of Apartment Finder, the Company decided to cease providing certain Apartment Finder services. Additionally, in June 2015, the Company decided to cease development work related to a development project within Apartment Finder. The Company evaluated the acquired customer base and acquired database technology for impairment during the second quarter of 2015 and, based on that evaluation, determined that the customer base and database technology assets associated with the ceased services and development work were impaired as they were not expected to provide any economic benefit to the Company. The Company recorded an impairment charge of approximately $1.4 million , most of which was recorded in general and administrative expenses in the consolidated statements of operations within the Company's North America operating segment for the year ended December 31, 2015.

LONG-TERM DEBT

LONG-TERM DEBT	12 Months Ended
	Dec. 31, 2015
Long-term Debt, Current and Noncurrent [Abstract]
LONG-TERM DEBT	LONG-TERM DEBT On April 1, 2014, the Company entered into the 2014 Credit Agreement by and among the Company, as Borrower, CoStar Realty Information, Inc., as Co-Borrower, the Lenders from time to time party thereto and JPMorgan Chase Bank, N.A., as Administrative Agent. The 2014 Credit Agreement provides for a $400.0 million term loan facility and a $225.0 million revolving credit facility, each with a term of five years . The proceeds of the term loan facility and the initial borrowing of $150.0 million under the revolving credit facility on the Closing Date were used to refinance the 2012 Credit Agreement, including related fees and expenses, and to pay a portion of the consideration and transaction costs related to the acquisition of Apartments.com. The undrawn proceeds of the revolving credit facility are available for the Company's working capital needs and other general corporate purposes. During June 2014, the Company repaid the $150.0 million initial borrowing under the revolving credit facility. The carrying value of the term loan facility approximates fair value and can be estimated through Level 3 unobservable inputs using a valuation technique based on expected cash flows discounted using the current credit-adjusted risk-free rate, which approximates the rate of interest on the term loan facility at origination. Effective April 1, 2014, the Company terminated the 2012 Credit Agreement and repaid all amounts outstanding thereunder, which amounts totaled $148.8 million . The Company evaluated the execution of the 2014 Credit Agreement and termination of the 2012 Credit Agreement and determined that the transactions did not qualify as an extinguishment of debt because the change in the present value of future cash flows between the initial term loan facility under the 2012 Credit Agreement and the new term loan facility under the 2014 Credit Agreement was not considered a substantial modification. 9. LONG-TERM DEBT — (CONTINUED) The revolving credit facility includes a subfacility for swingline loans of up to $10.0 million , and up to $10.0 million of the revolving credit facility is available for the issuance of letters of credit. The term loan facility will amortize in quarterly installments in amounts resulting in an annual amortization of 5% during each of the first, second and third years, 10% during the fourth year and 15% during the fifth year after the Closing Date, with the remainder payable at final maturity. The loans under the 2014 Credit Agreement bear interest, at the Company's option, either (i) during any interest period selected by the Company, at the London interbank offered rate for deposits in U.S. dollars with a maturity comparable to such interest period, adjusted for statutory reserves (“LIBOR”), plus an initial spread of 2.00% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio (as defined in the 2014 Credit Agreement) of the Company, or (ii) at the greatest of (x) the prime rate from time to time announced by JPMorgan Chase Bank, N.A., (y) the federal funds effective rate plus 0.5% and (z) LIBOR for a one-month interest period plus 1.00% , plus an initial spread of 1.00% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio of the Company. If an event of default occurs under the 2014 Credit Agreement, the interest rate on overdue amounts will increase by 2.00% per annum. 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. The 2014 Credit Agreement requires the Company to maintain (i) a First Lien Secured Leverage Ratio (as defined in the 2014 Credit Agreement) not exceeding 4.00 to 1.00 during each full fiscal quarter after the Closing Date through the three months ended March 31, 2016, and 3.50 to 1.00 thereafter and (ii) after the incurrence of additional indebtedness under certain specified exceptions in the 2014 Credit Agreement, a Total Leverage Ratio (as defined in the 2014 Credit Agreement) not exceeding 5.00 to 1.00 during each full fiscal quarter after the Closing Date through the three months ended March 31, 2016, and 4.50 to 1.00 thereafter. The 2014 Credit Agreement also includes other covenants, including covenants that, subject to certain exceptions, restrict the ability of the Company and its subsidiaries to (i) incur additional indebtedness, (ii) create, incur, assume or permit to exist any liens, (iii) enter into mergers, consolidations or similar transactions, (iv) make investments and acquisitions, (v) make certain dispositions of assets, (vi) make dividends, distributions and prepayments of certain indebtedness, and (vii) enter into certain transactions with affiliates. The Company was in compliance with the covenants in the 2014 Credit Agreement as of December 31, 2015 . In connection with obtaining the term loan facility and revolving credit facility pursuant to the 2014 Credit Agreement, the Company incurred approximately $10.1 million in debt issuance costs as of April 1, 2014. The debt issuance costs were comprised of approximately $9.7 million in underwriting fees and approximately $400,000 primarily related to legal fees associated with the debt issuance. Approximately $10.0 million of the fees associated with the refinancing, along with the unamortized debt issuance cost from the 2012 Credit Agreement were capitalized and are amortized as interest expense over the term of the 2014 Credit Agreement using the effective interest method. As of December 31, 2014 and 2015 , no amounts were outstanding under the revolving credit facilities. Total interest expense for the term loan facilities and revolving credit facilities was approximately $6.9 million , $10.5 million and $9.4 million for the years ended December 31, 2013 , 2014 and 2015 , respectively. Interest expense included amortized debt issuance costs of approximately $3.0 million , $3.3 million and $3.3 million for the years ended December 31, 2013 , 2014 and 2015 , respectively. Total interest paid for the term loan facilities was approximately $4.3 million , $7.0 million and $6.1 million for the years ended December 31, 2013 , 2014 and 2015 , respectively. The following table represents the Company's long-term debt (in thousands): December 31, 2014 2015 Term loan facility $ 385,000 $ 365,000 Debt issuance costs, net (13,199 ) (9,888 ) Total debt 371,801 355,112 Current maturities of long-term debt (20,000 ) (20,000 ) Current debt issuance costs, net 3,335 3,254 Total long-term debt, less current portion $ 355,136 $ 338,366 9. LONG-TERM DEBT — (CONTINUED) Maturities of the Company's borrowings under the 2014 Credit Agreement for each of the next four years as of December 31, 2015 are as follows (in thousands): Year ending December 31, Maturities Due in: 2016 $ 20,000 2017 35,000 2018 55,000 2019 255,000 Long-term debt, including current maturities $ 365,000

INCOME TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The components of the provision for income taxes attributable to operations consist of the following (in thousands): Year Ended December 31, 2013 2014 2015 Current: Federal $ 26,516 $ 24,741 $ 10,295 State 3,996 2,761 1,503 Foreign 31 53 40 Total current 30,543 27,555 11,838 Deferred: Federal (10,919 ) (698 ) (7,475 ) State (1,849 ) (813 ) 1,683 Foreign 28 — — Total deferred (12,740 ) (1,511 ) (5,792 ) Total provision for income taxes $ 17,803 $ 26,044 $ 6,046 10. INCOME TAXES — (CONTINUED) The components of deferred tax assets and liabilities consist of the following (in thousands): December 31, 2014 2015 Deferred tax assets: Reserve for bad debts $ 1,825 $ 2,849 Accrued compensation 7,287 10,725 Stock compensation 8,758 12,686 Net operating losses 15,665 36,406 Accrued reserve and other 3,360 3,515 Unrealized loss on securities 491 377 Deferred rent 5,902 7,274 Deferred revenue 1,879 2,243 Deferred gain on the sale of building 10,690 9,128 Total deferred tax assets, prior to valuation allowance 55,857 85,203 Valuation allowance (7,783 ) (9,347 ) Total deferred tax assets, net of valuation allowance 48,074 75,856 Deferred tax liabilities: Prepaids (1,258 ) (1,335 ) Depreciation (9,806 ) (13,047 ) Intangibles (47,720 ) (56,952 ) Total deferred tax liabilities (58,784 ) (71,334 ) Net deferred tax assets (liabilities) $ (10,710 ) $ 4,522 As of December 31, 2014 and 2015 , a valuation allowance has been established for certain deferred tax assets due to the uncertainty of realization. The valuation allowance as of December 31, 2014 includes an allowance for unrealized losses on ARS investments and foreign deferred tax assets. The valuation allowance as of December 31, 2015 includes an allowance for unrealized losses on ARS investments, foreign deferred tax assets and state net operating losses and tax credits. The valuation allowance for the deferred tax asset for unrealized losses on ARS has been recorded as an adjustment to accumulated other comprehensive loss. The Company established the valuation allowance because it is more likely than not that a portion of the deferred tax asset for certain items will not be realized based on the weight of available evidence. A valuation allowance was established for the unrealized losses on securities as the Company has not historically generated capital gains, and it is uncertain whether the Company will generate sufficient capital gains in the future to absorb the capital losses. A valuation allowance was established for the foreign deferred tax assets due to the cumulative loss in recent years in those jurisdictions. The Company has not had sufficient taxable income historically to utilize the foreign deferred tax assets, and it is uncertain whether the Company will generate sufficient taxable income in the future to utilize the deferred tax assets. Similarly, the Company has established a valuation allowance for net operating losses and tax credits in certain states where it is uncertain whether the Company will generate sufficient taxable income to utilize the net operating losses and tax credits before they expire. The Company’s change in valuation allowance was a decrease of approximately $3.2 million for the year ended December 31, 2014 and an increase of approximately $1.6 million for the year ended December 31, 2015 . The increase for the year ended December 31, 2015 is due to an increase in the valuation allowance for U.S. deferred tax assets of approximately $2.2 million primarily related to the change in local tax law that occurred during the first quarter of 2015, partially offset by the decrease in the valuation allowance for foreign deferred tax assets of approximately $551,000 . 10. INCOME TAXES — (CONTINUED) The Company had U.S. income before income taxes of approximately $53.2 million , $70.6 million and $1.5 million for the years ended December 31, 2013 , 2014 and 2015 , respectively. The Company had foreign losses of approximately $5.6 million for the year ended December 31, 2013 . The Company had foreign income before income taxes of approximately $273,000 and $1.1 million for the years ended December 31, 2014 and 2015 , respectively. The Company’s provision for income taxes resulted in effective tax rates that varied from the statutory federal income tax rate as follows (in thousands): Year Ended December 31, 2013 2014 2015 Expected federal income tax provision at statutory rate $ 16,638 $ 24,820 $ 903 State income taxes, net of federal benefit 885 1,965 (678 ) Foreign income taxes, net effect (724 ) 336 469 Increase (decrease) in valuation allowance 588 (2,397 ) 1,956 Nondeductible compensation 431 554 574 Nondeductible transaction costs — — 229 Meals and entertainment 272 415 1,032 Tax rate changes 25 61 1,203 Other adjustments (312 ) 290 358 Income tax expense, net $ 17,803 $ 26,044 $ 6,046 The Company’s U.K. subsidiaries with foreign losses are disregarded entities for U.S. income tax purposes. Accordingly, the losses from these disregarded entities are included in the Company’s consolidated federal income tax provision at the statutory rate. Federal income taxes attributable to income from these disregarded entities are reduced by foreign taxes paid by those disregarded entities. The Company paid approximately $6.5 million , $3.0 million , and $1.3 million in income taxes for the years ended December 31, 2013 , 2014 and 2015 , respectively. The Company has net operating loss carryforwards for international income tax purposes of approximately $24.6 million , which do not expire. The Company has federal net operating loss carryforwards of approximately $64.3 million that begin to expire in 2020 , state net operating loss carryforwards with a tax value of approximately $6.9 million that begin to expire in 2020 and state income tax credit carryforwards with a tax value of approximately $2.3 million that begin to expire in 2020 . The Company realized a cash benefit relating to the use of its tax loss carryforwards of approximately $1.2 million and $1.3 million in 2014 and 2015 , respectively. 10. INCOME TAXES — (CONTINUED) The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands): Unrecognized tax benefits as of December 31, 2012 $ 3,909 Increase for current year tax positions 66 Increase for prior year tax positions 2,037 Expiration of the statute of limitation for assessment of taxes (55 ) Unrecognized tax benefits as of December 31, 2013 5,957 Increase for current year tax positions 51 Decrease for prior year tax positions (189 ) Expiration of the statute of limitation for assessment of taxes (70 ) Unrecognized tax benefits as of December 31, 2014 5,749 Increase for prior year tax positions 1,954 Expiration of the statute of limitation for assessment of taxes (39 ) Unrecognized tax benefits as of December 31, 2015 $ 7,664 Approximately $1.5 million and $1.4 million of the unrecognized tax benefits as of December 31, 2014 and 2015 , respectively, would favorably affect the annual effective tax rate, if recognized in future periods. The Company recognized $62,000 , $62,000 and $83,000 for interest and penalties in its consolidated statements of operations for the years ended December 31, 2013 , 2014 and 2015 , respectively. The Company had liabilities of $404,000 , $466,000 and $549,000 for interest and penalties in its consolidated balance sheets as of December 31, 2013 , 2014 and 2015 , respectively. The Company does not anticipate the amount of the unrecognized tax benefits to change significantly over the next twelve months. The Company is subject to taxation in the U.S. and various states and foreign jurisdictions. The Company’s federal income tax returns and most state income tax returns for tax years 2012 through 2014 remain open to examination. For states that have a four year statute of limitations, the state income tax returns for tax years 2011 through 2014 remain open to examination. The Company’s U.K. income tax returns for tax years 2009 through 2014 remain open to examination.

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES The Company leases office facilities and office equipment under various non-cancelable operating leases. The leases contain various renewal options. Rent expense for the years ended December 31, 2013 , 2014 and 2015 was approximately $18.3 million , $19.2 million and $21.4 million , respectively. Future minimum lease payments as of December 31, 2015 are as follows (in thousands): 2016 $ 22,896 2017 22,131 2018 20,477 2019 18,224 2020 18,130 2021 and thereafter 66,732 Total future minimum lease payments $ 168,590 On April 1, 2014, the Company entered into the 2014 Credit Agreement. The 2014 Credit Agreement provides for a $400.0 million term loan facility and a $225.0 million revolving credit facility, each with a term of five years . See Note 9 for additional information regarding the term loan facility and revolving credit facility. 11. COMMITMENTS AND CONTINGENCIES — (CONTINUED) Currently, and from time to time, the Company is involved in litigation incidental to the conduct of its business. In accordance with GAAP, the Company records a provision for a liability when it is both probable that a liability has been incurred and the amount can be reasonably estimated. At the present time, while it is reasonably possible that an unfavorable outcome may occur as a result of one or more of the Company’s current litigation matters, management has concluded that it is not probable that a loss has been incurred in connection with the Company’s current litigation. In addition, the Company is unable to estimate the possible loss or range of loss that could result from an unfavorable outcome in the Company’s current litigation and accordingly, the Company has not recognized any liability in the consolidated financial statements for unfavorable results, if any. Legal defense costs are expensed as incurred.

SEGMENT REPORTING

SEGMENT REPORTING	12 Months Ended
	Dec. 31, 2015
Segment Reporting [Abstract]
SEGMENT REPORTING	SEGMENT REPORTING The Company manages its business geographically in two operating segments, with the primary areas of measurement and decision-making being North America, which includes the U.S. and parts of Canada, and International, which includes parts of the U.K., Spain and France. The Company and its subsidiaries' subscription-based services consist primarily of information, analytics and online marketplace services offered over the Internet to commercial real estate industry and related professionals. The Company’s subscription-based information services consist primarily of CoStar Suite TM services. CoStar Suite is sold as a platform of service offerings consisting of CoStar Property Professional ® , CoStar COMPS Professional ® and CoStar Tenant ® and through the Company's mobile application, CoStarGo ® . CoStar Suite is the Company’s primary service offering in the North America and International operating segments. Management relies on an internal management reporting process that provides revenue and operating segment EBITDA, which is the Company’s net income (loss) before interest, income taxes, depreciation and amortization. Management believes that operating segment EBITDA is an appropriate measure for evaluating the operational performance of the Company’s operating segments. EBITDA is used by management to internally measure operating and management performance and to evaluate the performance of the business. However, this measure should be considered in addition to, not as a substitute for or superior to, income from operations or other measures of financial performance prepared in accordance with GAAP. Summarized information by operating segment consists of the following (in thousands): Year Ended December 31, 2013 2014 2015 Revenues North America $ 420,817 $ 552,141 $ 686,573 International External customers 20,126 23,795 25,191 Intersegment revenue 339 57 41 Total International revenue 20,465 23,852 25,232 Intersegment eliminations (339 ) (57 ) (41 ) Total revenues $ 440,943 $ 575,936 $ 711,764 EBITDA North America $ 97,348 $ 148,913 $ 87,092 International (3,136 ) 2,337 2,895 Total EBITDA $ 94,212 $ 151,250 $ 89,987 12. SEGMENT REPORTING — (CONTINUED) The reconciliation of EBITDA to net income (loss) consists of the following (in thousands): Year Ended December 31, 2013 2014 2015 EBITDA $ 94,212 $ 151,250 $ 89,987 Purchase amortization in cost of revenues (11,883 ) (26,290 ) (30,077 ) Purchase amortization in operating expenses (15,183 ) (28,432 ) (27,931 ) Depreciation and other amortization (12,992 ) (15,650 ) (20,524 ) Interest income 326 516 537 Interest expense (6,943 ) (10,481 ) (9,411 ) Income tax expense, net (17,803 ) (26,044 ) (6,046 ) Net income (loss) $ 29,734 $ 44,869 $ (3,465 ) Intersegment revenue recorded during 2013 was attributable to services performed for the Company’s wholly owned subsidiary, CoStar Portfolio Strategy, by Property and Portfolio Research Ltd., a wholly owned subsidiary of CoStar Portfolio Strategy. Intersegment revenue recorded during 2014 and 2015 was attributable to services performed for CoStar Portfolio Strategy by Grecam S.A.S. (“Grecam”), a wholly owned subsidiary of CoStar Limited, the Company's wholly owned U.K. holding company. Intersegment revenue is recorded at an amount the Company believes approximates fair value. North America EBITDA includes a corresponding cost for the services performed by Grecam and Property and Portfolio Research Ltd. North America EBITDA includes an allocation of approximately $844,000 , $1.1 million and $954,000 for the years ended December 31, 2013 , 2014 and 2015 , respectively. This allocation represents costs incurred for International employees involved in development activities of the Company's North America operating segment. International EBITDA includes a corporate allocation of approximately $411,000 , $261,000 and $256,000 for the years ended December 31, 2013 , 2014 and 2015 , respectively. This corporate allocation represents costs incurred for North America employees involved in management and expansion activities of the Company’s International operating segment. 12. SEGMENT REPORTING — (CONTINUED) Summarized information by operating segment consists of the following (in thousands): December 31, 2014 2015 Property and equipment, net North America $ 71,209 $ 86,191 International 2,544 2,120 Total property and equipment, net $ 73,753 $ 88,311 Goodwill North America $ 1,114,363 $ 1,227,310 International 24,442 25,635 Total goodwill $ 1,138,805 $ 1,252,945 Assets North America $ 2,125,569 $ 2,130,202 International 41,896 41,370 Total operating segment assets $ 2,167,465 $ 2,171,572 Reconciliation of operating segment assets to total assets Total operating segment assets $ 2,167,465 $ 2,171,572 Investment in subsidiaries (18,344 ) (18,344 ) Intersegment receivables (78,638 ) (73,657 ) Total assets $ 2,070,483 $ 2,079,571 Liabilities North America $ 551,633 $ 525,566 International 75,584 72,544 Total operating segment liabilities $ 627,217 $ 598,110 Reconciliation of operating segment liabilities to total liabilities Total operating segment liabilities $ 627,217 $ 598,110 Intersegment payables (70,280 ) (62,319 ) Total liabilities $ 556,937 $ 535,791

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	12 Months Ended
	Dec. 31, 2015
Stockholders' Equity Note [Abstract]
STOCKHOLDERS' EQUITY	STOCKHOLDERS’ EQUITY Preferred Stock The Company has 2,000,000 shares of preferred stock, $0.01 par value, authorized for issuance as of December 31, 2015 . The Board of Directors may issue the preferred stock from time to time as shares of one or more classes or series. Common Stock The Company has 60,000,000 shares of common stock, $0.01 par value, authorized for issuance. Dividends may be declared and paid on the common stock, subject in all cases to the rights and preferences of the holders of preferred stock and authorization by the Board of Directors. In the event of liquidation or winding up of the Company and after the payment of all preferential amounts required to be paid to the holders of any series of preferred stock, any remaining funds shall be distributed among the holders of the issued and outstanding common stock. Equity Offering During June 2014, the Company completed a public equity offering of 3,450,000 shares of common stock for $160.00 per share. Net proceeds from the public equity offering were approximately $529.4 million , after deducting approximately $22.1 million of underwriting discounts and commissions and offering expenses of approximately $500,000 . The Company has used and intends to continue to use the net proceeds from the sale of the securities to fund all or a portion of the costs of any strategic acquisitions it determines to pursue, to finance the growth of its business and for general corporate purposes. General corporate purposes may include additions to working capital, capital expenditures, repayment of debt, investments in the Company’s subsidiaries, possible acquisitions and the repurchase, redemption or retirement of securities, including the Company’s common stock.

NET INCOME (LOSS) PER SHARE

NET INCOME (LOSS) PER SHARE	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
NET INCOME (LOSS) PER SHARE	NET INCOME (LOSS) PER SHARE The following table sets forth the calculation of basic and diluted net income (loss) per share (in thousands except per share data): Year Ended December 31, 2013 2014 2015 Numerator: Net income (loss) $ 29,734 $ 44,869 $ (3,465 ) Denominator: Denominator for basic net income (loss) per share — weighted-average outstanding shares 27,670 30,215 31,950 Effect of dilutive securities: Stock options and restricted stock 542 426 — Denominator for diluted net income (loss) per share — weighted-average outstanding shares 28,212 30,641 31,950 Net income (loss) per share — basic $ 1.07 $ 1.48 $ (0.11 ) Net income (loss) per share — diluted $ 1.05 $ 1.46 $ (0.11 ) Employee stock options with exercise prices greater than the average market price of the Company’s common stock for the period are excluded from the calculation of diluted net income per share as their inclusion would be anti-dilutive. Additionally, shares of restricted common stock that vest based on Company performance and service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. Finally, shares of restricted common stock units that vest based on Company service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. 14. NET INCOME (LOSS) PER SHARE — (CONTINUED) Other than the shares of restricted common stock that vest based on Company performance conditions, no other potential common shares were excluded from the calculation of diluted net income for the year ended December 31, 2013 . Stock options to purchase approximately 80,000 shares that were outstanding for the year ended December 31, 2014 were not included in the computation of diluted net income per share because the exercise price of the stock options was greater than the average market share price of the common stock during the period. The Company did not consider the impact of potentially dilutive securities for the year ended December 31, 2015 when calculating the diluted net loss per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Shares underlying restricted common stock awards and restricted stock units that vest based on Company performance and/or service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. The following table summarizes the shares underlying the performance-based restricted stock awards and service-based restricted stock units excluded from the basic and diluted calculation (in thousands): Year Ended December 31, 2013 2014 2015 Performance-based restricted stock awards 379 23 55 Service-based restricted stock units — 1 1 Total shares excluded from computation 379 24 56

EMPLOYEE BENEFIT PLANS

EMPLOYEE BENEFIT PLANS	12 Months Ended
	Dec. 31, 2015
Compensation Related Costs [Abstract]
EMPLOYEE BENEFIT PLANS	EMPLOYEE BENEFIT PLANS Stock Incentive Plans In June 1998, the Company’s Board of Directors adopted the 1998 Stock Incentive Plan (as amended, the “1998 Plan”) prior to consummation of the Company’s initial public offering. In April 2007, the Company’s Board of Directors adopted the CoStar Group, Inc. 2007 Stock Incentive Plan (as amended, the “2007 Plan”), subject to stockholder approval, which was obtained on June 7, 2007. All shares of common stock that were authorized for issuance under the 1998 Plan that, as of June 7, 2007, remained available for issuance under the 1998 Plan (excluding shares subject to outstanding awards) were rolled into the 2007 Plan and, as of that date, no shares of common stock were available for new awards under the 1998 Plan. The 1998 Plan continues to govern unexercised and unexpired awards issued under the 1998 Plan prior to June 7, 2007. The 1998 Plan provided for the grant of stock and stock options to officers, directors and employees of the Company and its subsidiaries. Stock options granted under the 1998 Plan could be incentive or non-qualified, and the exercise price for an incentive stock option may not be less than the fair market value of the Company’s common stock on the date of grant. The vesting period of the options and restricted stock grants under the 1998 Plan was determined by the Board of Directors or a committee thereof and was generally three to four years . Upon the occurrence of a Change of Control, as defined in the 1998 Plan, all outstanding unexercisable options and restricted stock grants under the 1998 Plan immediately become exercisable. 15. EMPLOYEE BENEFIT PLANS — (CONTINUED) Stock Incentive Plans — (Continued) The 2007 Plan provides for the grant of stock options, restricted stock, restricted stock units, and stock appreciation rights to officers, employees, directors and consultants of the Company and its subsidiaries. Stock options granted under the 2007 Plan may be non-qualified or may qualify as incentive stock options. Except in limited circumstances related to a merger or other acquisition, the exercise price for an option may not be less than the fair market value of the Company’s common stock on the date of grant. The vesting period for each grant of options, restricted stock, restricted stock units and stock appreciation rights under the 2007 Plan is determined by the Board of Directors or a committee thereof and is generally three to four years , subject to minimum vesting periods for restricted stock and restricted stock units of at least one year . In some cases, vesting of awards under the 2007 Plan may be based on performance conditions. The Company has issued and/or reserved the following shares of common stock for issuance under the 2007 Plan (including an increase of 1,300,000 shares of common stock pursuant to an amendment to the 2007 Plan approved by the Company’s stockholders on June 2, 2010 and an increase of 900,000 shares of common stock pursuant to an amendment to the 2007 Plan approved by the Company’s stockholders on June 5, 2012): (a) 3,200,000 shares of common stock, plus (b) 121,875 shares of common stock that were authorized for issuance under the 1998 Plan that, as of June 7, 2007, remained available for issuance under the 1998 Plan (not including any Shares that were subject as of such date to outstanding awards under the 1998 Plan), and (c) any shares of common stock subject to outstanding awards under the 1998 Plan as of June 7, 2007, that on or after such date cease for any reason to be subject to such awards (other than by reason of exercise or settlement of the awards to the extent they are exercised for or settled in vested and nonforfeitable shares). Unless terminated sooner, the 2007 Plan will terminate in April 2017, but will continue to govern unexercised and unexpired awards issued under the 2007 Plan prior to that date. Approximately 1.2 million and 1.0 million shares were available for future grant under the 2007 Plan as of December 31, 2014 and 2015 , respectively. At December 31, 2015 , there was $61.7 million of unrecognized compensation cost related to stock incentive plans, net of forfeitures, which the Company expects to recognize over a weighted-average-period of 2.4 years . Stock Options Option activity was as follows: Number of Shares Range of Exercise Price Weighted- Average Exercise Price Weighted- Average Remaining Contract Life (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2012 673,719 $25.00 - $60.23 $ 45.20 Granted 126,800 $102.16 - $102.16 $ 102.16 Exercised (409,799 ) $25.00 - $58.95 $ 41.05 Canceled or expired (16,380 ) $36.48 - $58.95 $ 47.54 Outstanding at December 31, 2013 374,340 $36.48 - $102.16 $ 68.94 Granted 87,700 $201.04 - $201.04 $ 201.04 Exercised (68,126 ) $39.00 - $102.16 $ 55.81 Canceled or expired (23,735 ) $58.95 - $201.04 $ 124.09 Outstanding at December 31, 2014 370,179 $36.48 - $201.04 $ 99.12 Granted 89,500 $193.69 - $193.69 $ 193.69 Exercised (59,602 ) $36.48 - $201.04 $ 85.48 Outstanding at December 31, 2015 400,077 $36.48 - $201.04 $ 122.30 6.98 $ 33,761 Exercisable at December 31, 2013 146,161 $36.48 - $60.23 $ 47.72 Exercisable at December 31, 2014 188,656 $36.48 - $102.16 $ 60.54 Exercisable at December 31, 2015 220,107 $36.48 - $201.04 $ 77.63 5.76 $ 28,408 15. EMPLOYEE BENEFIT PLANS — (CONTINUED) Stock Options — (Continued) The aggregate intrinsic value is calculated as the difference between (i) the closing price of the common stock at the end of the period and (ii) the exercise prices of the underlying awards, multiplied by the shares underlying options as of the end of the period that had an exercise price less than the closing price on that date. Options to purchase 409,799 , 68,126 and 59,602 shares were exercised during the years ended December 31, 2013 , 2014 , and 2015 , respectively. The aggregate intrinsic value of options exercised, determined as of the date of option exercise, was $39.0 million , $8.9 million and $6.8 million for the years ended December 31, 2013 , 2014 , and 2015 , respectively. The weighted-average grant date fair value of each option granted during the years ended December 31, 2013 , 2014 and 2015 using the Black-Scholes option-pricing model was $34.10 , $58.12 and $56.53 , respectively. The Company estimated the fair value of each option granted on the date of grant using the Black-Scholes option-pricing model, using the assumptions in the following table: Year Ended December 31, 2013 2014 2015 Dividend yield 0 % 0 % 0 % Expected volatility 37 % 30 % 30 % Risk-free interest rate 0.9 % 1.5 % 1.6 % Expected life (in years) 5 5 5 The assumptions above and the estimation of expected forfeitures are based on multiple factors, including historical employee behavior patterns of exercising options and post-employment termination behavior, expected future employee option exercise patterns, and the historical volatility of the Company’s stock price. The following table summarizes information regarding options outstanding at December 31, 2015 : Options Outstanding Options Exercisable Range of Exercise Price Number of Shares Weighted-Average Remaining Contractual Life (in years) Weighted- Average Exercise Price Number of Shares Weighted- Average Exercise Price $36.48 - $42.50 55,051 4.06 $ 41.25 55,051 $ 41.25 $42.51 - $58.06 48,638 4.88 $ 55.94 48,638 $ 55.94 $58.07 - $59.59 45,567 6.14 $ 58.95 45,567 $ 58.95 $59.60 - $81.19 2,320 5.42 $ 60.23 2,320 $ 60.23 $81.20 - $147.93 82,801 7.19 $ 102.16 45,599 $ 102.16 $147.94 - $197.37 89,500 9.17 $ 193.69 — $ — $197.38 - $201.04 76,200 8.16 $ 201.04 22,932 $ 201.04 $36.48 - $201.04 400,077 6.98 $ 122.30 220,107 $ 77.63 15. EMPLOYEE BENEFIT PLANS — (CONTINUED) Restricted Stock Awards In February 2012, the Compensation Committee of the Board of Directors of the Company approved grants of restricted common stock to the executive officers that vest based on the achievement of certain performance conditions. These awards support the Company’s goals of aligning executive incentives with long-term stockholder value and ensuring that executive officers have a continuing stake in the long-term success of the Company. In May and December of 2012, the Company granted additional shares of restricted common stock that vest based on the achievement of certain performance conditions to other employees. These shares of performance-based restricted common stock vest upon the Company’s achievement of $90.0 million of cumulative EBITDA over a period of four consecutive calendar quarters, and are subject to forfeiture in the event the foregoing performance condition is not met by March 31, 2017. The Company granted a total of 399,413 shares of performance-based restricted common stock during the year ended December 31, 2012. All of the awards were made under the 2007 Plan and pursuant to the Company’s standard form of restricted stock grant agreement. The number of shares granted was based on the fair market value of the Company’s common stock on the grant date. As of March 31, 2014, the Company had satisfied all performance conditions and the award recipients had satisfied all service conditions, and as a result, the restricted common stock granted under these awards vested. The Company recorded approximately $21.8 million , $2.2 million and $0 of stock-based compensation expense related to the 2012 performance-based restricted common stock for the years ended December 31, 2013 , 2014 and 2015 , respectively. In February 2014, the Compensation Committee of the Board of Directors of the Company approved grants of restricted common stock to the executive officers that vest based on the Company’s achievement of a three-year cumulative revenue goal established at the grant date, and are subject to forfeiture in the event the foregoing performance condition is not met by December 31, 2016. In March 2015, the Compensation Committee of the Board of Directors of the Company approved grants of restricted common stock to the executive officers that vest based on the Company’s achievement of a three-year cumulative revenue goal established at the grant date, and are subject to forfeiture in the event the foregoing performance condition is not met by December 31, 2017. The number of shares that may be earned ranges between 0% (if the specified threshold performance level is not attained) and 200% (if performance meets or exceeds the maximum achievement level) of the target award. If actual performance exceeds the pre-established threshold, the number of shares earned is calculated based on the relative performance between specified levels of achievement. These awards support the Company’s goals of aligning executive incentives with long-term stockholder value and ensuring that executive officers have a continuing stake in the long-term success of the Company. The 2014 and 2015 performance-based restricted common stock awards are subject to continuing employment requirements and to a market condition. The actual number of shares that vest at the end of the respective three-year period is determined based on the Company’s achievement of the three -year performance goals described above, as well as its TSR relative to the Russell 1000 Index over the same three-year performance period. At the end of the three-year performance period, if the performance condition is achieved at or above the pre-established threshold, the number of shares earned is further adjusted by a TSR payout percentage, which ranges between 80.0% and 120.0% , based on the Company’s TSR performance relative to that of the Russell 1000 Index over the respective three-year period. The Company granted a total of 24,720 shares of 2014 performance-based restricted common stock during the year ended December 31, 2014 and a total of 32,400 shares of 2015 performance-based restricted common stock during the year ended December 31, 2015 . The Company estimates the fair value of its performance-based restricted common stock awards with a market condition on the date of grant using a Monte-Carlo simulation valuation model. This pricing model uses multiple simulations to evaluate the probability of the Company achieving various stock price levels to determine the expected TSR performance ranking. Expense is only recorded for awards that are expected to vest, net of estimated forfeitures. The assumptions used to estimate the fair value of performance-based restricted common stock awards with a market condition granted were as follows: Year Ended December 31, Year Ended December 31, 2014 2015 Dividend yield 0 % 0 % Expected volatility 30 % 26 % Risk-free interest rate 0.6 % 1.0 % Expected life (in years) 3 3 Weighted-average grant date fair value $ 216.20 $ 208.08 15. EMPLOYEE BENEFIT PLANS — (CONTINUED) Restricted Stock Awards — (Continued) As of December 31, 2015 , the Company reassessed the probability of achieving the performance and market conditions and determined that it was probable that the performance and market conditions for the 2014 and 2015 performance-based restricted common stock awards would be met by their forfeiture dates. As a result, the Company recorded a total of approximately $1.1 million and $2.8 million of stock-based compensation expense related to the performance-based restricted common stock awards with a market condition for the years ended December 31, 2014 and 2015 , respectively. The Company expects to record an estimated stock-based compensation expense related to the performance-based restricted common stock awards of approximately $5.3 million over the periods 2016, 2017 and 2018. The following table presents unvested restricted stock awards activity without a market condition and performance-based restricted common stock awards activity with a market condition for the year ended December 31, 2015 : Restricted Stock Awards — without Market Condition Restricted Stock Awards — with Market Condition Number of Shares Weighted-Average Grant Date Fair Value per Share Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock awards at December 31, 2014 556,793 $ 126.01 22,560 $ 216.20 Granted 207,442 $ 201.88 32,400 $ 208.08 Vested (221,546 ) $ 102.02 — $ — Canceled (39,576 ) $ 144.28 — $ — Unvested restricted stock awards at December 31, 2015 503,113 $ 166.42 54,960 $ 211.41 Restricted Stock Units The following table presents unvested restricted stock units activity for the year ended December 31, 2015 : Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock units at December 31, 2014 887 $ 169.16 Granted 543 $ 211.98 Vested — $ — Canceled — $ — Unvested restricted stock units at December 31, 2015 1,430 $ 185.42 Employee 401(k) Plan The Company maintains a 401(k) Plan (the “401(k)”) as a defined contribution retirement plan for all eligible employees. The 401(k) provides for tax-deferred contributions of employees’ salaries, limited to a maximum annual amount as established by the IRS. In addition to the traditional 401(k), effective January 1, 2015, eligible employees have the option of making an after-tax contribution to a Roth 401(k) plan or a combination of both. In 2013 , 2014 and 2015 , the Company matched 100% of employee contributions up to a maximum of 4% of total compensation. Amounts contributed to the 401(k) by the Company to match employee contributions for the years ended December 31, 2013 , 2014 and 2015 were approximately $5.1 million , $6.1 million and $7.5 million , respectively. The Company had no administrative expenses in connection with the 401(k) plan for the years ended December 31, 2013 , 2014 and 2015 , respectively. 15. EMPLOYEE BENEFIT PLANS — (CONTINUED) Employee Pension Plan The Company maintains a Group Personal Pension Plan (the “Plan”) for all eligible employees in the Company’s U.K. offices. The Plan is a defined contribution plan. Employees are eligible to contribute a portion of their salaries, subject to a maximum annual amount as established by Her Majesty's Revenue and Customs. In 2013 , 2014 and 2015 , the Company's matching contribution was based on the percentage contributed by the employee, up to a maximum of 6% of total compensation. Amounts contributed to the Plan by the Company to match employee contributions for the years ended December 31, 2013 , 2014 and 2015 were approximately $280,000 , $390,000 and $420,000 , respectively. Registered Retirement Savings Plan As of January 1, 2015, the Company introduced a registered retirement savings plan (“RRSP”) for all eligible employees in the Company’s Canadian offices. In 2015, the Company matched 100% of employee contributions up to a maximum of 4% of total compensation. Amounts contributed to the RRSP by the Company to match employee contributions for the year ended December 31, 2015 were approximately $ 40,000 . Employee Stock Purchase Plan As of August 1, 2006, the Company introduced an Employee Stock Purchase Plan (“ESPP”), pursuant to which eligible employees participating in the plan authorize the Company to withhold specified amounts from the employees’ compensation and use the withheld amounts to purchase shares of the Company's common stock at 90% of the market price. Participating employees are able to purchase common stock under this plan during each offering period. An offering period begins the second Saturday before each of the Company’s regular pay dates and ends on each of the Company’s regular pay dates. On June 3, 2015, the Company’s stockholders approved an amendment to the ESPP to increase the number of shares available for purchase under the ESPP by 100,000 shares. On September 14, 2015, the Company registered the issuance of these additional shares under the ESPP pursuant to the registration statement filed September 14, 2015. There were 21,774 and 108,547 shares available for purchase under the ESPP as of December 31, 2014 and 2015 , respectively, and approximately 13,121 and 13,227 shares of the Company’s common stock were purchased under the ESPP during 2014 and 2015 , respectively.

Schedule II Valuation and Quali

Schedule II Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2015
Valuation and Qualifying Accounts [Abstract]
Schedule II - Valuation and Qualifying Accounts	Schedule II – Valuation and Qualifying Accounts Years Ended December 31, 2013 , 2014 , and 2015 (in thousands): Allowance for Doubtful Accounts and Billing Adjustments (1) Balance at Beginning of Year Charged to Expense Charged to Other Accounts (2) Write-offs, Net of Recoveries Balance at End of Year Year ended December 31, 2013 $ 2,935 $ 2,317 $ — $ 1,855 $ 3,397 Year ended December 31, 2014 $ 3,397 $ 4,822 $ 881 $ 4,285 $ 4,815 Year ended December 31, 2015 $ 4,815 $ 7,002 $ 1,470 $ 5,809 $ 7,478 (1) Additions to the allowance for doubtful accounts are charged to bad debt expense. (2) Amounts represent opening balances from acquired businesses.

SUMMARY OF SIGNIFICANT ACCOUN24

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Accounting policies are consistent for each operating segment.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company evaluates its estimates and assumptions, including those related to revenue recognition, allowance for doubtful accounts, useful lives of property and equipment and intangible assets, recoverability of long-lived assets and intangible assets with definite lives, goodwill, income taxes, fair value of equity instruments, fair value of auction rate securities, accounting for business combinations and contingencies, among others. The Company bases these estimates on historical and anticipated results, trends, and various other assumptions that it believes are reasonable, including assumptions as to future events. These estimates form the basis for making judgments about the carrying values of assets and liabilities and recorded revenue and expenses. Actual results could differ from these estimates.
Reclassifications	Reclassifications Certain previously reported amounts in the consolidated balance sheets as of December 31, 2014 and Note 12 have been reclassified to conform to the Company's current presentation as a result of the retrospective application of the authoritative guidance to simplify the presentation of debt issuance costs. Additionally, certain previously reported amounts in Note 10 have been reclassified to conform to the Company's current presentation within the reconciliation of the Company’s provision for income taxes and the amount computed at the statutory federal income tax rate.
Revenue Recognition	Revenue Recognition The Company primarily derives revenues by providing access to its proprietary database of commercial real estate information. The Company generally charges a fixed monthly amount for its subscription-based services. The Company and its subsidiaries' subscription-based services consist primarily of information, analytics and online marketplace services offered over the Internet to commercial real estate industry and related professionals. Subscription contract rates are based on the number of sites, number of users, organization size, the client’s business focus, geography, the number and types of services to which a client subscribes, the number of properties a client advertises and the prominence and placement of a client's advertised properties in the search results. A majority of the subscription-based license agreements have a term of one year and renew automatically. Revenue is recognized when (1) there is persuasive evidence of an arrangement, (2) the fee is fixed and determinable, (3) services have been rendered and payment has been contractually earned and (4) collectability is reasonably assured. Revenues from subscription-based services are recognized on a straight-line basis over the term of the agreement. Deferred revenue results from advance cash receipts from customers or amounts billed in advance to customers from the sale of subscription licenses and is recognized over the term of the license agreement.
Cost of Revenues	Cost of Revenues Cost of revenues principally consists of salaries and related expenses for the Company’s researchers who collect and analyze the commercial real estate data that is the basis for the Company’s information, analytics and online marketplaces. Additionally, cost of revenues includes the cost of data from third-party data sources, credit card and other transaction fees relating to processing customer transactions, which are expensed as incurred, and the amortization of acquired trade names and other intangible assets and database technology.
Foreign Currency Translation	Foreign Currency Translation The Company’s functional currency in its foreign locations is the local currency. Assets and liabilities are translated into U.S. dollars using the exchange rates as of the balance sheet dates. Revenues, expenses, gains and losses are translated at the average exchange rates in effect during each period. Gains and losses resulting from translation are included in accumulated other comprehensive loss. Net gains or losses resulting from foreign currency exchange transactions are included in the consolidated statements of operations.
Advertising Costs	Advertising Costs The Company expenses advertising costs as incurred. Advertising costs include e-commerce, television, radio, print and other media advertising.
Income Taxes	Income Taxes Deferred income taxes result from temporary differences between the tax basis of assets and liabilities and the basis reported in the Company’s consolidated financial statements. Deferred tax liabilities and assets are determined based on the difference between the financial statement and the tax basis of assets and liabilities using enacted rates in effect during the year in which the Company expects differences to reverse. Valuation allowances are provided against assets, including net operating losses, if it is anticipated that some or all of an asset may not be realized through future taxable earnings or implementation of tax planning strategies. Interest and penalties related to income tax matters are recognized in income tax expense.
Net Income (Loss) Per Share	Net Income (Loss) Per Share Net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding during the period on a basic and diluted basis. The Company’s potentially dilutive securities include stock options and restricted stock. Diluted net income (loss) per share considers the impact of potentially dilutive securities except in periods in which there is a net loss, as the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Employee stock options with exercise prices greater than the average market price of the Company’s common stock for the period are excluded from the calculation of diluted net income per share as their inclusion would be anti-dilutive. Additionally, shares of restricted common stock that vest based on Company performance and service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. Finally, shares of restricted common stock units that vest based on Company service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share.
Stock-Based Compensation	Stock-Based Compensation Equity instruments issued in exchange for employee services are accounted for using a fair-value based method and the fair value of such equity instruments is recognized as expense in the consolidated statements of operations. Stock-based compensation expense is measured at the grant date of the stock-based awards that vest over set time periods based on their fair values, and is recognized on a straight-line basis as expense over the vesting periods of the awards, net of an estimated forfeiture rate. For equity instruments that vest based on performance, the Company assesses the probability of the achievement of the performance conditions at the end of each reporting period, or more frequently based upon the occurrence of events that may change the probability of whether the performance conditions would be met. If the Company's initial estimates of the achievement of the performance conditions change, the related stock-based compensation expense and timing of recognition may fluctuate from period to period based on those estimates. For equity instruments that vest based on a performance condition and a market condition, the Company estimates the fair value of each equity instrument granted on the date of grant using a Monte-Carlo simulation model. This pricing model uses multiple simulations to evaluate the probability of achieving the market condition to calculate the fair value of the awards. Stock-based compensation expense is updated based on the expected achievement of the related performance conditions at the end of each reporting period. If the performance conditions are not met, no stock-based compensation expense will be recognized, and any previously recognized stock-based compensation expense will be reversed. The Company estimates the fair value of its performance-based restricted common stock awards with a market condition on the date of grant using a Monte-Carlo simulation valuation model. This pricing model uses multiple simulations to evaluate the probability of the Company achieving various stock price levels to determine the expected TSR performance ranking. Expense is only recorded for awards that are expected to vest, net of estimated forfeitures.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents. Cash equivalents consist of money market fund investments and commercial paper.
Investments	Investments The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities (“ARS”). Investments are carried at fair value. The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as ARS. Investments are carried at fair value.
Concentration of Credit Risk and Financial Instruments	Concentration of Credit Risk and Financial Instruments The Company performs ongoing credit evaluations of its customers’ financial conditions and generally does not require that its customers’ obligations to the Company be secured. The Company maintains reserves for estimated inherent credit losses, and such losses have been within management’s expectations. The large size and widespread nature of the Company’s customer base and the Company’s lack of dependence on any individual customer mitigates the risk of nonpayment of the Company’s accounts receivable. No single customer accounted for more than 5% of the Company’s revenues for each of the years ended December 31, 2013 , 2014 and 2015 . The carrying amount of the accounts receivable approximates the net realizable value. The carrying value of the accounts receivable, accounts payable, accrued expenses and long-term debt approximates fair value. The Company holds cash at major financial institutions that often exceed Federal Deposit Insurance Corporation insured limits. The Company believes its credit risk is minimal. The Company manages its credit risk associated with cash concentrations by concentrating its cash deposits in high quality financial institutions and by periodically evaluating the credit quality of the primary financial institutions holding such deposits.
Accounts Receivable, Net of Allowance for Doubtful Accounts	Accounts Receivable, Net of Allowance for Doubtful Accounts Accounts receivable are recorded at the invoiced amount. Accounts receivable payment terms vary and amounts due from customers are stated in the financial statements net of an allowance for doubtful accounts. The allowance for doubtful accounts is based on the Company’s assessment of the collectability of customer accounts. The Company regularly reviews the allowance by considering factors such as historical experience, the aging of the balances, and current economic conditions that may affect a customer’s ability to pay.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost, net of accumulated depreciation and amortization. All repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated on a straight-line basis over the following estimated useful lives of the assets: Leasehold improvements Shorter of lease term or useful life Furniture and office equipment Five to ten years Research vehicles Five to ten years Computer hardware and software Two to five years Qualifying internal-use software costs incurred during the application development stage, which consist primarily of internal product development costs, outside services and purchased software license costs are capitalized and amortized over the estimated useful life of the asset. All other costs are expensed as incurred.
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill represents the excess of costs over the fair value of assets of acquired businesses. Goodwill and intangible assets acquired in a business combination and determined to have an indefinite useful life are not amortized, but instead tested for impairment at least annually by each reporting unit. The Company’s operating segments, North America and International, are the reporting units tested for potential impairment. To determine whether it is necessary to perform the two-step goodwill impairment test, the Company may first assess qualitative factors to evaluate whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount. If the Company concludes that it is more likely than not that the fair value of a reporting unit is less than its carrying amount or if the Company elects not to assess qualitative factors, then the Company performs the two-step process. The first step is to determine the fair value of each reporting unit. The estimate of the fair value of each reporting unit is based on a projected discounted cash flow model that includes significant assumptions and estimates including the Company's discount rate, growth rate and future financial performance. Assumptions about the discount rate are based on a weighted average cost of capital for comparable companies. Assumptions about the growth rate and future financial performance of a reporting unit are based on the Company's forecasts, business plans, economic projections and anticipated future cash flows. The fair value of each reporting unit is compared to the carrying amount of the reporting unit. If the carrying value of the reporting unit exceeds the fair value, then the second step of the process is performed to measure the impairment loss. To determine whether it is necessary to perform the quantitative impairment test for indefinite-lived intangible assets, the Company may first assess qualitative factors to evaluate whether it is more likely than not that the fair value of the indefinite-lived intangible assets is less than the carrying amount. If the Company concludes that it is more likely than not that the fair value of the indefinite-lived intangible assets is less than the carrying amount or if the Company elects not to assess qualitative factors, then the Company performs a quantitative impairment test. The Company estimates the fair value of its existing indefinite-lived intangible assets using the relief from royalty method that includes significant assumptions and estimates including the Company's discount rate, revenue growth rate and royalty rate. Assumptions about the discount rate are based on a weighted average cost of capital for comparable companies. Assumptions about the revenue growth rate are based on the Company's forecasts, business plans and economic projections. Assumptions about the royalty rate are based on royalty agreements for comparable companies with similar intangible assets. Intangible assets with estimable useful lives that arose from acquisitions on or after July 1, 2001 are amortized over their respective estimated useful lives using a method of amortization that reflects the pattern in which the economic benefits of the intangible assets are consumed or otherwise used up. Intangible assets are reviewed for impairment at least annually, and more frequently whenever events or changes in circumstances indicate that the carrying value may not be recoverable . 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Goodwill and Intangible Assets — (Continued) Acquired database technology, customer base and trade names and other intangible assets are related to the Company’s acquisitions (see Notes 3, 7 and 8). Acquired database technology is amortized on a straight-line basis over periods ranging from five months to eight years . With the exception of the acquired trade name recorded in connection with the acquisition of LoopNet, acquired trade names and other intangible assets are amortized on a straight-line basis over periods ranging from seven months to fifteen years . The acquired trade name recorded in connection with the LoopNet acquisition has an indefinite estimated useful life and is not amortized, but is subject to annual impairment tests. Acquired intangible assets characterized as customer base consists of acquired customer contracts and the related customer relationships and are amortized over periods ranging from ten years to thirteen years . Acquired customer bases are typically amortized on an accelerated basis related to the expected economic benefit of the intangible asset. The cost of capitalized building photography is amortized on a straight-line basis over periods ranging from five months to five years .
Long-Lived Assets	Long-Lived Assets Long-lived assets, such as property and equipment, and purchased intangibles subject to amortization, are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to estimated undiscounted future cash flows expected to be generated by the asset or asset group. If the carrying amount of an asset exceeds its estimated future cash flows, an impairment charge is recognized in the amount by which the carrying amount of the asset exceeds the fair value of the asset. Assets to be disposed of would be separately presented in the balance sheet and reported at the lower of the carrying amount or fair value less costs to sell, and would no longer be depreciated. The assets and liabilities of a disposal group classified as held for sale would be presented separately in the appropriate asset and liability sections of the balance sheet.
Debt Issuance Costs	Debt Issuance Costs Costs incurred in connection with the issuance of long-term debt are capitalized and amortized as interest expense over the term of the related debt using the effective interest method. These amounts are reflected in the consolidated balance sheets as a direct deduction from a combination of the current and long-term portion of debt. Upon a refinancing, previously capitalized debt issuance costs are expensed and included in loss on extinguishment of debt if the Company determines that there has been a substantial modification of the related debt. If the Company determines that there has not been a substantial modification of the related debt, any previously capitalized debt issuance costs are amortized as interest expense over the term of the new debt instrument using the effective interest method.
Business Combinations	Business Combinations The Company allocates the purchase consideration to the tangible assets acquired, liabilities assumed and intangible assets acquired based on their estimated fair values. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is recorded as goodwill. Such valuations require management to make significant estimates and assumptions, especially with respect to intangible assets. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired database technology, and acquired trade names from a market participant's perspective, useful lives and discount rates. During the measurement period, the Company may record adjustments to the assets acquired and liabilities assumed. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings. See Note 3 for additional information regarding the Company's recent business combinations.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) and International Accounting Standards Board (“IASB”) jointly issued a new revenue recognition standard that is designed to improve financial reporting by creating common recognition guidance for U.S. GAAP and International Financial Reporting Standards (“IFRS”). This guidance provides a more robust framework for addressing revenue issues, improves the comparability of revenue recognition practices across industries, provides more useful information to users of financial statements through improved disclosure requirements and simplifies the presentation of financial statements. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. This guidance permits the use of either a full retrospective method or a modified retrospective approach. The modified retrospective approach would be applied only to the most current period presented along with a cumulative-effect adjustment at the date of adoption. The original effective date of the new standard was for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. In August 2015, the FASB issued an accounting standards update that defers by one year the effective date of this new revenue recognition standard. As a result, the new standard will be effective for annual reporting periods beginning after December 15, 2017, although companies may adopt the standard as early as the original effective date. Early application prior to the original effective date is not permitted. The Company has not yet determined when it will adopt the standard or selected a transition method and is currently evaluating the impact this guidance will have on its financial statements. In April 2015, the FASB issued authoritative guidance to simplify the presentation of debt issuance costs. This guidance requires a company to present debt issuance costs related to a recognized debt liability in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The recognition and measurement guidance for debt issuance costs are not affected by this guidance. In August 2015, the FASB issued an accounting standards update to address the presentation and subsequent measurement of debt issuance costs related to line-of-credit arrangements. The guidance allows an entity to present debt issuance costs related to a line-of-credit arrangement as an asset and subsequently amortize the deferred debt issuance costs ratably over the term of the line-of-credit arrangement, regardless of whether there are any outstanding borrowings under the line-of-credit arrangement. The April 2015 guidance and the August 2015 guidance are effective on a retrospective basis for annual reporting periods beginning after December 15, 2015, including interim periods within that reporting period. Early application is permitted. The Company has elected to early adopt this guidance on a retrospective basis. This guidance did not have a material impact on the Company's results of operations for the years ended December 31, 2013, 2014 and 2015, but it did require changes to the presentation of the 2014 and 2015 consolidated balance sheets and the notes to the consolidated financial statements. The Company had capitalized debt issuance costs, net of amortization, of approximately $13.2 million and $9.9 million as of December 31, 2014 and December 31, 2015 , respectively. These amounts are reflected in the consolidated balance sheets as a direct deduction from a combination of the current and long-term portion of debt, rather than as an asset, in accordance with the authoritative guidance. See Note 9 for further details on the presentation of debt issuance costs. In November 2015, the FASB issued authoritative guidance to simplify the presentation of deferred income taxes. This guidance requires a company to present deferred income tax assets and liabilities as non-current in the balance sheets. The guidance requiring a tax-paying component of a company to offset deferred tax liabilities and assets and present the deferred taxes as a single amount is not affected by this guidance. This guidance is effective on either a prospective or retrospective basis for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. Early application is permitted. The Company has elected to early adopt this guidance on a prospective basis and as a result, prior periods were not retrospectively adjusted. This guidance did not have a material impact on the Company's results of operations for the year ended December 31, 2015.

ACQUISITIONS (Policies)

ACQUISITIONS (Policies)	12 Months Ended
	Dec. 31, 2015
Business Acquisition [Line Items]
Acquisitions	Business Combinations The Company allocates the purchase consideration to the tangible assets acquired, liabilities assumed and intangible assets acquired based on their estimated fair values. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is recorded as goodwill. Such valuations require management to make significant estimates and assumptions, especially with respect to intangible assets. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired database technology, and acquired trade names from a market participant's perspective, useful lives and discount rates. During the measurement period, the Company may record adjustments to the assets acquired and liabilities assumed. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings. See Note 3 for additional information regarding the Company's recent business combinations.
Apartments.com [Member]
Business Acquisition [Line Items]
Acquisitions	The Company applied the acquisition method to account for the Apartments.com transaction, which requires that, among other things, assets acquired and liabilities assumed be recorded at their fair values as of the acquisition date.
Apartment Finder [Member]
Business Acquisition [Line Items]
Acquisitions	The Company applied the acquisition method to account for the Apartment Finder transaction, which requires that, among other things, assets acquired and liabilities assumed be recorded at their fair values as of the acquisition date.

INVESTMENTS (Policies)

INVESTMENTS (Policies)	12 Months Ended
	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Investments	Investments The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities (“ARS”). Investments are carried at fair value. The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as ARS. Investments are carried at fair value.

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Policies)	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Currently, and from time to time, the Company is involved in litigation incidental to the conduct of its business. In accordance with GAAP, the Company records a provision for a liability when it is both probable that a liability has been incurred and the amount can be reasonably estimated.

NET INCOME (LOSS) PER SHARE (Po

NET INCOME (LOSS) PER SHARE (Policies)	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Net Income (Loss) Per Share	Net Income (Loss) Per Share Net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding during the period on a basic and diluted basis. The Company’s potentially dilutive securities include stock options and restricted stock. Diluted net income (loss) per share considers the impact of potentially dilutive securities except in periods in which there is a net loss, as the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Employee stock options with exercise prices greater than the average market price of the Company’s common stock for the period are excluded from the calculation of diluted net income per share as their inclusion would be anti-dilutive. Additionally, shares of restricted common stock that vest based on Company performance and service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. Finally, shares of restricted common stock units that vest based on Company service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share.

EMPLOYEE BENEFIT PLANS (Policie

EMPLOYEE BENEFIT PLANS (Policies)	12 Months Ended
	Dec. 31, 2015
Compensation Related Costs [Abstract]
Stock-Based Compensation	Stock-Based Compensation Equity instruments issued in exchange for employee services are accounted for using a fair-value based method and the fair value of such equity instruments is recognized as expense in the consolidated statements of operations. Stock-based compensation expense is measured at the grant date of the stock-based awards that vest over set time periods based on their fair values, and is recognized on a straight-line basis as expense over the vesting periods of the awards, net of an estimated forfeiture rate. For equity instruments that vest based on performance, the Company assesses the probability of the achievement of the performance conditions at the end of each reporting period, or more frequently based upon the occurrence of events that may change the probability of whether the performance conditions would be met. If the Company's initial estimates of the achievement of the performance conditions change, the related stock-based compensation expense and timing of recognition may fluctuate from period to period based on those estimates. For equity instruments that vest based on a performance condition and a market condition, the Company estimates the fair value of each equity instrument granted on the date of grant using a Monte-Carlo simulation model. This pricing model uses multiple simulations to evaluate the probability of achieving the market condition to calculate the fair value of the awards. Stock-based compensation expense is updated based on the expected achievement of the related performance conditions at the end of each reporting period. If the performance conditions are not met, no stock-based compensation expense will be recognized, and any previously recognized stock-based compensation expense will be reversed. The Company estimates the fair value of its performance-based restricted common stock awards with a market condition on the date of grant using a Monte-Carlo simulation valuation model. This pricing model uses multiple simulations to evaluate the probability of the Company achieving various stock price levels to determine the expected TSR performance ranking. Expense is only recorded for awards that are expected to vest, net of estimated forfeitures.

SUMMARY OF SIGNIFICANT ACCOUN30

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Schedule of accumulated other comprehensive loss	The components of accumulated other comprehensive loss were as follows (in thousands): As of December 31, 2014 2015 Foreign currency translation adjustment $ (5,693 ) $ (7,159 ) Accumulated net unrealized loss on investments, net of tax (691 ) (435 ) Total accumulated other comprehensive loss $ (6,384 ) $ (7,594 )
Stock-based compensation expense for stock options and restricted stock	Stock-based compensation expense for stock options and restricted stock issued under equity incentive plans and stock purchases under the ESPP included in the Company’s results of operations were as follows (in thousands): Year Ended December 31, 2013 2014 2015 Cost of revenues $ 4,553 $ 4,759 $ 5,815 Selling and marketing 4,954 3,776 5,114 Software development 7,244 5,095 5,712 General and administrative 24,798 14,637 17,896 Total stock-based compensation $ 41,549 $ 28,267 $ 34,537
Property and equipment, net	Property and equipment are stated at cost, net of accumulated depreciation and amortization. All repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated on a straight-line basis over the following estimated useful lives of the assets: Leasehold improvements Shorter of lease term or useful life Furniture and office equipment Five to ten years Research vehicles Five to ten years Computer hardware and software Two to five years Property and equipment consists of the following (in thousands): December 31, 2014 2015 Leasehold improvements $ 42,026 $ 49,752 Furniture, office equipment and research vehicles 31,016 38,129 Computer hardware and software 49,655 56,656 Property and equipment, gross 122,697 144,537 Accumulated depreciation and amortization (48,944 ) (56,226 ) Property and equipment, net $ 73,753 $ 88,311

ACQUISITIONS (Tables)

ACQUISITIONS (Tables)	12 Months Ended
	Dec. 31, 2015
Apartments.com [Member]
Business Acquisition [Line Items]
Schedule of assets acquired and liabilities assumed	The following table summarizes the amounts for acquired assets and liabilities recorded at their fair values as of the acquisition date (in thousands): Accounts receivable $ 11,402 Goodwill 421,724 Acquired trade names and other intangible assets 71,779 Acquired customer base 69,684 Acquired database technology 11,489 Acquired building photography 1,006 Other assets and liabilities (2,866 ) Fair value of identifiable net assets acquired $ 584,218
Apartment Finder [Member]
Business Acquisition [Line Items]
Schedule of assets acquired and liabilities assumed	The following table summarizes the preliminary amounts for acquired assets and liabilities recorded at their fair values as of the acquisition date (in thousands): Cash and cash equivalents $ 39 Accounts receivable 4,556 Goodwill 107,692 Acquired trade names and other intangible assets 23,642 Acquired customer base 21,856 Acquired database technology 4,076 Acquired building photography 2,425 Deferred income taxes, net 9,290 Other assets and liabilities (849 ) Fair value of identifiable net assets acquired $ 172,727

INVESTMENTS (Tables)

INVESTMENTS (Tables)	12 Months Ended
	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Scheduled maturities of investments classified as available-for-sale	Scheduled maturities of investments classified as available-for-sale as of December 31, 2015 are as follows (in thousands): Maturity Fair Value Due in: 2016 $ — 2017 — 2020 1,052 2021 — 2025 — 2026 and thereafter 14,455 Available-for-sale investments $ 15,507
Schedule of available for sale securities reconciliation	As of December 31, 2015 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 15,942 $ 610 $ (1,045 ) $ 15,507 Available-for-sale investments $ 15,942 $ 610 $ (1,045 ) $ 15,507 As of December 31, 2014 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 17,842 $ 380 $ (1,071 ) $ 17,151 Available-for-sale investments $ 17,842 $ 380 $ (1,071 ) $ 17,151
Schedule of unrealized loss on investments	The components of the Company’s investments in an unrealized loss position for twelve months or longer were as follows (in thousands): December 31, 2014 2015 Aggregate Fair Value Gross Unrealized Losses Aggregate Fair Value Gross Unrealized Losses Auction rate securities $ 16,329 $ (1,071 ) $ 14,455 $ (1,045 ) Investments in an unrealized loss position $ 16,329 $ (1,071 ) $ 14,455 $ (1,045 )

FAIR VALUE (Tables)

FAIR VALUE (Tables)	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Summary of fair value hierarchy for the company's financial assets and liabilities measured at fair value on a recurring basis	The following table represents the Company's fair value hierarchy for its financial assets (cash, cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2015 (in thousands): Level 1 Level 2 Level 3 Total Assets: Cash $ 405,597 $ — $ — $ 405,597 Money market funds 5,043 — — 5,043 Commercial paper 11,178 — — 11,178 Auction rate securities — — 15,507 15,507 Total assets measured at fair value $ 421,818 $ — $ 15,507 $ 437,325 5. FAIR VALUE — (CONTINUED) The following table represents the Company's fair value hierarchy for its financial assets (cash, cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2014 (in thousands): Level 1 Level 2 Level 3 Total Assets: Cash $ 160,275 $ — $ — $ 160,275 Money market funds 310,482 — — 310,482 Commercial paper 56,255 — — 56,255 Auction rate securities — — 17,151 17,151 Total assets measured at fair value $ 527,012 $ — $ 17,151 $ 544,163
Summary of changes in the fair value of the company's level 3 assets	The following table summarizes changes in fair value of the Company’s Level 3 assets from December 31, 2013 to December 31, 2015 (in thousands): Auction Rate Securities Balance at December 31, 2013 $ 21,990 Decrease in unrealized loss included in accumulated other comprehensive loss 836 Settlements (5,675 ) Balance at December 31, 2014 $ 17,151 Decrease in unrealized loss included in accumulated other comprehensive loss 256 Settlements (1,900 ) Balance at December 31, 2015 $ 15,507

PROPERTY AND EQUIPMENT (Tables)

PROPERTY AND EQUIPMENT (Tables)	12 Months Ended
	Dec. 31, 2015
Property, Plant and Equipment [Abstract]
Schedule of property, plant and equipment	Property and equipment are stated at cost, net of accumulated depreciation and amortization. All repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated on a straight-line basis over the following estimated useful lives of the assets: Leasehold improvements Shorter of lease term or useful life Furniture and office equipment Five to ten years Research vehicles Five to ten years Computer hardware and software Two to five years Property and equipment consists of the following (in thousands): December 31, 2014 2015 Leasehold improvements $ 42,026 $ 49,752 Furniture, office equipment and research vehicles 31,016 38,129 Computer hardware and software 49,655 56,656 Property and equipment, gross 122,697 144,537 Accumulated depreciation and amortization (48,944 ) (56,226 ) Property and equipment, net $ 73,753 $ 88,311

GOODWILL (Tables)

GOODWILL (Tables)	12 Months Ended
	Dec. 31, 2015
Goodwill [Abstract]
Schedule of goodwill	The changes in the carrying amount of goodwill by operating segment consist of the following (in thousands): North America International Total Goodwill, December 31, 2013 $ 692,639 $ 25,948 $ 718,587 Acquisition 421,724 — 421,724 Effect of foreign currency translation — (1,506 ) (1,506 ) Goodwill, December 31, 2014 1,114,363 24,442 1,138,805 Acquisitions 112,947 2,400 115,347 Effect of foreign currency translation — (1,207 ) (1,207 ) Goodwill, December 31, 2015 $ 1,227,310 $ 25,635 $ 1,252,945

INTANGIBLE ASSETS (Tables)

INTANGIBLE ASSETS (Tables)	12 Months Ended
	Dec. 31, 2015
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Schedule of acquired finite-lived intangible assets by major class	Intangible assets consist of the following (in thousands, except amortization period data): December 31, Weighted- Average Amortization Period (in years) 2014 2015 Capitalized product development cost $ 2,140 $ 2,243 4 Accumulated amortization (2,140 ) (2,172 ) Capitalized product development cost, net — 71 Building photography 14,943 17,677 4 Accumulated amortization (12,665 ) (15,875 ) Building photography, net 2,278 1,802 Acquired database technology 88,739 77,905 5 Accumulated amortization (60,498 ) (62,818 ) Acquired database technology, net 28,241 15,087 Acquired customer base 199,826 221,409 10 Accumulated amortization (102,443 ) (129,782 ) Acquired customer base, net 97,383 91,627 Acquired trade names and other intangible assets (1) 128,171 153,910 12 Accumulated amortization (14,451 ) (24,179 ) Acquired trade names and other intangible assets, net 113,720 129,731 Intangible assets, net $ 241,622 $ 238,318 (1) The weighted-average amortization period for acquired trade names excludes $48.7 million for acquired trade names recorded in connection with the LoopNet acquisition on April 30, 2012, which amount is not amortized, but is subject to annual impairment tests.

LONG-TERM DEBT (Tables)

LONG-TERM DEBT (Tables)	12 Months Ended
	Dec. 31, 2015
Long-term Debt, Current and Noncurrent [Abstract]
Components of long-term debt	The following table represents the Company's long-term debt (in thousands): December 31, 2014 2015 Term loan facility $ 385,000 $ 365,000 Debt issuance costs, net (13,199 ) (9,888 ) Total debt 371,801 355,112 Current maturities of long-term debt (20,000 ) (20,000 ) Current debt issuance costs, net 3,335 3,254 Total long-term debt, less current portion $ 355,136 $ 338,366
Schedule of maturities of long-term debt	Maturities of the Company's borrowings under the 2014 Credit Agreement for each of the next four years as of December 31, 2015 are as follows (in thousands): Year ending December 31, Maturities Due in: 2016 $ 20,000 2017 35,000 2018 55,000 2019 255,000 Long-term debt, including current maturities $ 365,000

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Schedule of components of income tax expense	The components of the provision for income taxes attributable to operations consist of the following (in thousands): Year Ended December 31, 2013 2014 2015 Current: Federal $ 26,516 $ 24,741 $ 10,295 State 3,996 2,761 1,503 Foreign 31 53 40 Total current 30,543 27,555 11,838 Deferred: Federal (10,919 ) (698 ) (7,475 ) State (1,849 ) (813 ) 1,683 Foreign 28 — — Total deferred (12,740 ) (1,511 ) (5,792 ) Total provision for income taxes $ 17,803 $ 26,044 $ 6,046
Schedule of deferred tax assets and liabilities	The components of deferred tax assets and liabilities consist of the following (in thousands): December 31, 2014 2015 Deferred tax assets: Reserve for bad debts $ 1,825 $ 2,849 Accrued compensation 7,287 10,725 Stock compensation 8,758 12,686 Net operating losses 15,665 36,406 Accrued reserve and other 3,360 3,515 Unrealized loss on securities 491 377 Deferred rent 5,902 7,274 Deferred revenue 1,879 2,243 Deferred gain on the sale of building 10,690 9,128 Total deferred tax assets, prior to valuation allowance 55,857 85,203 Valuation allowance (7,783 ) (9,347 ) Total deferred tax assets, net of valuation allowance 48,074 75,856 Deferred tax liabilities: Prepaids (1,258 ) (1,335 ) Depreciation (9,806 ) (13,047 ) Intangibles (47,720 ) (56,952 ) Total deferred tax liabilities (58,784 ) (71,334 ) Net deferred tax assets (liabilities) $ (10,710 ) $ 4,522
Schedule of effective income tax rate reconciliation	The Company’s provision for income taxes resulted in effective tax rates that varied from the statutory federal income tax rate as follows (in thousands): Year Ended December 31, 2013 2014 2015 Expected federal income tax provision at statutory rate $ 16,638 $ 24,820 $ 903 State income taxes, net of federal benefit 885 1,965 (678 ) Foreign income taxes, net effect (724 ) 336 469 Increase (decrease) in valuation allowance 588 (2,397 ) 1,956 Nondeductible compensation 431 554 574 Nondeductible transaction costs — — 229 Meals and entertainment 272 415 1,032 Tax rate changes 25 61 1,203 Other adjustments (312 ) 290 358 Income tax expense, net $ 17,803 $ 26,044 $ 6,046
Schedule of unrecognized tax benefits	The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands): Unrecognized tax benefits as of December 31, 2012 $ 3,909 Increase for current year tax positions 66 Increase for prior year tax positions 2,037 Expiration of the statute of limitation for assessment of taxes (55 ) Unrecognized tax benefits as of December 31, 2013 5,957 Increase for current year tax positions 51 Decrease for prior year tax positions (189 ) Expiration of the statute of limitation for assessment of taxes (70 ) Unrecognized tax benefits as of December 31, 2014 5,749 Increase for prior year tax positions 1,954 Expiration of the statute of limitation for assessment of taxes (39 ) Unrecognized tax benefits as of December 31, 2015 $ 7,664

COMMITMENTS AND CONTINGENCIES39

COMMITMENTS AND CONTINGENCIES (Tables)	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Future minimum lease payments	Future minimum lease payments as of December 31, 2015 are as follows (in thousands): 2016 $ 22,896 2017 22,131 2018 20,477 2019 18,224 2020 18,130 2021 and thereafter 66,732 Total future minimum lease payments $ 168,590

SEGMENT REPORTING (Tables)

SEGMENT REPORTING (Tables)	12 Months Ended
	Dec. 31, 2015
Segment Reporting [Abstract]
Summarized information by operating segment	Summarized information by operating segment consists of the following (in thousands): Year Ended December 31, 2013 2014 2015 Revenues North America $ 420,817 $ 552,141 $ 686,573 International External customers 20,126 23,795 25,191 Intersegment revenue 339 57 41 Total International revenue 20,465 23,852 25,232 Intersegment eliminations (339 ) (57 ) (41 ) Total revenues $ 440,943 $ 575,936 $ 711,764 EBITDA North America $ 97,348 $ 148,913 $ 87,092 International (3,136 ) 2,337 2,895 Total EBITDA $ 94,212 $ 151,250 $ 89,987
Reconciliation of EBITDA to net income (loss)	The reconciliation of EBITDA to net income (loss) consists of the following (in thousands): Year Ended December 31, 2013 2014 2015 EBITDA $ 94,212 $ 151,250 $ 89,987 Purchase amortization in cost of revenues (11,883 ) (26,290 ) (30,077 ) Purchase amortization in operating expenses (15,183 ) (28,432 ) (27,931 ) Depreciation and other amortization (12,992 ) (15,650 ) (20,524 ) Interest income 326 516 537 Interest expense (6,943 ) (10,481 ) (9,411 ) Income tax expense, net (17,803 ) (26,044 ) (6,046 ) Net income (loss) $ 29,734 $ 44,869 $ (3,465 )
Summarized information by operating segment, assets and liabilities	Summarized information by operating segment consists of the following (in thousands): December 31, 2014 2015 Property and equipment, net North America $ 71,209 $ 86,191 International 2,544 2,120 Total property and equipment, net $ 73,753 $ 88,311 Goodwill North America $ 1,114,363 $ 1,227,310 International 24,442 25,635 Total goodwill $ 1,138,805 $ 1,252,945 Assets North America $ 2,125,569 $ 2,130,202 International 41,896 41,370 Total operating segment assets $ 2,167,465 $ 2,171,572 Reconciliation of operating segment assets to total assets Total operating segment assets $ 2,167,465 $ 2,171,572 Investment in subsidiaries (18,344 ) (18,344 ) Intersegment receivables (78,638 ) (73,657 ) Total assets $ 2,070,483 $ 2,079,571 Liabilities North America $ 551,633 $ 525,566 International 75,584 72,544 Total operating segment liabilities $ 627,217 $ 598,110 Reconciliation of operating segment liabilities to total liabilities Total operating segment liabilities $ 627,217 $ 598,110 Intersegment payables (70,280 ) (62,319 ) Total liabilities $ 556,937 $ 535,791

NET INCOME (LOSS) PER SHARE (Ta

NET INCOME (LOSS) PER SHARE (Tables)	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Calculation of basic and diluted net income (loss) per share	The following table sets forth the calculation of basic and diluted net income (loss) per share (in thousands except per share data): Year Ended December 31, 2013 2014 2015 Numerator: Net income (loss) $ 29,734 $ 44,869 $ (3,465 ) Denominator: Denominator for basic net income (loss) per share — weighted-average outstanding shares 27,670 30,215 31,950 Effect of dilutive securities: Stock options and restricted stock 542 426 — Denominator for diluted net income (loss) per share — weighted-average outstanding shares 28,212 30,641 31,950 Net income (loss) per share — basic $ 1.07 $ 1.48 $ (0.11 ) Net income (loss) per share — diluted $ 1.05 $ 1.46 $ (0.11 )
Schedule of anti-dilutive securities excluded from computation of earnings per share	The following table summarizes the shares underlying the performance-based restricted stock awards and service-based restricted stock units excluded from the basic and diluted calculation (in thousands): Year Ended December 31, 2013 2014 2015 Performance-based restricted stock awards 379 23 55 Service-based restricted stock units — 1 1 Total shares excluded from computation 379 24 56

EMPLOYEE BENEFIT PLANS (Tables)

EMPLOYEE BENEFIT PLANS (Tables)	12 Months Ended
	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of option activity	Option activity was as follows: Number of Shares Range of Exercise Price Weighted- Average Exercise Price Weighted- Average Remaining Contract Life (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2012 673,719 $25.00 - $60.23 $ 45.20 Granted 126,800 $102.16 - $102.16 $ 102.16 Exercised (409,799 ) $25.00 - $58.95 $ 41.05 Canceled or expired (16,380 ) $36.48 - $58.95 $ 47.54 Outstanding at December 31, 2013 374,340 $36.48 - $102.16 $ 68.94 Granted 87,700 $201.04 - $201.04 $ 201.04 Exercised (68,126 ) $39.00 - $102.16 $ 55.81 Canceled or expired (23,735 ) $58.95 - $201.04 $ 124.09 Outstanding at December 31, 2014 370,179 $36.48 - $201.04 $ 99.12 Granted 89,500 $193.69 - $193.69 $ 193.69 Exercised (59,602 ) $36.48 - $201.04 $ 85.48 Outstanding at December 31, 2015 400,077 $36.48 - $201.04 $ 122.30 6.98 $ 33,761 Exercisable at December 31, 2013 146,161 $36.48 - $60.23 $ 47.72 Exercisable at December 31, 2014 188,656 $36.48 - $102.16 $ 60.54 Exercisable at December 31, 2015 220,107 $36.48 - $201.04 $ 77.63 5.76 $ 28,408
Summarized information regarding options outstanding	The following table summarizes information regarding options outstanding at December 31, 2015 : Options Outstanding Options Exercisable Range of Exercise Price Number of Shares Weighted-Average Remaining Contractual Life (in years) Weighted- Average Exercise Price Number of Shares Weighted- Average Exercise Price $36.48 - $42.50 55,051 4.06 $ 41.25 55,051 $ 41.25 $42.51 - $58.06 48,638 4.88 $ 55.94 48,638 $ 55.94 $58.07 - $59.59 45,567 6.14 $ 58.95 45,567 $ 58.95 $59.60 - $81.19 2,320 5.42 $ 60.23 2,320 $ 60.23 $81.20 - $147.93 82,801 7.19 $ 102.16 45,599 $ 102.16 $147.94 - $197.37 89,500 9.17 $ 193.69 — $ — $197.38 - $201.04 76,200 8.16 $ 201.04 22,932 $ 201.04 $36.48 - $201.04 400,077 6.98 $ 122.30 220,107 $ 77.63
Unvested restricted stock awards activity	The following table presents unvested restricted stock awards activity without a market condition and performance-based restricted common stock awards activity with a market condition for the year ended December 31, 2015 : Restricted Stock Awards — without Market Condition Restricted Stock Awards — with Market Condition Number of Shares Weighted-Average Grant Date Fair Value per Share Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock awards at December 31, 2014 556,793 $ 126.01 22,560 $ 216.20 Granted 207,442 $ 201.88 32,400 $ 208.08 Vested (221,546 ) $ 102.02 — $ — Canceled (39,576 ) $ 144.28 — $ — Unvested restricted stock awards at December 31, 2015 503,113 $ 166.42 54,960 $ 211.41
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value assumption for options granted	The Company estimated the fair value of each option granted on the date of grant using the Black-Scholes option-pricing model, using the assumptions in the following table: Year Ended December 31, 2013 2014 2015 Dividend yield 0 % 0 % 0 % Expected volatility 37 % 30 % 30 % Risk-free interest rate 0.9 % 1.5 % 1.6 % Expected life (in years) 5 5 5
Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value assumption for options granted	The assumptions used to estimate the fair value of performance-based restricted common stock awards with a market condition granted were as follows: Year Ended December 31, Year Ended December 31, 2014 2015 Dividend yield 0 % 0 % Expected volatility 30 % 26 % Risk-free interest rate 0.6 % 1.0 % Expected life (in years) 3 3 Weighted-average grant date fair value $ 216.20 $ 208.08
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted stock awards activity	The following table presents unvested restricted stock units activity for the year ended December 31, 2015 : Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock units at December 31, 2014 887 $ 169.16 Granted 543 $ 211.98 Vested — $ — Canceled — $ — Unvested restricted stock units at December 31, 2015 1,430 $ 185.42

Schedule II Valuation and Qua43

Schedule II Valuation and Qualifying Accounts (Tables)	12 Months Ended
	Dec. 31, 2015
Valuation and Qualifying Accounts [Abstract]
Allowance for doubtful accounts and billing adjustments	Years Ended December 31, 2013 , 2014 , and 2015 (in thousands): Allowance for Doubtful Accounts and Billing Adjustments (1) Balance at Beginning of Year Charged to Expense Charged to Other Accounts (2) Write-offs, Net of Recoveries Balance at End of Year Year ended December 31, 2013 $ 2,935 $ 2,317 $ — $ 1,855 $ 3,397 Year ended December 31, 2014 $ 3,397 $ 4,822 $ 881 $ 4,285 $ 4,815 Year ended December 31, 2015 $ 4,815 $ 7,002 $ 1,470 $ 5,809 $ 7,478 (1) Additions to the allowance for doubtful accounts are charged to bad debt expense. (2) Amounts represent opening balances from acquired businesses.

ORGANIZATION (Details)

ORGANIZATION (Details)	12 Months Ended
	Dec. 31, 2015operating_segments
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of business segments (in segments)	2
Term of subscription-based license agreements (in years)	1 year

SUMMARY OF SIGNIFICANT ACCOUN45

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - revenue Recognition and Advertising Costs) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Abstract]
Term of subscription-based license agreements (in years)	1 year
Advertising costs	$ 132.1	$ 28.7	$ 7.9

SUMMARY OF SIGNIFICANT ACCOUN46

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Foreign Currency Translation and Accumulated Other Comprehensive Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) Net of Tax [Abstract]
Material gains or losses from foreign currency exchange transactions	$ 0	$ 0	$ 0
Foreign currency translation adjustment	(7,159)	(5,693)
Accumulated net unrealized loss on investments, net of tax	(435)	(691)
Total accumulated other comprehensive loss	(7,594)	(6,384)
Reclassifications out of accumulated other comprehensive loss	$ 0	$ 0	$ 0

SUMMARY OF SIGNIFICANT ACCOUN47

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Stock-Based Compensation) $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)quarters	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Stock-Based Compensation Expense [Abstract]
Compensation expense	$ 34,537	$ 28,267	$ 41,549
Performance service period	3 years
Net cash proceeds from the exercise of stock options and ESPP	$ 7,404	5,740	18,133
Excess tax benefit from stock-based compensation	8,528	28,406	19,585
Cost of revenues
Stock-Based Compensation Expense [Abstract]
Compensation expense	5,815	4,759	4,553
Selling and marketing
Stock-Based Compensation Expense [Abstract]
Compensation expense	5,114	3,776	4,954
Software development
Stock-Based Compensation Expense [Abstract]
Compensation expense	5,712	5,095	7,244
General and administrative
Stock-Based Compensation Expense [Abstract]
Compensation expense	17,896	14,637	24,798
Performance-Based Restricted Common Stock [Member]
Stock-Based Compensation Expense [Abstract]
Cumulative EBITDA required for award shares to vest	$ 90,000
Number of consecutive quarters to mantain cumulative EBITDA required for award shares to vest (in quarters) | quarters	4
Compensation expense	$ 0	2,200	$ 21,800
Performance-based RSAs - with Market Condition [Member]
Stock-Based Compensation Expense [Abstract]
Compensation expense	2,800	$ 1,100
Compensation cost expected to be recognized in future years	$ 5,300

SUMMARY OF SIGNIFICANT ACCOUN48

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Cash and Cash Equivalents and Concentration of Credit Risk)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Restricted Cash and Cash Equivalents Items [Line Items]
Maximum original maturity of highly liquid investments to be considered cash equivalents (in months)	3 months
Number of customers accounting for more than 5% of revenue (in customers)	0	0	0
Significant customer revenue threshold percent	5.00%	5.00%	5.00%

SUMMARY OF SIGNIFICANT ACCOUN49

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Property and Equipment)	12 Months Ended
	Dec. 31, 2015
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	Shorter of lease term or useful life
Furniture and Office Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Furniture and Office Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	10 years
Research Vehicles [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Research Vehicles [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	10 years
Computer Hardware and Software [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	2 years
Computer Hardware and Software [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years

SUMMARY OF SIGNIFICANT ACCOUN50

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Goodwill and Intangible Assets)	12 Months Ended
	Dec. 31, 2015
Acquired Database Technology [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	5 months
Acquired Database Technology [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	8 years
Acquired Trade Names and Other Intangible Assets [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	7 months
Acquired Trade Names and Other Intangible Assets [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	15 years
Acquired Building Photography [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	5 months
Acquired Building Photography [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	5 years
Acquired Customer Base [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	10 years
Acquired Customer Base [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	13 years

SUMMARY OF SIGNIFICANT ACCOUN51

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Debt Issuance Costs and Recent Accounting Pronouncements) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Apr. 01, 2014
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Capitalized debt issuance costs, net	$ 9,888	$ 13,199		$ 10,000
Amortization of debt issuance costs	$ 3,311	$ 3,312	$ 3,014

ACQUISITIONS (Details for Apart

ACQUISITIONS (Details for Apartments.com) $ in Thousands	Jun. 01, 2015USD ($)distinct_intangible_asset	Apr. 03, 2014USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Apr. 01, 2014USD ($)distinct_intangible_asset
Business Acquisition [Line Items]
Payments to acquire businesses		$ 587,104
Closing purchase price adjustments		2,104
Post closing purchase price adjustments		$ (2,886)
Goodwill			$ 1,252,945	$ 1,138,805	$ 718,587
Apartments.com [Member]
Business Acquisition [Line Items]
Goodwill						$ 421,724
Goodwill tax deductible amount						$ 421,724
Acquisition related costs			$ 0	$ 1,375	$ 0
Apartment Finder [Member]
Business Acquisition [Line Items]
Payments to acquire businesses	$ 172,706
Closing purchase price adjustments	2,706
Post closing purchase price adjustments	21
Goodwill	107,692
Goodwill tax deductible amount	$ 0
Acquired Trade Names and Other Intangible Assets [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets			12 years
Acquired Trade Names and Other Intangible Assets [Member] | Apartments.com [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		13 years
Acquired Trade Names and Other Intangible Assets [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	9 years
Acquired Customer Base [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets			10 years
Acquired Customer Base [Member] | Apartments.com [Member]
Business Acquisition [Line Items]
Number of distinct intangible assets | distinct_intangible_asset						1
Estimated useful life of acquired assets		10 years
Acquired Customer Base [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Number of distinct intangible assets | distinct_intangible_asset	3
Estimated useful life of acquired assets	10 years
Acquired Database Technology [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets			5 years
Acquired Database Technology [Member] | Apartments.com [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		1 year
Acquired Database Technology [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	5 months
Acquired Building Photography [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets			4 years
Acquired Building Photography [Member] | Apartments.com [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		3 years
Acquired Building Photography [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	5 months

ACQUISITIONS (Details - Schedul

ACQUISITIONS (Details - Schedule of Assets Acquired and Liabilities Assumed for Apartments.com) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Apr. 01, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Goodwill	$ 1,252,945	$ 1,138,805		$ 718,587
Apartments.com [Member]
Business Acquisition [Line Items]
Accounts receivable			$ 11,402
Goodwill			421,724
Other assets and liabilities			(2,866)
Fair value of identifiable net assets acquired			584,218
Apartments.com [Member] | Acquired Trade Names and Other Intangible Assets [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill			71,779
Apartments.com [Member] | Acquired Customer Base [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill			69,684
Apartments.com [Member] | Acquired Database Technology [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill			11,489
Apartments.com [Member] | Acquired Building Photography [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill			$ 1,006

ACQUISITIONS (Details for Apa54

ACQUISITIONS (Details for Apartment Finder) $ in Thousands	Jun. 01, 2015USD ($)distinct_intangible_asset	Apr. 03, 2014USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Business Acquisition [Line Items]
Payments to acquire businesses		$ 587,104
Closing purchase price adjustments		2,104
Post closing purchase price adjustments		$ (2,886)
Goodwill			$ 1,252,945	$ 1,138,805	$ 718,587
Apartment Finder [Member]
Business Acquisition [Line Items]
Percentage of outstanding stock acquired	100.00%
Payments to acquire businesses	$ 172,706
Closing purchase price adjustments	2,706
Post closing purchase price adjustments	21
Goodwill	107,692
Goodwill tax deductible amount	$ 0
Acquired Trade Names and Other Intangible Assets [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets			12 years
Acquired Trade Names and Other Intangible Assets [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	9 years
Acquired Customer Base [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets			10 years
Acquired Customer Base [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Number of distinct intangible assets | distinct_intangible_asset	3
Estimated useful life of acquired assets	10 years
Acquired Database Technology [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets			5 years
Acquired Database Technology [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	5 months
Acquired Building Photography [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets			4 years
Acquired Building Photography [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	5 months

ACQUISITIONS ACQUISITIONS (Deta

ACQUISITIONS ACQUISITIONS (Details - Schedule of Assets Acquired and Liabilities Assumed for Apartment Finder) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Jun. 01, 2015	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Goodwill	$ 1,252,945		$ 1,138,805	$ 718,587
Apartment Finder [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		$ 39
Accounts receivable		4,556
Goodwill		107,692
Deferred income taxes, net		9,290
Other assets and liabilities		(849)
Fair value of identifiable net assets acquired		172,727
Acquired Trade Names and Other Intangible Assets [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill		23,642
Acquired Customer Base [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill		21,856
Acquired Database Technology [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill		4,076
Acquired Building Photography [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill		$ 2,425

INVESTMENTS, SCHEDULED MATURITI

INVESTMENTS, SCHEDULED MATURITIES AND REALIZED GAINS AND LOSSES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt Maturities Fair Value [Abstract]
2,016	$ 0
2017 — 2020	1,052,000
2021 — 2025	0
2026 and thereafter	14,455,000
Available-for-sale investments	15,507,000	$ 17,151,000
Available-for-sale securities, gross realized gains	0	0	$ 0
Available-for-sale securities, gross realized losses	$ 0	$ 0	$ 0

INVESTMENTS, AVAILABLE-FOR-SALE

INVESTMENTS, AVAILABLE-FOR-SALE SECURITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than 12 months	$ 0	$ 0
Available-for-sale Securities Reconciliation [Abstract]
Amortized cost	15,942	17,842
Gross unrealized gains	610	380
Gross unrealized losses	1,045	1,071
Available-for-sale investments	15,507	17,151
Available-for-sale securities, unrealized loss positions
Aggregate fair value	14,455	16,329
Auction Rate Securities [Member]
Available-for-sale Securities Reconciliation [Abstract]
Amortized cost	15,942	17,842
Gross unrealized gains	610	380
Gross unrealized losses	1,045	1,071
Available-for-sale investments	15,507	17,151
Available-for-sale securities, unrealized loss positions
Aggregate fair value	$ 14,455	$ 16,329

FAIR VALUE (Details)

FAIR VALUE (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Unobservable inputs assets (level 3) [Roll forward]
Auction rate securities variable rate debt instruments interest rate reset period	28 days
The minimum contractual maturities on the underlying securities involved in the auction rate securities (in years)	20 years
Temporary impairment of the auction rates security investments	$ (435,000)	$ (691,000)
Fair Value, Measurements, Recurring [Member]
Assets:
Total assets measured at fair value	437,325,000	544,163,000
Fair Value, Measurements, Recurring [Member] | Cash [Member]
Assets:
Total assets measured at fair value	405,597,000	160,275,000
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	5,043,000	310,482,000
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Assets:
Total assets measured at fair value	11,178,000	56,255,000
Fair Value, Measurements, Recurring [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	15,507,000	17,151,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Total assets measured at fair value	421,818,000	527,012,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Assets:
Total assets measured at fair value	405,597,000	160,275,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	5,043,000	310,482,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Assets:
Total assets measured at fair value	11,178,000	56,255,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Cash [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets:
Total assets measured at fair value	15,507,000	17,151,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Cash [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Commercial Paper [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	15,507,000	17,151,000
Auction Rate Securities [Member]
Unobservable inputs assets (level 3) [Roll forward]
Beginning balance	17,151,000	21,990,000
Decrease in unrealized loss included in accumulated other comprehensive loss	256,000	836,000
Settlements	(1,900,000)	(5,675,000)
Ending balance	15,507,000	$ 17,151,000
Par value of company held auction rate securities	$ 16,800,000
Discount rate (in percent)	4.70%	4.10%
Temporary impairment of the auction rates security investments	$ (435,000)

PROPERTY AND EQUIPMENT (Details

PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 144,537	$ 122,697
Accumulated depreciation and amortization	(56,226)	(48,944)
Property and equipment, net	88,311	73,753
Depreciation expense for property and equipment	19,967	15,111	$ 12,495
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	49,752	42,026
Furniture, Office Equipment and Research Vehicles [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	38,129	31,016
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 56,656	$ 49,655

GOODWILL (Details)

GOODWILL (Details) - USD ($) $ in Thousands	Jul. 01, 2015	Jun. 01, 2015	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Roll Forward]
Goodwill, beginning balance			$ 1,138,805	$ 718,587
Acquisitions			115,347	421,724
Effect of foreign currency translation			(1,207)	(1,506)
Goodwill, ending balance			1,252,945	1,138,805
Apartment Finder [Member]
Goodwill [Roll Forward]
Acquisitions		$ 107,692
Goodwill, ending balance		$ 107,692
Belbex [Member]
Goodwill [Roll Forward]
Acquisitions	$ 2,400
Independent Distributor Buyout [Member]
Goodwill [Roll Forward]
Acquisitions			5,255
North America [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance			1,114,363	692,639
Acquisitions			112,947	421,724
Effect of foreign currency translation			0	0
Goodwill, ending balance			1,227,310	1,114,363
International [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance			24,442	25,948
Acquisitions			2,400	0
Effect of foreign currency translation			(1,207)	(1,506)
Goodwill, ending balance			$ 25,635	$ 24,442

INTANGIBLE ASSETS (Details)

INTANGIBLE ASSETS (Details) - USD ($) $ in Thousands	Jun. 01, 2015	Apr. 03, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Apr. 30, 2012
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net			$ 238,318	$ 241,622
Amortization of intangible assets			58,565	55,261	$ 27,563
Impairment of intangible assets, finite-lived			2,778	1,799	$ 0
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Amortization expense for 2016			43,000
Amortization expense for 2017			28,500
Amortization expense for 2018			20,300
Amortization expense for 2019			17,400
Amortization expense for 2020			15,800
Capitalized Product Development Costs [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross			2,243	2,140
Finite-lived intangible assets, accumulated amortization			(2,172)	(2,140)
Finite-lived intangible assets, net			$ 71	0
Weighted-average amortization period (in years}			4 years
Acquired Building Photography [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross			$ 17,677	14,943
Finite-lived intangible assets, accumulated amortization			(15,875)	(12,665)
Finite-lived intangible assets, net			$ 1,802	2,278
Weighted-average amortization period (in years}			4 years
Acquired Database Technology [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross			$ 77,905	88,739
Finite-lived intangible assets, accumulated amortization			(62,818)	(60,498)
Finite-lived intangible assets, net			$ 15,087	28,241
Weighted-average amortization period (in years}			5 years
Acquired Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross			$ 221,409	199,826
Finite-lived intangible assets, accumulated amortization			(129,782)	(102,443)
Finite-lived intangible assets, net			$ 91,627	97,383
Weighted-average amortization period (in years}			10 years
Acquired Trade Names and Other Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross			$ 153,910	128,171
Finite-lived intangible assets, accumulated amortization			(24,179)	(14,451)
Finite-lived intangible assets, net			$ 129,731	$ 113,720
Weighted-average amortization period (in years}			12 years
LoopNet [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived acquired trade names and other intangible assets						$ 48,700
Apartments.com [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Impairment of intangible assets, finite-lived			$ 1,402
Apartments.com [Member] | Acquired Building Photography [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average amortization period (in years}		3 years
Apartments.com [Member] | Acquired Database Technology [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average amortization period (in years}		1 year
Apartments.com [Member] | Acquired Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average amortization period (in years}		10 years
Apartments.com [Member] | Acquired Trade Names and Other Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average amortization period (in years}		13 years
Apartment Finder [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Impairment of intangible assets, finite-lived			$ 1,376
Apartment Finder [Member] | Acquired Building Photography [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average amortization period (in years}	5 months
Apartment Finder [Member] | Acquired Database Technology [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average amortization period (in years}	5 months
Apartment Finder [Member] | Acquired Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average amortization period (in years}	10 years
Apartment Finder [Member] | Acquired Trade Names and Other Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average amortization period (in years}	9 years

LONG-TERM DEBT (Details)

LONG-TERM DEBT (Details) - USD ($)	Apr. 03, 2014	Jun. 30, 2014	Apr. 04, 2014	Dec. 31, 2015	Dec. 31, 2014	Apr. 01, 2014
Business Acquisition [Line Items]
Repayments of term loan	$ 148,800,000
LIBOR 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 cost			$ 10,100,000
Debt issuance underwriting fees			9,700,000
Legal fees associated with the debt issuance			$ 400,000
Capitalized debt issuance costs, net				$ 9,888,000	$ 13,199,000	$ 10,000,000
Term Loan [Member]
Business Acquisition [Line Items]
Debt instrument borrowing capacity						400,000,000
Term of loan (in years)	5 years
Revolving Credit Facility [Member]
Business Acquisition [Line Items]
Debt instrument borrowing capacity						225,000,000
Term of loan (in years)	5 years
Line of credit facility, amount outstanding				$ 0	$ 0	150,000,000
Repayments of revolving credit facility		$ 150,000,000
CoStar Group [Member] | Swingline Loan [Member]
Business Acquisition [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]
Business Acquisition [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]
Business Acquisition [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]
Business Acquisition [Line Items]
Basis spread on federal funds rate (in percent)	0.50%
LIBOR [Member]
Business Acquisition [Line Items]
Basis spread on federal funds rate (in percent)	2.00%
Basis spread on variable rate, one month interest period (in percent)						1.00%
Basis spread on variable rate, per annum (in percent)						1.00%

LONG-TERM DEBT (Details - Coven

LONG-TERM DEBT (Details - Covenant Calculations)	Apr. 01, 2014
Debt Disclosure [Abstract]
Maximum first lien secured leverage ratio for first eight quarters after closing date	400.00%
Maximum first lien secured leverage ratio after eight quarters after closing date	350.00%
Maximum total leverage ratio for first eight quarters after closing date	500.00%
Maximum total leverage ratio after eight fiscal quarters	450.00%

LONG-TERM DEBT (Details - Inter

LONG-TERM DEBT (Details - Interest Expense and Amortization of Debt Issuance Costs) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Apr. 01, 2014
Debt Instrument [Line Items]
Interest expense, debt	$ 9,411	$ 10,481	$ 6,943
Amortization of debt issuance costs	3,311	3,312	3,014
Interest paid	6,096	7,007	$ 4,291
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	$ 0	$ 0		$ 150,000

LONG-TERM DEBT (Details - Compo

LONG-TERM DEBT (Details - Components of Long-Term Debt) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Apr. 01, 2014
Debt Disclosure [Abstract]
Term loan facility	$ 365,000	$ 385,000
Debt issuance costs, net	(9,888)	(13,199)	$ (10,000)
Total debt	355,112	371,801
Current maturities of long-term debt	(20,000)	(20,000)
Current debt issuance costs, net	3,254	3,335
Total long-term debt, less current portion	$ 338,366	$ 355,136

LONG-TERM DEBT (Details - Sched

LONG-TERM DEBT (Details - Schedule of Maturities of Long-Term Debt) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Long-term Debt, Current and Noncurrent [Abstract]
2,016	$ 20,000
2,017	35,000
2,018	55,000
2,019	255,000
Long-term debt, including current maturities	$ 365,000	$ 385,000

INCOME TAXES (Details - Compone

INCOME TAXES (Details - Components for Provision for Income Taxes) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current:
Federal	$ 10,295	$ 24,741	$ 26,516
State	1,503	2,761	3,996
Foreign	40	53	31
Total current	11,838	27,555	30,543
Deferred:
Federal	(7,475)	(698)	(10,919)
State	1,683	(813)	(1,849)
Foreign	0	0	28
Total deferred	(5,792)	(1,511)	(12,740)
Income tax expense, net	$ 6,046	$ 26,044	$ 17,803

INCOME TAXES (Details - Compo68

INCOME TAXES (Details - Components of Deferred Tax Assets and Liabilities) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Reserve for bad debts	$ 2,849	$ 1,825
Accrued compensation	10,725	7,287
Stock compensation	12,686	8,758
Net operating losses	36,406	15,665
Accrued reserve and other	3,515	3,360
Unrealized loss on securities	377	491
Deferred rent	7,274	5,902
Deferred revenue	2,243	1,879
Deferred gain on the sale of building	9,128	10,690
Total deferred tax assets, prior to valuation allowance	85,203	55,857
Valuation allowance	(9,347)	(7,783)
Total deferred tax assets, net of valuation allowance	75,856	48,074
Deferred tax liabilities:
Prepaids	(1,335)	(1,258)
Depreciation	(13,047)	(9,806)
Intangibles	(56,952)	(47,720)
Total deferred tax liabilities	71,334	58,784
Net deferred tax liabilities		$ (10,710)
Net deferred tax assets	$ 4,522

INCOME TAXES (Details - Reconci

INCOME TAXES (Details - Reconciliation of Provision for Income Taxes) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective tax rate reconciliation [Abstract]
Expected federal income tax provision at statutory rate	$ 903	$ 24,820	$ 16,638
State income taxes, net of federal benefit	(678)	1,965	885
Foreign income taxes, net effect	469	336	(724)
Increase (decrease) in valuation allowance	1,956	(2,397)	588
Nondeductible compensation	574	554	431
Nondeductible transaction costs	229	0	0
Meals and entertainment	1,032	415	272
Tax law changes	1,203	61	25
Other adjustments	358	290	(312)
Income tax expense, net	$ 6,046	$ 26,044	$ 17,803

INCOME TAXES (Details - Other)

INCOME TAXES (Details - Other) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	$ 1,564,000	$ (3,153,000)
Cash tax benefits resulting in net operating loss carryforward	1,300,000	1,200,000
Income from U.S. sources	1,500,000	70,600,000	$ 53,200,000
Income (loss) from foreign sources	1,100,000	273,000	(5,600,000)
Income taxes paid	1,300,000	$ 3,000,000	$ 6,500,000
Foreign Country [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	(551,000)
Net operating loss carryforward	24,600,000
Domestic Country [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	$ 64,300,000
Operating loss carryforwards, first expiration date	Dec. 31, 2020
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	$ 2,180,000
Net operating loss carryforward	$ 6,900,000
Operating loss carryforwards, first expiration date	Dec. 31, 2020
Income tax credit carryforward	$ 2,300,000

INCOME TAXES (Details - Unrecog

INCOME TAXES (Details - Unrecognized Tax Benefits) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits beginning balance	$ 5,749,000	$ 5,957,000	$ 3,909,000
Increase for current year tax positions		51,000	66,000
Increase for prior year tax positions	1,954,000		2,037,000
Decrease for prior period tax positions		(189,000)
Expiration of the statute of limitation for assessment of taxes	(39,000)	(70,000)	(55,000)
Unrecognized tax benefits ending balance	7,664,000	5,749,000	5,957,000
Unrecognized tax benefits that would favorably affect the annual effective tax rate if recognized in future periods	1,400,000	1,500,000
Interest and penalties on income taxes recognized	83,000	62,000	62,000
Interest and penalties accrued on income taxes	$ 549,000	$ 466,000	$ 404,000

COMMITMENTS AND CONTINGENCIES72

COMMITMENTS AND CONTINGENCIES (Details) - USD ($) $ in Thousands	Apr. 03, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Apr. 01, 2014
Operating Leases Rent Expense [Abstract]
Rent expense for operating leases		$ 21,400	$ 19,200	$ 18,300
Future Minimum Lease Payments [Abstract]
2,016		22,896
2,017		22,131
2,018		20,477
2,019		18,224
2,020		18,130
2021 and thereafter		66,732
Total future minimum lease payments		$ 168,590
Term Loan [Member]
Business Acquisition [Line Items]
Debt instrument borrowing capacity					$ 400,000
Term of loan (in years)	5 years
Revolving Credit Facility [Member]
Business Acquisition [Line Items]
Debt instrument borrowing capacity					$ 225,000
Term of loan (in years)	5 years

SEGMENT REPORTING (Details)

SEGMENT REPORTING (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)operating_segments	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Segment Reporting Information [Line Items]
Number of business segments (in segments) | operating_segments	2
Segment Reporting Information, Revenue [Abstract]
Revenues	$ 711,764	$ 575,936	$ 440,943
Reconciliation of EBITDA to net income [Abstract]
EBITDA	89,987	151,250	94,212
Purchase amortization in cost of revenues	(30,077)	(26,290)	(11,883)
Purchase amortization in operating expenses	(27,931)	(28,432)	(15,183)
Depreciation and other amortization	(20,524)	(15,650)	(12,992)
Interest income	537	516	326
Interest expense	(9,411)	(10,481)	(6,943)
Income tax expense, net	(6,046)	(26,044)	(17,803)
Net income (loss)	(3,465)	44,869	29,734
North America [Member]
Segment Reporting Information [Line Items]
North America corporate allocation	954	1,109	844
Segment Reporting Information, Revenue [Abstract]
Revenues	686,573	552,141	420,817
Reconciliation of EBITDA to net income [Abstract]
EBITDA	87,092	148,913	97,348
International [Member]
Segment Reporting Information [Line Items]
International corporate allocation	256	261	411
Segment Reporting Information, Revenue [Abstract]
Revenues	25,232	23,852	20,465
Reconciliation of EBITDA to net income [Abstract]
EBITDA	2,895	2,337	(3,136)
Intersegment Revenue [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	41	57	339
External Customers [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	25,191	23,795	20,126
Intersegment Eliminations [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues	$ (41)	$ (57)	$ (339)

SEGMENT REPORTING, ASSETS AND L

SEGMENT REPORTING, ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Property and equipment, net	$ 88,311	$ 73,753
Goodwill	1,252,945	1,138,805	$ 718,587
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	2,079,571	2,070,483
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	535,791	556,937
North America [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	86,191	71,209
Goodwill	1,227,310	1,114,363	692,639
International [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	2,120	2,544
Goodwill	25,635	24,442	$ 25,948
Operating Segments [Member]
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	2,171,572	2,167,465
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	598,110	627,217
Operating Segments [Member] | North America [Member]
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	2,130,202	2,125,569
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	525,566	551,633
Operating Segments [Member] | International [Member]
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	41,370	41,896
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	72,544	75,584
Consolidation Eliminations [Member]
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	(18,344)	(18,344)
Intersegment Eliminations [Member]
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	(73,657)	(78,638)
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	$ (62,319)	$ (70,280)

STOCKHOLDERS' EQUITY (Details)

STOCKHOLDERS' EQUITY (Details) - USD ($)	1 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Preferred stock
Preferred stock authorized for issuance (in shares)		2,000,000	2,000,000
Preferred stock, par value (in dollars per share)		$ 0.01	$ 0.01
Common stock
Common stock authorized for issuance (in shares)		60,000,000	60,000,000
Common stock par value (in dollars per share)		$ 0.01	$ 0.01
Equity offering
Equity offering in period, new shares (in shares)	3,450,000
New shares issued, price per share (in dollars per share)	$ 160
Proceeds from equity offering, net of transaction costs	$ 529,360,000	$ 0	$ 529,360,000	$ 0
Payments for underwriting expense	22,100,000
Payments of equity offering costs	$ 500,000

NET INCOME (LOSS) PER SHARE (De

NET INCOME (LOSS) PER SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income (loss)	$ (3,465)	$ 44,869	$ 29,734
Denominator:
Denominator for basic net income (loss) per share — weighted-average outstanding shares	31,950,000	30,215,000	27,670,000
Effect of dilutive securities:
Stock options and restricted stock (in shares)	0	426,000	542,000
Denominator for diluted net income (loss) per share — weighted-average outstanding shares	31,950,000	30,641,000	28,212,000
Net income (loss) per share — basic (in dollars per share)	$ (0.11)	$ 1.48	$ 1.07
Net income (loss) per share — diluted (in dollars per share)	$ (0.11)	$ 1.46	$ 1.05
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	56,000	24,000	379,000
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)		80,000	0
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	55,000	23,000	379,000
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	1,000	1,000	0

EMPLOYEE BENEFIT PLANS (Details

EMPLOYEE BENEFIT PLANS (Details for Stock Incentive Plans) $ / shares in Units, $ in Thousands	Jun. 05, 2012shares	Jun. 02, 2010shares	Dec. 31, 2015USD ($)quarters$ / sharesshares	Dec. 31, 2014USD ($)$ / sharesshares	Dec. 31, 2013USD ($)$ / sharesshares	Dec. 31, 2012$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense | $			$ 34,537	$ 28,267	$ 41,549
Performance service period			3 years
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)			30.00%	30.00%	37.00%
Risk-free interest rate (in hundredths)			1.60%	1.50%	0.90%
Expected life (in years)			5 years	5 years	5 years
Options Outstanding 1 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			$ 36.48
Range of exercise price, maximum, (in dollars per share)			$ 42.50
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			55,051
Weighted-average remaining contractual life (in years)			4 years 23 days
Weighted- average exercise price (in dollars per share)			$ 41.25
Options Outstanding 2 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			42.51
Range of exercise price, maximum, (in dollars per share)			$ 58.06
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			48,638
Weighted-average remaining contractual life (in years)			4 years 10 months 18 days
Weighted- average exercise price (in dollars per share)			$ 55.94
Options Outstanding 3 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			58.07
Range of exercise price, maximum, (in dollars per share)			$ 59.59
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			45,567
Weighted-average remaining contractual life (in years)			6 years 1 month 20 days
Weighted- average exercise price (in dollars per share)			$ 58.95
Options Outstanding 4 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			59.60
Range of exercise price, maximum, (in dollars per share)			$ 81.19
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			2,320
Weighted-average remaining contractual life (in years)			5 years 5 months 1 day
Weighted- average exercise price (in dollars per share)			$ 60.23
Options Outstanding 5 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			81.20
Range of exercise price, maximum, (in dollars per share)			$ 147.93
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			82,801
Weighted-average remaining contractual life (in years)			7 years 2 months 10 days
Weighted- average exercise price (in dollars per share)			$ 102.16
Options Outstanding 6 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			147.94
Range of exercise price, maximum, (in dollars per share)			$ 197.37
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			89,500
Weighted-average remaining contractual life (in years)			9 years 2 months 1 day
Weighted- average exercise price (in dollars per share)			$ 193.69
Options Outstanding 7 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			197.38
Range of exercise price, maximum, (in dollars per share)			$ 201.04
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			76,200
Weighted-average remaining contractual life (in years)			8 years 1 month 28 days
Weighted- average exercise price (in dollars per share)			$ 201.04
Options Outstanding 8 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)			36.48
Range of exercise price, maximum, (in dollars per share)			$ 201.04
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares			400,077
Weighted-average remaining contractual life (in years)			6 years 11 months 22 days
Weighted- average exercise price (in dollars per share)			$ 122.30
Options Exercisable 1 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			55,051
Weighted-average exercise price (in dollars per share)			$ 41.25
Options Exercisable 2 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			48,638
Weighted-average exercise price (in dollars per share)			$ 55.94
Options Exercisable 3 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			45,567
Weighted-average exercise price (in dollars per share)			$ 58.95
Options Exercisable 4 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			2,320
Weighted-average exercise price (in dollars per share)			$ 60.23
Options Exercisable 5 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			45,599
Weighted-average exercise price (in dollars per share)			$ 102.16
Options Exercisable 6 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			0
Weighted-average exercise price (in dollars per share)			$ 0
Options Exercisable 7 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			22,932
Weighted-average exercise price (in dollars per share)			$ 201.04
Options Exercisable 8 [Member]
Options Exercisable [Abstract]
Number of shares (in shares) | shares			220,107
Weighted-average exercise price (in dollars per share)			$ 77.63
1998 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant under the plan (in shares) | shares			0
Shares of common stock authorized for issuance under the plan (in shares) | shares			121,875
CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant under the plan (in shares) | shares			1,000,000	1,200,000
Increase in shares of common stock pursuant to amendment to the plan (in shares) | shares		1,300,000
Increase in shares of common stock issued pursuant to stock plan (in shares) | shares	900,000
Shares of common stock authorized for issuance under the plan (in shares) | shares			3,200,000
Restricted Stock Units (RSUs) [Member]
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) | shares			887
Granted (in shares) | shares			543
Vested (in shares) | shares			0
Canceled (in shares) | shares			0
Unvested restricted stock at end of period (in shares) | shares			1,430	887
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)			$ 169.16
Granted (in dollars per share)			211.98
Vested (in dollars per share)			0
Canceled (in dollars per share)			0
Unvested restricted stock at end of period (in dollars per share)			$ 185.42	$ 169.16
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period (in shares) | shares			370,179	374,340	673,719
Granted (in shares) | shares			89,500	87,700	126,800
Exercised (in shares) | shares			(59,602)	(68,126)	(409,799)
Canceled or expired (in shares) | shares				(23,735)	(16,380)
Outstanding at end of period (in shares) | shares			400,077	370,179	374,340	673,719
Exercisable at end of period (in shares) | shares			220,107	188,656	146,161
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average exercise price, outstanding at beginning of period (in dollars per share)			$ 99.12	$ 68.94	$ 45.20
Weighted-average exercise price, granted (in dollars per share)			193.69	201.04	102.16
Weighted-average exercise price, exercised (in dollars per share)			85.48	55.81	41.05
Weighted-average exercise price, canceled or expired (in dollars per share)				124.09	47.54
Weighted-average exercise price, outstanding at end of period (in dollars per share)			122.30	99.12	68.94	$ 45.20
Weighted-average exercise price, exercisable at end of period (in dollars per share)			$ 77.63	$ 60.54	$ 47.72
Weighted-average remaining contract life of options outstanding at end of period			6 years 11 months 24 days
Weighted-average remaining contract life of options exercisable at end of period			5 years 9 months 2 days
Aggregate intrinsic value of options outstanding at end of period | $			$ 33,761
Aggregate intrinsic value of options exercisable at end of period | $			28,408
Aggregate intrinsic value of options exercised | $			6,800	$ 8,900	$ 39,000
Compensation cost expected to be recognized in future years | $			$ 61,700
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)			2 years 5 months 11 days
Weighted-average grant date fair value of each option granted during the period (in dollars per share)			$ 56.53	$ 58.12	$ 34.10
Restricted Stock [Member]
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) | shares			556,793
Granted (in shares) | shares			207,442
Vested (in shares) | shares			(221,546)
Canceled (in shares) | shares			(39,576)
Unvested restricted stock at end of period (in shares) | shares			503,113	556,793
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)			$ 126.01
Granted (in dollars per share)			201.88
Vested (in dollars per share)			102.02
Canceled (in dollars per share)			144.28
Unvested restricted stock at end of period (in dollars per share)			$ 166.42	$ 126.01
Performance-Based Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative EBITDA required for award shares to vest | $			$ 90,000
Number of consecutive quarters to mantain cumulative EBITDA required for award shares to vest (in quarters) | quarters			4
Compensation expense | $			$ 0	$ 2,200	$ 21,800
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted (in shares) | shares						399,413
Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense | $			2,800	$ 1,100
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Compensation cost expected to be recognized in future years | $			$ 5,300
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (in hundredths)			0.00%	0.00%
Expected volatility (in hundredths)			26.00%	30.00%
Risk-free interest rate (in hundredths)			1.00%	0.60%
Expected life (in years)			3 years	3 years
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) | shares			22,560
Granted (in shares) | shares			32,400	24,720
Vested (in shares) | shares			0
Canceled (in shares) | shares			0
Unvested restricted stock at end of period (in shares) | shares			54,960	22,560
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)			$ 216.20
Granted (in dollars per share)			208.08	$ 216.20
Vested (in dollars per share)			0
Canceled (in dollars per share)			0
Unvested restricted stock at end of period (in dollars per share)			$ 211.41	216.20
Minimum [Member] | 1998 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants			3 years
Minimum [Member] | CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants			3 years
Minimum [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period, (in dollars per share)			$ 36.48	36.48	$ 25
Granted (in dollars per share)			193.69	201.04	102.16
Exercised (in dollars per share)			36.48	39	25
Canceled or expired (in dollars per share)				58.95	36.48
Outstanding at end of period, (in dollars per share)			36.48	36.48	36.48	$ 25
Exercisable at end of period (in dollars per share)			$ 36.48	36.48	36.48
Minimum [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants			1 year
Minimum [Member] | Performance-Based Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage			0.00%
Minimum [Member] | Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage			80.00%
Maximum [Member] | 1998 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants			4 years
Maximum [Member] | CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants			4 years
Maximum [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period, (in dollars per share)			$ 201.04	102.16	60.23
Granted (in dollars per share)			193.69	201.04	102.16
Exercised (in dollars per share)			201.04	102.16	58.95
Canceled or expired (in dollars per share)				201.04	58.95
Outstanding at end of period, (in dollars per share)			201.04	201.04	102.16	$ 60.23
Exercisable at end of period (in dollars per share)			$ 201.04	$ 102.16	$ 60.23
Maximum [Member] | Performance-Based Restricted Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage			200.00%
Maximum [Member] | Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage			120.00%

EMPLOYEE BENEFIT PLANS (Detai78

EMPLOYEE BENEFIT PLANS (Details for Employee 401(k) Plan, Employee Pension Plan, Registered Retirement Savings Plan and Employee Stock Purchase Plan) - USD ($)	Sep. 15, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee 401(k) Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of company match of employee contribution		100.00%	100.00%	100.00%
Maximum percentage of employee total compensation matched by employer (in hundredths)		4.00%	4.00%	4.00%
Company match to employee contributions		$ 7,500,000	$ 6,100,000	$ 5,100,000
Administrative expense		$ 0	$ 0	$ 0
Employee Pension Plan, London Office [Member]
Defined Contribution Plan Disclosure [Line Items]
Maximum percentage of employee total compensation matched by employer (in hundredths)		6.00%	6.00%	6.00%
Company match to employee contributions		$ 420,000	$ 390,000	$ 280,000
Registered Retirement Savings Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of company match of employee contribution		100.00%
Maximum percentage of employee total compensation matched by employer (in hundredths)		4.00%
Company match to employee contributions		$ 40,000
Employee Stock Purchase Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of purchase price of company's common stock to the market price		90.00%
Increase in shares of common stock issued pursuant to stock plan (in shares)	100,000
Shares available, employee stock purchase plan		108,547	21,774
Shares of Company's common stock purchased during the period (in shares)		13,227	13,121

Schedule II Valuation and Qua79

Schedule II Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Valuation and Qualifying Accounts [Abstract]
Balance at beginning of year	[1]	$ 4,815	$ 3,397	$ 2,935
Charged to expense	[1]	7,002	4,822	2,317
Charged to other accounts	[1],[2]	1,470	881	0
Write-offs, net of recoveries	[1]	5,809	4,285	1,855
Balance at end of year	[1]	$ 7,478	$ 4,815	$ 3,397
[1]	Additions to the allowance for doubtful accounts are charged to bad debt expense.
[2]	Amounts represent opening balances from acquired businesses.