CoStar (CSGP) 10-K 26 Feb 15 2014 FY Annual report Financial data

DOCUMENT_AND_ENTITY_INFORMATIO

DOCUMENT AND ENTITY INFORMATION DOCUMENT AND ENTITY INFORMATION (USD $)	12 Months Ended
In Billions, except Share data, unless otherwise specified	Dec. 31, 2014	Feb. 20, 2015	Jun. 30, 2014
Document and Entity Information [Abstract]
Entity Registrant Name	COSTAR GROUP INC
Entity Central Index Key	1057352
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$4.90
Entity Common Stock, Shares Outstanding (in shares)		32,311,866
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14

CONSOLIDATED_STATEMENTS_OF_OPE

CONSOLIDATED STATEMENTS OF OPERATIONS (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenues	$575,936	$440,943	$349,936
Cost of revenues	156,979	129,185	114,866
Gross margin	418,957	311,758	235,070
Operating expenses:
Selling and marketing	150,305	98,708	84,113
Software development	55,426	46,757	32,756
General and administrative	103,916	96,956	77,154
Purchase amortization	28,432	15,183	13,607
Total operating expenses	338,079	257,604	207,630
Income from operations	80,878	54,154	27,440
Interest and other income	516	326	526
Interest and other expense	-10,481	-6,943	-4,832
Income before income taxes	70,913	47,537	23,134
Income tax expense, net	26,044	17,803	13,219
Net income	$44,869	$29,734	$9,915
Net income per share — basic (in dollars per share)	$1.48	$1.07	$0.37
Net income per share — diluted (in dollars per share)	$1.46	$1.05	$0.37
Weighted average outstanding shares — basic (in shares)	30,215	27,670	26,533
Weighted average outstanding shares — diluted (in shares)	30,641	28,212	26,949

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$44,869	$29,734	$9,915
Other comprehensive income (loss), net of tax
Foreign currency translation adjustment	-1,690	610	1,277
Net decrease in unrealized loss on investments	836	378	773
Total other comprehensive income (loss)	-854	988	2,050
Total comprehensive income	$44,015	$30,722	$11,965

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$527,012	$255,953
Accounts receivable, net of allowance for doubtful accounts of approximately $3,397 and $4,815 as of December 31, 2013 and 2014, respectively	38,694	20,761
Deferred and other income taxes, net	20,007	22,506
Income tax receivable	1,027	0
Prepaid expenses and other current assets	9,736	6,597
Debt issuance costs, net	3,335	2,649
Total current assets	599,811	308,466
Long-term investments	17,151	21,990
Property and equipment, net	73,753	57,719
Goodwill	1,138,805	718,587
Intangible assets, net	241,622	144,472
Deposits and other assets	2,676	1,855
Debt issuance costs, net	9,864	3,893
Total assets	2,083,682	1,256,982
Current liabilities:
Current portion of long-term debt	20,000	24,063
Accounts payable	8,608	4,939
Accrued wages and commissions	23,155	20,104
Accrued expenses	27,001	23,200
Deferred gain on the sale of building	2,523	2,523
Income taxes payable	0	2,362
Deferred revenue	38,003	34,362
Total current liabilities	119,290	111,553
Long-term debt, less current portion	365,000	129,062
Deferred gain on the sale of building	23,762	26,286
Deferred rent	27,032	22,828
Deferred income taxes, net	30,349	34,582
Income taxes payable	4,703	4,809
Total liabilities	570,136	329,120
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; 28,848 and 32,318 issued and outstanding as of December 31, 2013 and 2014, respectively	323	288
Additional paid-in capital	1,405,414	863,780
Accumulated other comprehensive loss	-6,384	-5,530
Retained earnings	114,193	69,324
Total stockholders’ equity	1,513,546	927,862
Total liabilities and stockholders’ equity	$2,083,682	$1,256,982

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified
Allowance for doubtful accounts	$4,815	$3,397
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)	32,318,000	28,848,000
Common stock, shares outstanding (in shares)	32,318,000	28,848,000

CONSOLIDATED_STATEMENTS_OF_STO

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (USD $)	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings
In Thousands, except Share data
Balance at Dec. 31, 2011	$659,177	$254	$637,816	($8,568)	$29,675
Balance (in shares) at Dec. 31, 2011		25,426,000
Net income	9,915				9,915
Foreign currency translation adjustment	1,277			1,277
Net decrease in unrealized loss on investments	773			773
Exercise of stock options (in shares)		273,000
Exercise of stock options	9,196	2	9,194
Restricted stock grants (in shares)		855,000
Restricted stock grants	0	8	-8
Restricted stock grants surrendered (in shares)		-96,000
Restricted stock grants surrendered	-4,204	0	-4,204
Stock compensation expense, net of forfeitures	12,207		12,207
Employee stock purchase plan (in shares)		10,000
Employee stock purchase plan	749		749
Consideration for LoopNet (in shares)		1,880,000
Consideration for LoopNet	137,055	19	137,036
Excess tax benefit from stock-based compensation	198		198
Balance at Dec. 31, 2012	826,343	283	792,988	-6,518	39,590
Balance (in shares) at Dec. 31, 2012		28,348,000
Net income	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)		409,000
Exercise of stock options	16,823	3	16,820
Restricted stock grants (in shares)		238,000
Restricted stock grants	0	2	-2
Restricted stock grants surrendered (in shares)		-158,000
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)		11,000
Employee stock purchase plan	1,455		1,455
Excess tax benefit from stock-based compensation	19,585		19,585
Balance at Dec. 31, 2013	927,862	288	863,780	-5,530	69,324
Balance (in shares) at Dec. 31, 2013		28,848,000
Net income	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)		68,000
Exercise of stock options	3,803	1	3,802
Restricted stock grants (in shares)		260,000
Restricted stock grants	0	2	-2
Restricted stock grants surrendered (in shares)		-321,000
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)		3,450,000
Stock issued for equity offering	529,360	34	529,326
Employee stock purchase plan (in shares)		13,000
Employee stock purchase plan	2,152		2,152
Excess tax benefit from stock-based compensation	28,406		28,406
Balance at Dec. 31, 2014	$1,513,546	$323	$1,405,414	($6,384)	$114,193
Balance (in shares) at Dec. 31, 2014		32,318,000

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities:
Net income	$44,869	$29,734	$9,915
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	15,111	12,495	10,053
Amortization	55,261	27,563	22,699
Amortization of debt issuance costs	3,312	3,014	1,989
Impairment loss	1,799	0	0
Property and equipment write-off	1,004	104	122
Excess tax benefit from stock-based compensation	-28,406	-19,585	-198
Stock-based compensation expense	28,267	41,549	12,282
Deferred income tax expense (benefit), net	-1,366	-12,740	13,643
Provision for losses on accounts receivable	4,822	2,317	1,456
Changes in operating assets and liabilities, net of acquisitions:
Accounts receivable	-12,353	-6,607	1,295
Income taxes payable	24,542	29,295	7,598
Prepaid expenses and other current assets	-2,846	2,934	-3,316
Deposits and other assets	-157	399	1,172
Accounts payable and other liabilities	6,078	-3,882	1,629
Deferred revenue	3,972	1,708	5,787
Net cash provided by operating activities	143,909	108,298	86,126
Investing activities:
Proceeds from sale and settlement of investments	5,675	76	15,365
Purchases of property and equipment and other assets	-27,444	-19,042	-14,834
Acquisitions, net of cash acquired	-584,218	0	-640,929
Net cash used in investing activities	-605,987	-18,966	-640,398
Financing activities:
Proceeds from long-term debt	550,000	0	175,000
Payments of long-term debt	-318,125	-17,500	-4,375
Payments of debt issuance costs	-9,969	0	-11,546
Payments of deferred consideration	-1,344	-1,344	0
Excess tax benefit from stock-based compensation	28,406	19,585	198
Repurchase of restricted stock to satisfy tax withholding obligations	-50,555	-8,469	-4,204
Proceeds from equity offering, net of transaction costs	529,360	0	0
Proceeds from exercise of stock options and employee stock purchase plan	5,740	18,133	9,868
Net cash provided by financing activities	733,513	10,405	164,941
Effect of foreign currency exchange rates on cash and cash equivalents	-376	189	78
Net increase (decrease) in cash and cash equivalents	271,059	99,926	-389,253
Cash and cash equivalents at beginning of year	255,953	156,027	545,280
Cash and cash equivalents at end of year	$527,012	$255,953	$156,027

ORGANIZATION

ORGANIZATION	12 Months Ended
	Dec. 31, 2014
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.”), the United Kingdom (“U.K.”), Toronto, Canada and parts of France. The Company provides online marketplaces for commercial real estate listings, 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_ACCOUNT

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2014
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. Actual results could differ from those estimates.
	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. Subscription contract rates are based on the number of sites, number of users, organization size, the client’s business focus, geography and the number and types of services to which a client subscribes. A majority of the subscription-based license agreements typically 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 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 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, 2012, 2013 and 2014.
	2	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)
	Accumulated Other Comprehensive Loss
	The components of accumulated other comprehensive loss were as follows (in thousands):
		Year Ended December 31,
		2013			2014
	Foreign currency translation adjustment, net of tax	$	(4,003	)	$	(5,693	)
	Accumulated net unrealized loss on investments, net of tax	(1,527		)	(691		)
	Total accumulated other comprehensive loss	$	(5,530	)	$	(6,384	)
	There were no amounts reclassified out of accumulated other comprehensive loss to the consolidated statements of operations for the years ended December 31, 2012, 2013 and 2014, respectively.
	Advertising Costs
	The Company expenses advertising costs as incurred. Advertising costs, which includes e-commerce, television, radio, print and other media advertising, were approximately $6.2 million, $7.9 million and $28.7 million for the years ended December 31, 2012, 2013 and 2014, 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 differences are expected 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 Per Share
	Net income per share is computed by dividing net income 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 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.
	2	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)
	Stock-Based Compensation — (Continued)
	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 and service conditions, and as a result, the restricted common stock granted under these awards vested. The Company recorded approximately $0, $21.8 million and $2.2 million of stock-based compensation expense related to the 2012 performance-based restricted common stock awards for the years ended December 31, 2012, 2013 and 2014, respectively.
	In February 2014, the Compensation Committee (the “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. 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 related three-year performance period. As of December 31, 2014, 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 these awards would be met by the December 31, 2016 forfeiture date. As a result, the Company recorded a total of approximately $1.1 million of stock-based compensation expense related to the performance-based restricted common stock awards with a market condition for the year ended December 31, 2014. No stock-based compensation expense related to the grant of 2014 performance-based restricted common stock was recorded for the years ended December 31, 2012 and 2013. The Company expects to record an estimated unrecognized stock-based compensation expense related to the performance-based restricted common stock awards of approximately $2.7 million over the periods 2015, 2016 and 2017.
	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 $9.9 million, $18.1 million and $5.7 million for the years ended December 31, 2012, 2013 and 2014, respectively. The Company realized approximately $198,000, $19.6 million and $28.4 million of excess tax benefits from stock options exercised and restricted stock awards vested for the years ended December 31, 2012, 2013 and 2014, respectively.
	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,
		2012			2013			2014
	Cost of revenues	$	2,556		$	4,553		$	4,759
	Selling and marketing	1,966			4,954			3,776
	Software development	2,241			7,244			5,095
	General and administrative	5,519			24,798			14,637
	Total stock-based compensation	$	12,282		$	41,549		$	28,267
	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. As of December 31, 2013 and 2014, cash of approximately $105,000 and $0, respectively, was held to support letters of credit for security deposits.
	2	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)
	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, 2012, 2013 and 2014. 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.
	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 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 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.
	2	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)
	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. The impairment loss is measured based on a projected discounted cash flow method using a discount rate determined by the Company’s management to be commensurate with the risk in its current business model.
	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 the quantitative impairment test similar to the test performed on goodwill discussed above.
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, and are reviewed at least annually for impairment.
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 one 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 two 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. The acquired intangible asset characterized as customer base consists of one distinct intangible asset composed of acquired customer contracts and the related customer relationships. 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 three 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.
2	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED)
Capitalized Product Development Costs
Product development costs are expensed as incurred until technological feasibility has been established, at which time such costs are capitalized. Costs are capitalized, to the extent that the capitalizable costs do not exceed the realizable value of such costs, until the product is available for general release to customers. The Company defines the establishment of technological feasibility as the completion of all planning, designing, coding and testing activities that are necessary to establish products that meet design specifications including functions, features and technical performance requirements. The Company’s capitalized product development costs had a total net book value of approximately $111,000 and $0 as of December 31, 2013 and 2014, respectively. These capitalized product development costs are included in intangible and other assets in the Company’s consolidated balance sheets. Amortization is computed using a straight-line method over the remaining estimated economic life of the product, typically three to five years after the software is ready for its intended use. The Company amortized capitalized product development costs of approximately $191,000, $191,000 and $111,000 for the years ended December 31, 2012, 2013 and 2014, respectively.
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. 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 of approximately $6.5 million and $13.2 million as of December 31, 2013 and 2014, 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 $2.0 million, $3.0 million and $3.3 million for the years ended December 31, 2012, 2013 and 2014, respectively.
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 will 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 is effective on a retrospective basis for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. Early application is not permitted. This guidance permits the use of either a full retrospective method or a modified retrospective approach in which it would be applied only to the most current period presented along with a cumulative-effect adjustment at the date of adoption. The Company has not yet selected a transition method and is currently evaluating the impact this guidance will have on its financial statements.

ACQUISITION

ACQUISITION	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
ACQUISITIONS	ACQUISITION
	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 business (collectively, the “Apartments.com Business”). Apartments.com is a national online apartment rentals resource for renters, property managers and owners. Apartments.com offers renters a 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 the Apartments.com Business, 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 the Apartments.com Business 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 has an estimated useful life of one year due to the Company's intent to replace the existing database technology in 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 are recognized on a straight-line basis over the estimated useful life. 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 entire amount of goodwill is expected 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: (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)
	The Company's consolidated revenue for the year ended December 31, 2014, included $76.8 million from the Apartments.com Business. The Company's consolidated income before income taxes for the year ended December 31, 2014, included a $23.9 million loss before income taxes from the Apartments.com Business. The Company's consolidated revenue and income before income taxes for the years ended December 31, 2012 and 2013 did not include any amount from the Apartments.com Business.
	The following unaudited pro forma amounts present consolidated information as if the acquisition had been completed as of January 1, 2013 (in thousands except per share data):
		Year Ended December 31,
		2013			2014
	Revenue	$	526,811		$	598,340
	Net income	$	14,432		$	51,649
	Net income per share — basic	$	0.52		$	1.71
	Net income per share — diluted	$	0.51		$	1.69
	This information is based on historical results of operations, adjusted for the allocation of purchase price and other acquisition accounting adjustments, including: (i) the amortization associated with the acquired intangible assets; (ii) interest expense associated with debt used to fund a portion of the acquisition; and (iii) income tax expense associated with pro forma adjustments and the historical results of Apartments.com calculated at a tax rate of 38%. The unaudited pro forma results do not include: (i) any potential synergies, cost savings or other expected benefits of the acquisition and (ii) the non-recurring acquisition costs incurred through the date of acquisition. Accordingly, the unaudited pro forma amounts are for comparative purposes only and may not necessarily reflect the results of operations which would have resulted had the acquisition been completed at the beginning of the applicable period and may not be indicative of the results that will be attained in the future.
	As a result of the acquisition of the Apartments.com Business, 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 the Apartments.com Business, are expensed as incurred and are recorded in general and administrative expense. The Company did not record any acquisition-related costs for the year ended December 31, 2013 and recorded approximately $5.2 million in acquisition-related costs for the year ended December 31, 2012 as a result of the acquisition of LoopNet on April 30, 2012.

INVESTMENTS

INVESTMENTS	12 Months Ended
	Dec. 31, 2014
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, 2014 are as follows (in thousands):
	Maturity		Fair Value
	Due in:
	2015		$	—
	2016 — 2019		822
	2020 — 2024		—
	2025 and thereafter		16,329
	Available-for-sale investments		$	17,151
	The Company had no realized gains on its investments for the years ended December 31, 2012, 2013 and 2014, respectively. The Company had no realized losses on its investments for the years ended December 31, 2012, 2013 and 2014, respectively. Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis.
	4	INVESTMENTS — (CONTINUED)
	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, 2014, the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands):
		Amortized			Gross			Gross			Fair Value
		Cost			Unrealized			Unrealized
					Gains			Losses
	Auction rate securities	$	17,842		$	380		$	(1,071	)	$	17,151
	Available-for-sale investments	$	17,842		$	380		$	(1,071	)	$	17,151
	As of December 31, 2013, the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands):
		Amortized			Gross			Gross			Fair Value
		Cost			Unrealized			Unrealized
					Gains			Losses
	Auction rate securities	$	23,517		$	411		$	(1,938	)	$	21,990
	Available-for-sale investments	$	23,517		$	411		$	(1,938	)	$	21,990
	The unrealized losses on the Company’s investments as of December 31, 2013 and 2014 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, 2013 and 2014. 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,
		2013						2014
		Aggregate			Gross			Aggregate			Gross
		Fair			Unrealized			Fair			Unrealized
		Value			Losses			Value			Losses
	Auction rate securities	$	21,137		$	(1,938	)	$	16,329		$	(1,071	)
	Investments in an unrealized loss position	$	21,137		$	(1,938	)	$	16,329		$	(1,071	)
	The Company did not have any investments in an unrealized loss position for less than twelve months as of December 31, 2013 and 2014, respectively.

FAIR_VALUE

FAIR VALUE	12 Months Ended
	Dec. 31, 2014
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, 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 following table represents the Company's fair value hierarchy for its financial assets (cash, cash equivalents and investments) and liabilities measured at fair value on a recurring basis as of December 31, 2013 (in thousands):
		Level 1			Level 2		Level 3		Total
	Assets:
	Cash	$	134,989		$	—	$	—	$	134,989
	Money market funds	50,593			—		—		50,593
	Commercial paper	70,371			—		—		70,371
	Auction rate securities	—			—		21,990		21,990
	Total assets measured at fair value	$	255,953		$	—	$	21,990	$	277,943
	Liabilities:
	Deferred consideration	$	—		$	—	$	1,344	$	1,344
	Total liabilities measured at fair value	$	—		$	—	$	1,344	$	1,344
	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.
	5	FAIR VALUE — (CONTINUED)
	The following table summarizes changes in fair value of the Company’s Level 3 assets from December 31, 2008 to December 31, 2014 (in thousands):
		Auction
		Rate
		Securities
	Balance at December 31, 2008	$	29,340
	Decrease in unrealized loss included in accumulated other comprehensive loss	684
	Settlements	(300		)
	Balance at December 31, 2009	29,724
	Decrease in unrealized loss included in accumulated other comprehensive loss	40
	Settlements	(575		)
	Balance at December 31, 2010	29,189
	Decrease in unrealized loss included in accumulated other comprehensive loss	245
	Settlements	(4,850		)
	Balance at December 31, 2011	24,584
	Auction rate securities upon acquisition	442
	Decrease in unrealized loss included in accumulated other comprehensive loss	836
	Settlements	(4,200		)
	Balance at December 31, 2012	21,662
	Decrease in unrealized loss included in accumulated other comprehensive loss	378
	Settlements	(50		)
	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
	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, 2014, the Company held ARS with $18.7 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, 2014.
	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, 2014. 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, 2013 and 2014 was approximately 4.9% and 4.1%, 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, 2014, the Company determined there was a decline in the fair value of its ARS investments of approximately $691,000. 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.
	As of December 31, 2014, the Company had no Level 3 liabilities. As of December 31, 2013, the Company held Level 3 liabilities for deferred consideration that it acquired as a result of the April 30, 2012 acquisition of LoopNet. The deferred consideration included potential deferred cash payments in connection with acquisitions LoopNet completed in 2010 including: (i) potential deferred cash payments due to the sellers of LandsofAmerica.com, LLC ("LandsofAmerica") on March 31, 2014 based on LandsofAmerica's achievement of financial and operational milestones, resulting in undiscounted deferred consideration as of December 31, 2013 of approximately $1.0 million; and (ii) potential deferred cash payments due to the sellers of CoStar Private Sale NetworkTM (formerly known as Reaction Corp. (“Reaction Web”)) on March 31, 2014 based on CoStar Private Sale Network's achievement of revenue milestones, resulting in undiscounted deferred consideration as of December 31, 2013 of approximately $344,000. On March 28, 2013, the Company paid $1.0 million to the sellers of LandsofAmerica for the achievement of financial and operational milestones in 2012 and paid approximately $344,000 to the sellers of CoStar Private Sale Network for the achievement of revenue milestones in 2012. On March 31, 2014, the Company paid $1.0 million to the sellers of LandsofAmerica for the achievement of financial and operational milestones in 2013 and paid approximately $344,000 to the sellers of CoStar Private Sale Network for the achievement of revenue milestones in 2013.
	The following table summarizes changes in fair value of the Company’s Level 3 liabilities from December 31, 2012 to December 31, 2014 (in thousands):
		Deferred
		Consideration
	Balance at December 31, 2012	$	2,304
	Accretion for 2013	384
	Payments made in 2013	(1,344		)
	Balance at December 31, 2013	1,344
	Payments made in 2014	(1,344		)
	Balance at December 31, 2014	$	—
	The Company used a discounted cash flow model to determine the estimated fair value of its Level 3 liabilities. The assumptions used in preparing the discounted cash flow model include the discount rate and probabilities for completion of financial and operational milestones.
The only significant unobservable input in the discounted cash flow model used to determine the estimated fair value of the Company's Level 3 liabilities was the discount rate. The discount rate used represented LoopNet's cost of equity at the time of each acquisition plus a margin for counterparty risk.

PROPERTY_AND_EQUIPMENT

PROPERTY AND EQUIPMENT	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
PROPERTY AND EQUIPMENT	PROPERTY AND EQUIPMENT
	Property and equipment consists of the following (in thousands):
		December 31,
		2013			2014
	Leasehold improvements	$	36,933		$	42,026
	Furniture, office equipment and research vehicles	27,395			31,016
	Computer hardware and software	36,391			49,655
	Property and equipment, gross	100,719			122,697
	Accumulated depreciation and amortization	(43,000		)	(48,944		)
	Property and equipment, net	$	57,719		$	73,753
	Depreciation expense for property and equipment was approximately $10.1 million, $12.5 million and $15.1 million for the years ended December 31, 2012, 2013 and 2014, respectively.

GOODWILL

GOODWILL	12 Months Ended
	Dec. 31, 2014
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, 2012	$	692,639	$	25,439		$	718,078
	Effect of foreign currency translation	—		509			509
	Goodwill, December 31, 2013	692,639		25,948			718,587
	Acquisitions	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
	The Company recorded goodwill of approximately $421.7 million in connection with the April 1, 2014 acquisition of the Apartments.com Business.
	During the fourth quarters of 2012, 2013 and 2014, 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, 2014
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
		2013			2014			(in years)
	Capitalized product development cost	$	2,140		$	2,140		4
	Accumulated amortization	(2,029		)	(2,140		)
	Capitalized product development cost, net	111			—
	Building photography	13,743			14,943			5
	Accumulated amortization	(12,005		)	(12,665		)
	Building photography, net	1,738			2,278
	Acquired database technology	77,368			88,739			4
	Accumulated amortization	(41,073		)	(60,498		)
	Acquired database technology, net	36,295			28,241
	Acquired customer base	130,960			199,826			10
	Accumulated amortization	(74,734		)	(102,443		)
	Acquired customer base, net	56,226			97,383
	Acquired trade names and other intangible assets(1)	59,336			128,171			13
	Accumulated amortization	(9,234		)	(14,451		)
	Acquired trade names and other intangible assets, net	50,102			113,720
	Intangible assets, net	$	144,472		$	241,622
	(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 $22.7 million, $27.6 million and $55.3 million for the years ended December 31, 2012, 2013 and 2014, respectively.
	In the aggregate, amortization for intangible assets existing as of December 31, 2014 for future periods is expected to be approximately $48.5 million, $36.2 million, $22.4 million, $14.9 million and $12.9 million for the years ending December 31, 2015, 2016, 2017, 2018 and 2019, respectively.
	During the fourth quarter of 2014, 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.

LONGTERM_DEBT

LONG-TERM DEBT	12 Months Ended
	Dec. 31, 2014
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 the Apartments.com Business. 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 an expected present value 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 debt modification and determined that the modification 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.
	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 ½ of 1% 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, 2014.
	9	LONG-TERM DEBT — (CONTINUED)
	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, 2013 and 2014, no amounts were outstanding under the revolving credit facilities. Total interest expense for the term loan facilities and revolving credit facilities was approximately $4.8 million, $6.9 million and $10.5 million for the years ended December 31, 2012, 2013 and 2014, respectively. Interest expense included amortized debt issuance costs of approximately $2.0 million, $3.0 million and $3.3 million for the years ended December 31, 2012, 2013 and 2014, respectively. Total interest paid for the term loan facilities was approximately $2.5 million, $4.3 million and $7.0 million for the years ended December 31, 2012, 2013 and 2014, respectively.
	Maturities of the Company's borrowings under the Credit Agreement for each of the next five years as of December 31, 2014 are as follows (in thousands):
	Year ending December 31,	Maturities
	Due in:
	2015	$	20,000
	2016	20,000
	2017	35,000
	2018	55,000
	2019	255,000
	Long-term debt, including current maturities	$	385,000

INCOME_TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES
	The components of the provision (benefit) for income taxes attributable to operations consist of the following (in thousands):
		Year Ended December 31,
		2012			2013			2014
	Current:
	Federal	$	(2,260	)	$	26,516		$	24,741
	State	1,974			3,996			2,761
	Foreign	55			31			53
	Total current	(231		)	30,543			27,555
	Deferred:
	Federal	15,512			(10,919		)	(698		)
	State	(2,067		)	(1,849		)	(813		)
	Foreign	5			28			—
	Total deferred	13,450			(12,740		)	(1,511		)
	Total provision for income taxes	$	13,219		$	17,803		$	26,044
	10	INCOME TAXES — (CONTINUED)
	The components of deferred tax assets and liabilities consists of the following (in thousands):
		December 31,
		2013			2014
	Deferred tax assets:
	Reserve for bad debts	$	1,274		$	1,825
	Accrued compensation	6,725			7,287
	Stock compensation	13,381			8,758
	Net operating losses	17,457			15,665
	Accrued reserve and other	4,284			3,360
	Unrealized loss on securities	786			491
	Deferred rent	4,329			5,902
	Deferred revenue	1,538			1,879
	Deferred gain on the sale of building	11,499			10,690
	Total deferred tax assets	61,273			55,857
	Deferred tax liabilities:
	Prepaids	(1,096		)	(1,258		)
	Depreciation	(6,033		)	(9,806		)
	Intangibles	(55,284		)	(47,720		)
	Total deferred tax liabilities	(62,413		)	(58,784		)
	Net deferred tax liabilities, prior to valuation allowance	(1,140		)	(2,927		)
	Valuation allowance	(10,936		)	(7,783		)
	Net deferred tax liabilities	$	(12,076	)	$	(10,710	)
	As of December 31, 2013 and 2014, a valuation allowance has been established for certain deferred tax assets due to the uncertainty of realization. The valuation allowance as of December 31, 2013 includes an allowance for unrealized losses on ARS investments, foreign deferred tax assets and certain state net operating loss carryforwards. 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 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 in certain states where it is uncertain whether the Company will generate sufficient taxable income to utilize the net operating losses before the losses expire.
	The Company’s change in valuation allowance was an increase of approximately $446,000 for the year ended December 31, 2013 and a decrease of approximately $3.2 million for the year ended December 31, 2014. The decrease for the year ended December 31, 2014 is due to a decrease in the valuation allowance for U.S. deferred tax assets of approximately $2.3 million primarily related to the write-off of tax credits and state net operating loss carryforwards and a decrease in the amount of unrealized losses on investments as well as the decrease in the valuation allowance for foreign deferred tax assets of approximately $835,000.
	10	INCOME TAXES — (CONTINUED)
	The Company had U.S. income before income taxes of approximately $36.1 million, $53.2 million and $70.6 million for the years ended December 31, 2012, 2013 and 2014, respectively. The Company had foreign losses of approximately $13.0 million and $5.6 million for the years ended December 31, 2012 and 2013, respectively. The Company had foreign income before income taxes of approximately $273,000 for the year ended December 31, 2014.
	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,
		2012			2013			2014
	Expected federal income tax provision at statutory rate	$	8,097		$	16,638		$	24,820
	State income taxes, net of federal benefit	(1,360		)	885			1,965
	Foreign income taxes, net effect	(2,971		)	(724		)	336
	Stock compensation	(313		)	(116		)	35
	Increase (decrease) in valuation allowance	2,978			588			(2,397		)
	Nondeductible compensation	656			431			554
	Nondeductible transaction costs	5,829			—			—
	Other adjustments	303			101			731
	Income tax expense, net	$	13,219		$	17,803		$	26,044
	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 $2.6 million, $6.5 million, and $3.0 million in income taxes for the years ended December 31, 2012, 2013 and 2014, respectively.
	The Company has net operating loss carryforwards for international income tax purposes of approximately $27.9 million, which do not expire. The Company has federal net operating loss carryforwards of approximately $11.0 million that begin to expire in 2020, state net operating loss carryforwards with a tax value of approximately $4.4 million that begin to expire in 2020 and state income tax credit carryforwards with a tax value of approximately $1.9 million that begin to expire in 2020. The Company realized a cash benefit relating to the use of its tax loss carryforwards of approximately $4.2 million and $1.2 million in 2013 and 2014, respectively.
	The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands):
	Unrecognized tax benefits as of December 31, 2011	$	3,347
	Increase for current year tax positions	792
	Decrease for prior year tax positions	(161		)
	Expiration of the statute of limitation for assessment of taxes	(69		)
	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
10	INCOME TAXES — (CONTINUED)
Approximately $1.6 million and $1.5 million of the unrecognized tax benefits as of December 31, 2013 and 2014, respectively, would favorably affect the annual effective tax rate, if recognized in future periods. The Company recognized $58,000, $62,000 and $62,000 for interest and penalties in its consolidated statements of operations for the years ended December 31, 2012, 2013 and 2014, respectively. The Company had liabilities of $342,000, $404,000 and $466,000 for interest and penalties in its consolidated balance sheets as of December 31, 2012, 2013 and 2014, 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 and state income tax returns for tax years 2011 through 2013 remain open to examination. The Company’s U.K. income tax returns for tax years 2008 through 2013 remain open to examination.
The Company was previously under an Internal Revenue Service (“IRS”) audit in the U.S. for tax year 2010. During September 2014, the statute of limitations for tax year 2010 expired with no adjustments proposed by the IRS.
During July 2014, the IRS completed its audit of the tax returns filed by LoopNet, the Company's subsidiary, for tax years 2009, 2010, 2011 and the four months ended April 30, 2012. No adjustments were made to the financial statements as a result of the completion of the IRS audit.

COMMITMENTS_AND_CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2014
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, 2012, 2013 and 2014 was approximately $16.7 million, $18.3 million and $19.2 million, respectively.
	Future minimum lease payments as of December 31, 2014 are as follows (in thousands):
	2015	$	19,442
	2016	18,783
	2017	18,607
	2018	17,680
	2019	15,907
	2020 and thereafter	75,878
	Total future minimum lease payments	$	166,297
	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)
	In May 2011, LoopNet, the Board of Directors of LoopNet (“the LoopNet Board”) and/or the Company were named as defendants in three purported class action lawsuits brought by alleged LoopNet stockholders challenging LoopNet's then-proposed merger with the Company. The stockholder actions alleged, among other things, that (i) each member of the LoopNet Board breached his fiduciary duties to LoopNet and its stockholders in authorizing the sale of LoopNet to the Company, (ii) the merger did not maximize value to LoopNet stockholders, (iii) LoopNet and the Company made incomplete or materially misleading disclosures about the transaction and (iv) LoopNet and the Company aided and abetted the breaches of fiduciary duty allegedly committed by the members of the LoopNet Board. The stockholder actions sought class action certification and equitable relief, including an injunction against consummation of the merger. The parties stipulated to the consolidation of the actions, and permitted the filing of a consolidated complaint. In June 2011, counsel for the parties entered into a memorandum of understanding in which they agreed on the terms of a settlement of this litigation, which could result in a loss to the Company of approximately $200,000. On March 20, 2013, the California Superior Court declined to grant preliminary approval to the proposed settlement and issued an order scheduling a hearing on June 11, 2013 to show good cause why the case should not be dismissed. Shortly before the hearing, the plaintiffs filed a third supplemental submission in support of their motion for preliminary approval of the proposed settlement. The show cause hearing took place on May 13, 2014 and a follow up hearing took place on July 16, 2014. At the July 16, 2014 hearing the Court again denied preliminary approval of the settlement and encouraged the parties to discuss a potential disposition of the case due to the mootness of plaintiffs’ disclosure claims. The parties engaged in such discussions, and on October 14, 2014, the plaintiffs requested that the Court dismiss their claims with prejudice. The Court dismissed the action and, pursuant to an agreement between the parties, the Company and its insurer reimbursed certain legal fees to plaintiffs’ counsel in the total amount of $300,000. The Company paid approximately $200,000 of this amount.
	On January 3, 2012, LoopNet, the Company’s wholly owned subsidiary, was sued by CIVIX-DDI, LLC (“Civix”) in the U.S. District Court for the Eastern District of Virginia for alleged infringement of U.S. Patent Nos. 6,385,622 and 6,415,291. The complaint sought unspecified damages, attorneys' fees and costs. On February 16, 2012, LoopNet filed an answer to Civix’s complaint and filed counterclaims against Civix seeking, among other things, declaratory relief that the asserted patents are invalid, not infringed, and that Civix committed inequitable conduct during the prosecution and re-examination of the asserted patents. On or about May 14, 2012, Civix filed a motion for leave to amend its complaint against LoopNet in the U.S. District Court for the Eastern District of Virginia seeking to add the Company as a defendant, alleging that the Company's products also infringe Civix’s patents. The Company filed a motion opposing Civix’s motion, and on June 21, 2012, the district court denied Civix’s motion to amend its complaint. On June 21, 2012, the Company filed an action in the U.S. District Court for the Northern District of Illinois seeking a declaratory judgment of non-infringement and invalidity against Civix. On August 14, 2012, the Company amended its complaint against Civix to assert an affirmative claim against Civix for breach of contract, alleging Civix violated its license agreement with, and covenant not to sue, one of the Company's technology licensors. On August 30, 2012, the Eastern District of Virginia transferred Civix’s case against LoopNet to the Northern District of Illinois,. On October 29, 2012, Civix filed a separate action against LoopNet in the Northern District of Illinois alleging infringement of U.S. Patent No. 8,296,335. That case was later consolidated with Civix’s original lawsuit against LoopNet. Civix amended its complaint against the Company on November 8, 2012 to add claims under Patent No. 8,296,335 as well. On November 15, 2012, LoopNet filed an amended answer and counterclaim against Civix, asserting an affirmative claim against Civix for breach of contract, alleging Civix violated its license agreement with, and covenant not to sue, one of LoopNet's technology licensors. The U.S. District Court for the Northern District of Illinois construed the language of the patent on September 23, 2013, but no trial date was set. On November 25, 2013, Civix submitted its expert’s report of damages, which estimated the payment it deemed appropriate in the event that the Company was found liable for infringement. The Company believed that Civix’s calculation of damages was based on improper assumptions and miscalculations, and was otherwise unsupported. The Company submitted its own expert’s report of damages, which concluded that the appropriate payment to be made in the event that the Company was found liable for infringement was significantly less than Civix’s estimate of appropriate damages. Moreover, the Company’s expert’s report of damages concluded that while Civix’s calculation of damages was fundamentally flawed and should not have been used to determine damages, simply applying certain necessary adjustments to Civix’s calculation as outlined in the Company’s report resulted in a significant reduction in Civix’s calculation of damages to approximately $3.7 million. The Court granted a motion submitted by the parties requesting a settlement conference, which took place on November 20, 2014. Subsequent to the settlement conference, on December 1, 2014 the Company settled all outstanding litigation with Civix for $2.9 million, which was paid on December 9, 2014.
	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 other than as described above. In addition, other than as described above, 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, other than described above. Legal defense costs are expensed as incurred.

SEGMENT_REPORTING

SEGMENT REPORTING	12 Months Ended
	Dec. 31, 2014
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 Canada, and International, which includes the U.K. and France. The Company’s subscription-based information services consist primarily of CoStar SuiteTM 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. Prior to the third quarter of 2014, FOCUSTM was the Company’s primary service offering in the International operating segment. The Company introduced CoStar Suite in the U.K. in the fourth quarter of 2012 and no longer offered FOCUS to new clients beginning in 2013. CoStar's and its subsidiaries' subscription-based services consist primarily of similar services offered over the Internet to commercial real estate industry and related professionals. Management relies on an internal management reporting process that provides revenue and operating segment EBITDA, which is the Company’s net income 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,
		2012			2013			2014
	Revenues
	North America	$	330,805		$	420,817		$	552,141
	International
	External customers	19,131			20,126			23,795
	Intersegment revenue	1,514			339			57
	Total International revenue	20,645			20,465			23,852
	Intersegment eliminations	(1,514		)	(339		)	(57		)
	Total revenues	$	349,936		$	440,943		$	575,936
	EBITDA
	North America	$	70,199		$	97,348		$	148,913
	International	(10,007		)	(3,136		)	2,337
	Total EBITDA	$	60,192		$	94,212		$	151,250
	12	SEGMENT REPORTING — (CONTINUED)
	The reconciliation of EBITDA to net income consists of the following (in thousands):
		Year Ended December 31,
		2012			2013			2014
	EBITDA	$	60,192		$	94,212		$	151,250
	Purchase amortization in cost of revenues	(8,634		)	(11,883		)	(26,290		)
	Purchase amortization in operating expenses	(13,607		)	(15,183		)	(28,432		)
	Depreciation and other amortization	(10,511		)	(12,992		)	(15,650		)
	Interest income	526			326			516
	Interest expense	(4,832		)	(6,943		)	(10,481		)
	Income tax expense, net	(13,219		)	(17,803		)	(26,044		)
	Net income	$	9,915		$	29,734		$	44,869
	Intersegment revenue recorded during 2014 was attributable to services performed for the Company’s wholly owned subsidiary, 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 recorded during 2012 and 2013 was attributable to services performed for CoStar Portfolio Strategy by Property and Portfolio Research Ltd., a wholly owned subsidiary of CoStar Portfolio Strategy. 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.
	There were no costs allocated to North America EBITDA for the year ended December 31, 2012. North America EBITDA includes an allocation of approximately $800,000 and $1.1 million for the years ended December 31, 2013 and 2014, 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 $5.3 million, $400,000 and $300,000 for the years ended December 31, 2012, 2013 and 2014, respectively. This 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,
		2013			2014
	Property and equipment, net
	North America	$	53,733		$	71,209
	International	3,986			2,544
	Total property and equipment, net	$	57,719		$	73,753
	Goodwill
	North America	$	692,639		$	1,114,363
	International	25,948			24,442
	Total goodwill	$	718,587		$	1,138,805
	Assets
	North America	$	1,311,292		$	2,138,768
	International	43,464			41,896
	Total operating segment assets	$	1,354,756		$	2,180,664
	Reconciliation of operating segment assets to total assets
	Total operating segment assets	$	1,354,756		$	2,180,664
	Investment in subsidiaries	(18,344		)	(18,344		)
	Intersegment receivables	(79,430		)	(78,638		)
	Total assets	$	1,256,982		$	2,083,682
	Liabilities
	North America	$	324,626		$	564,832
	International	79,266			75,584
	Total operating segment liabilities	$	403,892		$	640,416
	Reconciliation of operating segment liabilities to total liabilities
	Total operating segment liabilities	$	403,892		$	640,416
	Intersegment payables	(74,772		)	(70,280		)
	Total liabilities	$	329,120		$	570,136

STOCKHOLDERS_EQUITY

STOCKHOLDERS' EQUITY	12 Months Ended
	Dec. 31, 2014
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, 2014. 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 intends 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 in the future, 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_PER_SHARE

NET INCOME PER SHARE	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
NET INCOME PER SHARE	NET INCOME PER SHARE
	The following table sets forth the calculation of basic and diluted net income per share (in thousands except per share data):
		Year Ended December 31,
		2012			2013			2014
	Numerator:
	Net income	$	9,915		$	29,734		$	44,869
	Denominator:
	Denominator for basic net income per share — weighted-average outstanding shares	26,533			27,670			30,215
	Effect of dilutive securities:
	Stock options and restricted stock	416			542			426
	Denominator for diluted net income per share — weighted-average outstanding shares	26,949			28,212			30,641
	Net income per share — basic	$	0.37		$	1.07		$	1.48
	Net income per share — diluted	$	0.37		$	1.05		$	1.46
	14	NET INCOME PER SHARE — (CONTINUED)
	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. The following table summarizes the potential common shares excluded from the diluted calculation (in thousands):
		Year Ended December 31,
		2012		2013			2014
	Employee stock options	—		—			80
	Performance-based restricted stock awards	399		379			23
	Service-based restricted stock units	—		—			1
	Total shares excluded from computation	399		379			104

EMPLOYEE_BENEFIT_PLANS

EMPLOYEE BENEFIT PLANS	12 Months Ended
	Dec. 31, 2014
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.
	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 shares were available for future grant under the 2007 Plan as of December 31, 2013 and 2014.
	15	EMPLOYEE BENEFIT PLANS — (CONTINUED)
	Stock Incentive Plans — (Continued)
	At December 31, 2014, there was $51.3 million of unrecognized compensation cost related to stock-based payments, net of forfeitures, which is expected to be recognized over a weighted-average-period of 2.5 years.
	Stock Options
	Option activity was as follows:
		Number of			Range of		Weighted-				Weighted-		Aggregate
		Shares			Exercise Price		Average				Average		Intrinsic
							Exercise				Remaining		Value
							Price				Contract		(in thousands)
											Life (in years)
	Outstanding at December 31, 2011	847,102			$17.34 - $60.23		$	39.93
	Granted	102,000			$58.95 - $58.95		$	58.95
	Exercised	(274,842	)		$17.34 - $57.16		$	34.04
	Canceled or expired	(541	)		$54.51 - $54.51		$	54.51
	Outstanding at December 31, 2012	673,719			$25.00 - $60.23		$	45.2
	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			7.25		$	32,677
	Exercisable at December 31, 2012	432,196			$25.00 - $60.23		$	40.22
	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			6.09		$	23,221
	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 274,842, 409,799 and 68,126 shares were exercised during the years ended December 31, 2012, 2013, and 2014, respectively. The aggregate intrinsic value of options exercised, determined as of the date of option exercise, was $11.9 million, $39.0 million and $8.9 million for the years ended December 31, 2012, 2013, and 2014, respectively.
	The weighted-average grant date fair value of each option granted during the years ended December 31, 2012, 2013 and 2014 using the Black-Scholes option-pricing model was $20.99, $34.10 and $58.12 respectively.
	15	EMPLOYEE BENEFIT PLANS — (CONTINUED)
	Stock Options — (Continued)
	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,
		2012			2013			2014
	Dividend yield	0	%		0	%		0	%
	Expected volatility	40	%		37	%		30	%
	Risk-free interest rate	0.9	%		0.9	%		1.5	%
	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, 2014:
			Options Outstanding									Options Exercisable
	Range of		Number of			Weighted-Average Remaining Contractual Life (in years)		Weighted-				Number of			Weighted-
	Exercise Price		Shares					Average				Shares			Average
								Exercise Price							Exercise Price
	$36.48 - $42.50		58,603			5.05		$	41.2			58,603			$	41.2
	$42.51 - $55.83		19,666			3.47		$	49.24			19,666			$	49.24
	$55.84 - $57.61		43,549			6.17		$	57.16			43,549			$	57.16
	$57.62 - $58.51		473			6.09		$	58.06			100			$	58.06
	$58.52 - $59.59		54,068			7.14		$	58.95			27,800			$	58.95
	$59.60 - $81.19		2,320			6.42		$	60.23			1,740			$	60.23
	$81.20 - $151.60		111,600			8.19		$	102.16			37,198			$	102.16
	$151.61 - $201.04		79,900			9.15		$	201.04			—			$	—
	$36.48 - $201.04		370,179			7.25		$	99.12			188,656			$	60.54
	15	EMPLOYEE BENEFIT PLANS — (CONTINUED)
	Restricted Stock Awards
	In February 2012, the Committee approved grants of restricted common stock to the executive officers that vest based on the achievement of certain performance conditions. These awards support the Committee’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. No shares of the 2012 performance-based restricted common stock were granted by the Company during the years ended December 31, 2013 and 2014. 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, 2013, the Company initially determined that it was probable that the performance condition for these performance-based restricted common stock awards would be met by the March 31, 2017 forfeiture date. As of March 31, 2014, the Company had satisfied all performance and service conditions, and as a result, the restricted common stock granted under these awards vested. The Company recorded approximately $0, $21.8 million and $2.2 million of stock-based compensation expense related to the 2012 performance-based restricted common stock for the years ended December 31, 2012, 2013 and 2014, respectively.
	In February 2014, the Committee 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. 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 awards originally granted. 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 Committee’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 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 related 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.
	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 during the year ended December 31, 2014 were as follows:
		Year Ended December 31,
		2014
Dividend yield	0		%
Expected volatility	30		%
Risk-free interest rate	0.6		%
Expected life (in years)	3
Weighted-average grant date fair value	$	216.2
15	EMPLOYEE BENEFIT PLANS — (CONTINUED)
Restricted Stock Awards — (Continued)
As of December 31, 2014, 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 these awards would be met by December 31, 2016. As a result, the Company recorded a total of approximately $1.1 million of stock-based compensation expense related to the performance-based restricted common stock awards with a market condition for the year ended December 31, 2014. There was no stock-based compensation expense related to the grant of 2014 performance-based restricted common stock recorded for the years ended December 31, 2012 and 2013. The Company expects to record an estimated unrecognized stock-based compensation expense related to the performance-based restricted common stock awards of approximately $2.7 million over the periods 2015, 2016 and 2017.
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, 2014:
	Restricted Stock Awards — without Market Condition							Restricted Stock Awards — with Market Condition
	Number of			Weighted-Average				Number of			Weighted-Average
	Shares			Grant Date				Shares			Grant Date
				Fair Value per Share							Fair Value per Share
Unvested restricted stock awards at December 31, 2013	968,270			$	80.52			—			$	—
Granted	234,282			$	167.49			24,720			$	216.2
Vested	(586,842	)		$	68.34			—			$	—
Canceled	(58,917	)		$	117.7			(2,160	)		$	216.2
Unvested restricted stock awards at December 31, 2014	556,793			$	126.01			22,560			$	216.2
Restricted Stock Units
The following table presents unvested restricted stock units activity for the year ended December 31, 2014:
	Number of			Weighted-Average
	Shares			Grant Date
				Fair Value per Share
Unvested restricted stock units at December 31, 2013	—			$	—
Granted	887			$	169.16
Vested	—			$	—
Canceled	—			$	—
Unvested restricted stock units at December 31, 2014	887			$	169.16
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 2012, the Company matched 50% of employee contributions up to a maximum of 6% of total compensation. In 2013 and 2014, 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, 2012, 2013 and 2014 were approximately $2.7 million, $5.1 million and $6.1 million, respectively. The Company had no administrative expenses in connection with the 401(k) plan for the years ended December 31, 2012, 2013 and 2014, 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 2012, the Company matched 50% of employee contributions up to a maximum of 6% of total compensation. In 2013 and 2014, 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, 2012, 2013 and 2014 were approximately $180,000, $280,000 and $390,000, respectively.
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. There were 34,895 and 21,774 shares available for purchase under the ESPP as of December 31, 2013 and 2014, respectively and approximately 11,291 and 13,121 shares of the Company’s common stock were purchased under the ESPP during 2013 and 2014, respectively.

Schedule_II_Valuation_and_Qual

Schedule II Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2014
Valuation and Qualifying Accounts [Abstract]
Schedule II - Valuation and Qualifying Accounts	Schedule II – Valuation and Qualifying Accounts
	Years Ended December 31, 2012, 2013, and 2014 (in thousands):
	Allowance for Doubtful Accounts and Billing Adjustments (1)		Balance at		Charged to		Charged to		Write-offs,		Balance at
			Beginning		Expense		Other		Net of		End of Year
			of Year				Accounts (2)		Recoveries
	Year ended December 31, 2012		$	2,524	$	1,456	$	475	$	1,520	$	2,935
	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
	(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_ACCOUNT1

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2014
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. Actual results could differ from those estimates.
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. Subscription contract rates are based on the number of sites, number of users, organization size, the client’s business focus, geography and the number and types of services to which a client subscribes. A majority of the subscription-based license agreements typically 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 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, 2012, 2013 and 2014.
Accumulated Other Comprehensive Loss	Accumulated Other Comprehensive Loss
	The components of accumulated other comprehensive loss were as follows (in thousands):
		Year Ended December 31,
		2013			2014
	Foreign currency translation adjustment, net of tax	$	(4,003	)	$	(5,693	)
	Accumulated net unrealized loss on investments, net of tax	(1,527		)	(691		)
	Total accumulated other comprehensive loss	$	(5,530	)	$	(6,384	)
Advertising Costs	Advertising Costs
	The Company expenses advertising costs as incurred. Advertising costs, which includes e-commerce, television, radio, print and other media advertising, were approximately $6.2 million, $7.9 million and $28.7 million for the years ended December 31, 2012, 2013 and 2014, respectively.
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 differences are expected 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 Per Share	Net Income Per Share
	Net income per share is computed by dividing net income 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 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. As of December 31, 2013 and 2014, cash of approximately $105,000 and $0, respectively, was held to support letters of credit for security deposits.
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, 2012, 2013 and 2014. 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.
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 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. The impairment loss is measured based on a projected discounted cash flow method using a discount rate determined by the Company’s management to be commensurate with the risk in its current business model.
	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 the quantitative impairment test similar to the test performed on goodwill discussed above.
	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, and are reviewed at least annually for impairment.
	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 one 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 two 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. The acquired intangible asset characterized as customer base consists of one distinct intangible asset composed of acquired customer contracts and the related customer relationships. 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 three 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.
Capitalized Product Development Costs	Capitalized Product Development Costs
	Product development costs are expensed as incurred until technological feasibility has been established, at which time such costs are capitalized. Costs are capitalized, to the extent that the capitalizable costs do not exceed the realizable value of such costs, until the product is available for general release to customers. The Company defines the establishment of technological feasibility as the completion of all planning, designing, coding and testing activities that are necessary to establish products that meet design specifications including functions, features and technical performance requirements. The Company’s capitalized product development costs had a total net book value of approximately $111,000 and $0 as of December 31, 2013 and 2014, respectively. These capitalized product development costs are included in intangible and other assets in the Company’s consolidated balance sheets. Amortization is computed using a straight-line method over the remaining estimated economic life of the product, typically three to five years after the software is ready for its intended use. The Company amortized capitalized product development costs of approximately $191,000, $191,000 and $111,000 for the years ended December 31, 2012, 2013 and 2014, respectively.
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. 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 of approximately $6.5 million and $13.2 million as of December 31, 2013 and 2014, 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 $2.0 million, $3.0 million and $3.3 million for the years ended December 31, 2012, 2013 and 2014, respectively.
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 will 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 is effective on a retrospective basis for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. Early application is not permitted. This guidance permits the use of either a full retrospective method or a modified retrospective approach in which it would be applied only to the most current period presented along with a cumulative-effect adjustment at the date of adoption. The Company has not yet selected a transition method and is currently evaluating the impact this guidance will have on its financial statements.

ACQUISITION_Policies

ACQUISITION (Policies)	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
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.

INVESTMENTS_Policies

INVESTMENTS (Policies)	12 Months Ended
	Dec. 31, 2014
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, 2014
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_PER_SHARE_Policies

NET INCOME PER SHARE (Policies)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Net Income Per Share	Net Income Per Share
	Net income per share is computed by dividing net income 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 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, 2014
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_ACCOUNT2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Schedule of accumulated other comprehensive loss	The components of accumulated other comprehensive loss were as follows (in thousands):
		Year Ended December 31,
		2013			2014
	Foreign currency translation adjustment, net of tax	$	(4,003	)	$	(5,693	)
	Accumulated net unrealized loss on investments, net of tax	(1,527		)	(691		)
	Total accumulated other comprehensive loss	$	(5,530	)	$	(6,384	)
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,
		2012			2013			2014
	Cost of revenues	$	2,556		$	4,553		$	4,759
	Selling and marketing	1,966			4,954			3,776
	Software development	2,241			7,244			5,095
	General and administrative	5,519			24,798			14,637
	Total stock-based compensation	$	12,282		$	41,549		$	28,267
Property and equipment estimated useful lives	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 years
	Computer hardware and software		Two to five years

ACQUISITION_Tables

ACQUISITION (Tables)	12 Months Ended
	Dec. 31, 2014
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
Schedule of pro forma information	The following unaudited pro forma amounts present consolidated information as if the acquisition had been completed as of January 1, 2013 (in thousands except per share data):
		Year Ended December 31,
		2013			2014
	Revenue	$	526,811		$	598,340
	Net income	$	14,432		$	51,649
	Net income per share — basic	$	0.52		$	1.71
	Net income per share — diluted	$	0.51		$	1.69

INVESTMENTS_Tables

INVESTMENTS (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014 are as follows (in thousands):
	Maturity		Fair Value
	Due in:
	2015		$	—
	2016 — 2019		822
	2020 — 2024		—
	2025 and thereafter		16,329
	Available-for-sale investments		$	17,151
Schedule of available for sale securities reconciliation	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			Gross			Gross			Fair Value
		Cost			Unrealized			Unrealized
					Gains			Losses
	Auction rate securities	$	17,842		$	380		$	(1,071	)	$	17,151
	Available-for-sale investments	$	17,842		$	380		$	(1,071	)	$	17,151
	As of December 31, 2013, the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands):
		Amortized			Gross			Gross			Fair Value
		Cost			Unrealized			Unrealized
					Gains			Losses
	Auction rate securities	$	23,517		$	411		$	(1,938	)	$	21,990
	Available-for-sale investments	$	23,517		$	411		$	(1,938	)	$	21,990
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,
		2013						2014
		Aggregate			Gross			Aggregate			Gross
		Fair			Unrealized			Fair			Unrealized
		Value			Losses			Value			Losses
	Auction rate securities	$	21,137		$	(1,938	)	$	16,329		$	(1,071	)
	Investments in an unrealized loss position	$	21,137		$	(1,938	)	$	16,329		$	(1,071	)

FAIR_VALUE_Tables

FAIR VALUE (Tables)	12 Months Ended
	Dec. 31, 2014
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, 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 following table represents the Company's fair value hierarchy for its financial assets (cash, cash equivalents and investments) and liabilities measured at fair value on a recurring basis as of December 31, 2013 (in thousands):
		Level 1			Level 2		Level 3		Total
	Assets:
	Cash	$	134,989		$	—	$	—	$	134,989
	Money market funds	50,593			—		—		50,593
	Commercial paper	70,371			—		—		70,371
	Auction rate securities	—			—		21,990		21,990
	Total assets measured at fair value	$	255,953		$	—	$	21,990	$	277,943
	Liabilities:
	Deferred consideration	$	—		$	—	$	1,344	$	1,344
	Total liabilities measured at fair value	$	—		$	—	$	1,344	$	1,344
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, 2008 to December 31, 2014 (in thousands):
		Auction
		Rate
		Securities
	Balance at December 31, 2008	$	29,340
	Decrease in unrealized loss included in accumulated other comprehensive loss	684
	Settlements	(300		)
	Balance at December 31, 2009	29,724
	Decrease in unrealized loss included in accumulated other comprehensive loss	40
	Settlements	(575		)
	Balance at December 31, 2010	29,189
	Decrease in unrealized loss included in accumulated other comprehensive loss	245
	Settlements	(4,850		)
	Balance at December 31, 2011	24,584
	Auction rate securities upon acquisition	442
	Decrease in unrealized loss included in accumulated other comprehensive loss	836
	Settlements	(4,200		)
	Balance at December 31, 2012	21,662
	Decrease in unrealized loss included in accumulated other comprehensive loss	378
	Settlements	(50		)
	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
Summary of changes in the fair value of the company's level 3 liabilities	The following table summarizes changes in fair value of the Company’s Level 3 liabilities from December 31, 2012 to December 31, 2014 (in thousands):
		Deferred
		Consideration
	Balance at December 31, 2012	$	2,304
	Accretion for 2013	384
	Payments made in 2013	(1,344		)
	Balance at December 31, 2013	1,344
	Payments made in 2014	(1,344		)
	Balance at December 31, 2014	$	—

PROPERTY_AND_EQUIPMENT_Tables

PROPERTY AND EQUIPMENT (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Schedule of property, plant and equipment	Property and equipment consists of the following (in thousands):
		December 31,
		2013			2014
	Leasehold improvements	$	36,933		$	42,026
	Furniture, office equipment and research vehicles	27,395			31,016
	Computer hardware and software	36,391			49,655
	Property and equipment, gross	100,719			122,697
	Accumulated depreciation and amortization	(43,000		)	(48,944		)
	Property and equipment, net	$	57,719		$	73,753

GOODWILL_Tables

GOODWILL (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2012	$	692,639	$	25,439		$	718,078
	Effect of foreign currency translation	—		509			509
	Goodwill, December 31, 2013	692,639		25,948			718,587
	Acquisitions	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

INTANGIBLE_ASSETS_Tables

INTANGIBLE ASSETS (Tables)	12 Months Ended
	Dec. 31, 2014
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
		2013			2014			(in years)
	Capitalized product development cost	$	2,140		$	2,140		4
	Accumulated amortization	(2,029		)	(2,140		)
	Capitalized product development cost, net	111			—
	Building photography	13,743			14,943			5
	Accumulated amortization	(12,005		)	(12,665		)
	Building photography, net	1,738			2,278
	Acquired database technology	77,368			88,739			4
	Accumulated amortization	(41,073		)	(60,498		)
	Acquired database technology, net	36,295			28,241
	Acquired customer base	130,960			199,826			10
	Accumulated amortization	(74,734		)	(102,443		)
	Acquired customer base, net	56,226			97,383
	Acquired trade names and other intangible assets(1)	59,336			128,171			13
	Accumulated amortization	(9,234		)	(14,451		)
	Acquired trade names and other intangible assets, net	50,102			113,720
	Intangible assets, net	$	144,472		$	241,622
	(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.

LONGTERM_DEBT_Tables

LONG-TERM DEBT (Tables)	12 Months Ended
	Dec. 31, 2014
Long-term Debt, Current and Noncurrent [Abstract]
Schedule of maturities of long-term debt	Maturities of the Company's borrowings under the Credit Agreement for each of the next five years as of December 31, 2014 are as follows (in thousands):
	Year ending December 31,	Maturities
	Due in:
	2015	$	20,000
	2016	20,000
	2017	35,000
	2018	55,000
	2019	255,000
	Long-term debt, including current maturities	$	385,000

INCOME_TAXES_Tables

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Schedule of components of income tax expense (benefit)	The components of the provision (benefit) for income taxes attributable to operations consist of the following (in thousands):
		Year Ended December 31,
		2012			2013			2014
	Current:
	Federal	$	(2,260	)	$	26,516		$	24,741
	State	1,974			3,996			2,761
	Foreign	55			31			53
	Total current	(231		)	30,543			27,555
	Deferred:
	Federal	15,512			(10,919		)	(698		)
	State	(2,067		)	(1,849		)	(813		)
	Foreign	5			28			—
	Total deferred	13,450			(12,740		)	(1,511		)
	Total provision for income taxes	$	13,219		$	17,803		$	26,044
Schedule of deferred tax assets and liabilities	The components of deferred tax assets and liabilities consists of the following (in thousands):
		December 31,
		2013			2014
	Deferred tax assets:
	Reserve for bad debts	$	1,274		$	1,825
	Accrued compensation	6,725			7,287
	Stock compensation	13,381			8,758
	Net operating losses	17,457			15,665
	Accrued reserve and other	4,284			3,360
	Unrealized loss on securities	786			491
	Deferred rent	4,329			5,902
	Deferred revenue	1,538			1,879
	Deferred gain on the sale of building	11,499			10,690
	Total deferred tax assets	61,273			55,857
	Deferred tax liabilities:
	Prepaids	(1,096		)	(1,258		)
	Depreciation	(6,033		)	(9,806		)
	Intangibles	(55,284		)	(47,720		)
	Total deferred tax liabilities	(62,413		)	(58,784		)
	Net deferred tax liabilities, prior to valuation allowance	(1,140		)	(2,927		)
	Valuation allowance	(10,936		)	(7,783		)
	Net deferred tax liabilities	$	(12,076	)	$	(10,710	)
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,
		2012			2013			2014
	Expected federal income tax provision at statutory rate	$	8,097		$	16,638		$	24,820
	State income taxes, net of federal benefit	(1,360		)	885			1,965
	Foreign income taxes, net effect	(2,971		)	(724		)	336
	Stock compensation	(313		)	(116		)	35
	Increase (decrease) in valuation allowance	2,978			588			(2,397		)
	Nondeductible compensation	656			431			554
	Nondeductible transaction costs	5,829			—			—
	Other adjustments	303			101			731
	Income tax expense, net	$	13,219		$	17,803		$	26,044
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, 2011	$	3,347
	Increase for current year tax positions	792
	Decrease for prior year tax positions	(161		)
	Expiration of the statute of limitation for assessment of taxes	(69		)
	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

COMMITMENTS_AND_CONTINGENCIES_1

COMMITMENTS AND CONTINGENCIES (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Future minimum lease payments	Future minimum lease payments as of December 31, 2014 are as follows (in thousands):
	2015	$	19,442
	2016	18,783
	2017	18,607
	2018	17,680
	2019	15,907
	2020 and thereafter	75,878
	Total future minimum lease payments	$	166,297

SEGMENT_REPORTING_Tables

SEGMENT REPORTING (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Summarized information by operating segment	Summarized information by operating segment consists of the following (in thousands):
		Year Ended December 31,
		2012			2013			2014
	Revenues
	North America	$	330,805		$	420,817		$	552,141
	International
	External customers	19,131			20,126			23,795
	Intersegment revenue	1,514			339			57
	Total International revenue	20,645			20,465			23,852
	Intersegment eliminations	(1,514		)	(339		)	(57		)
	Total revenues	$	349,936		$	440,943		$	575,936
	EBITDA
	North America	$	70,199		$	97,348		$	148,913
	International	(10,007		)	(3,136		)	2,337
	Total EBITDA	$	60,192		$	94,212		$	151,250
Reconciliation of EBITDA to net income	The reconciliation of EBITDA to net income consists of the following (in thousands):
		Year Ended December 31,
		2012			2013			2014
	EBITDA	$	60,192		$	94,212		$	151,250
	Purchase amortization in cost of revenues	(8,634		)	(11,883		)	(26,290		)
	Purchase amortization in operating expenses	(13,607		)	(15,183		)	(28,432		)
	Depreciation and other amortization	(10,511		)	(12,992		)	(15,650		)
	Interest income	526			326			516
	Interest expense	(4,832		)	(6,943		)	(10,481		)
	Income tax expense, net	(13,219		)	(17,803		)	(26,044		)
	Net income	$	9,915		$	29,734		$	44,869
Summarized information by operating segment, assets and liabilities	Summarized information by operating segment consists of the following (in thousands):
		December 31,
		2013			2014
	Property and equipment, net
	North America	$	53,733		$	71,209
	International	3,986			2,544
	Total property and equipment, net	$	57,719		$	73,753
	Goodwill
	North America	$	692,639		$	1,114,363
	International	25,948			24,442
	Total goodwill	$	718,587		$	1,138,805
	Assets
	North America	$	1,311,292		$	2,138,768
	International	43,464			41,896
	Total operating segment assets	$	1,354,756		$	2,180,664
	Reconciliation of operating segment assets to total assets
	Total operating segment assets	$	1,354,756		$	2,180,664
	Investment in subsidiaries	(18,344		)	(18,344		)
	Intersegment receivables	(79,430		)	(78,638		)
	Total assets	$	1,256,982		$	2,083,682
	Liabilities
	North America	$	324,626		$	564,832
	International	79,266			75,584
	Total operating segment liabilities	$	403,892		$	640,416
	Reconciliation of operating segment liabilities to total liabilities
	Total operating segment liabilities	$	403,892		$	640,416
	Intersegment payables	(74,772		)	(70,280		)
	Total liabilities	$	329,120		$	570,136

NET_INCOME_PER_SHARE_Tables

NET INCOME PER SHARE (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Calculation of basic and diluted net income per share	The following table sets forth the calculation of basic and diluted net income per share (in thousands except per share data):
		Year Ended December 31,
		2012			2013			2014
	Numerator:
	Net income	$	9,915		$	29,734		$	44,869
	Denominator:
	Denominator for basic net income per share — weighted-average outstanding shares	26,533			27,670			30,215
	Effect of dilutive securities:
	Stock options and restricted stock	416			542			426
	Denominator for diluted net income per share — weighted-average outstanding shares	26,949			28,212			30,641
	Net income per share — basic	$	0.37		$	1.07		$	1.48
	Net income per share — diluted	$	0.37		$	1.05		$	1.46
Schedule of anti-dilutive securities excluded from computation of earnings per share	The following table summarizes the potential common shares excluded from the diluted calculation (in thousands):
		Year Ended December 31,
		2012		2013			2014
	Employee stock options	—		—			80
	Performance-based restricted stock awards	399		379			23
	Service-based restricted stock units	—		—			1
	Total shares excluded from computation	399		379			104

EMPLOYEE_BENEFIT_PLANS_Tables

EMPLOYEE BENEFIT PLANS (Tables)	12 Months Ended
	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of option activity	Option activity was as follows:
		Number of			Range of		Weighted-				Weighted-		Aggregate
		Shares			Exercise Price		Average				Average		Intrinsic
							Exercise				Remaining		Value
							Price				Contract		(in thousands)
											Life (in years)
	Outstanding at December 31, 2011	847,102			$17.34 - $60.23		$	39.93
	Granted	102,000			$58.95 - $58.95		$	58.95
	Exercised	(274,842	)		$17.34 - $57.16		$	34.04
	Canceled or expired	(541	)		$54.51 - $54.51		$	54.51
	Outstanding at December 31, 2012	673,719			$25.00 - $60.23		$	45.2
	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			7.25		$	32,677
	Exercisable at December 31, 2012	432,196			$25.00 - $60.23		$	40.22
	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			6.09		$	23,221
Summarized information regarding options outstanding	The following table summarizes information regarding options outstanding at December 31, 2014:
			Options Outstanding									Options Exercisable
	Range of		Number of			Weighted-Average Remaining Contractual Life (in years)		Weighted-				Number of			Weighted-
	Exercise Price		Shares					Average				Shares			Average
								Exercise Price							Exercise Price
	$36.48 - $42.50		58,603			5.05		$	41.2			58,603			$	41.2
	$42.51 - $55.83		19,666			3.47		$	49.24			19,666			$	49.24
	$55.84 - $57.61		43,549			6.17		$	57.16			43,549			$	57.16
	$57.62 - $58.51		473			6.09		$	58.06			100			$	58.06
	$58.52 - $59.59		54,068			7.14		$	58.95			27,800			$	58.95
	$59.60 - $81.19		2,320			6.42		$	60.23			1,740			$	60.23
	$81.20 - $151.60		111,600			8.19		$	102.16			37,198			$	102.16
	$151.61 - $201.04		79,900			9.15		$	201.04			—			$	—
	$36.48 - $201.04		370,179			7.25		$	99.12			188,656			$	60.54
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, 2014:
		Restricted Stock Awards — without Market Condition							Restricted Stock Awards — with Market Condition
		Number of			Weighted-Average				Number of			Weighted-Average
		Shares			Grant Date				Shares			Grant Date
					Fair Value per Share							Fair Value per Share
	Unvested restricted stock awards at December 31, 2013	968,270			$	80.52			—			$	—
	Granted	234,282			$	167.49			24,720			$	216.2
	Vested	(586,842	)		$	68.34			—			$	—
	Canceled	(58,917	)		$	117.7			(2,160	)		$	216.2
	Unvested restricted stock awards at December 31, 2014	556,793			$	126.01			22,560			$	216.2
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 during the year ended December 31, 2014 were as follows:
		Year Ended December 31,
		2014
	Dividend yield	0		%
	Expected volatility	30		%
	Risk-free interest rate	0.6		%
	Expected life (in years)	3
	Weighted-average grant date fair value	$	216.2
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,
		2012			2013			2014
	Dividend yield	0	%		0	%		0	%
	Expected volatility	40	%		37	%		30	%
	Risk-free interest rate	0.9	%		0.9	%		1.5	%
	Expected life (in years)	5			5			5
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, 2014:
		Number of			Weighted-Average
		Shares			Grant Date
					Fair Value per Share
	Unvested restricted stock units at December 31, 2013	—			$	—
	Granted	887			$	169.16
	Vested	—			$	—
	Canceled	—			$	—
	Unvested restricted stock units at December 31, 2014	887			$	169.16

Schedule_II_Valuation_and_Qual1

Schedule II Valuation and Qualifying Accounts (Tables)	12 Months Ended
	Dec. 31, 2014
Valuation and Qualifying Accounts [Abstract]
Allowance for doubtful accounts and billing adjustments	Years Ended December 31, 2012, 2013, and 2014 (in thousands):
	Allowance for Doubtful Accounts and Billing Adjustments (1)		Balance at		Charged to		Charged to		Write-offs,		Balance at
			Beginning		Expense		Other		Net of		End of Year
			of Year				Accounts (2)		Recoveries
	Year ended December 31, 2012		$	2,524	$	1,456	$	475	$	1,520	$	2,935
	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
	(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, 2014
	operating_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_ACCOUNT3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 01, 2014
Summary of Significant Accounting Policies [Line Items]
Term of subscription-based license agreements (in years)	1 year
Material gains or losses from foreign currency exchange transactions	$0	$0	$0
Reclassifications out of accumulated other comprehensive loss	0	0	0
Advertising costs	28,700,000	7,900,000	6,200,000
Maximum original maturity of highly liquid investments to be considered cash equivalents (in months)	3 months
Cash held to support letters of credit for security deposits	0	105,000
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%
Finite-lived intangible assets, net	241,622,000	144,472,000
Amortization expense	28,432,000	15,183,000	13,607,000
Capitalized debt issuance costs, net	13,199,000	6,542,000		10,000,000
Amortization of debt issuance costs	3,312,000	3,014,000	1,989,000
Leasehold Improvements [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)	Shorter of lease term or useful life
Furniture and Office Equipment [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Furniture and Office Equipment [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)	10 years
Vehicles [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Computer Hardware and Software [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)	2 years
Computer Hardware and Software [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Acquired Database Technology [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life (in years)	1 year
Acquired Database Technology [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life (in years)	8 years
Acquired Trade Names and Other Intangible Assets [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible assets, net	113,720,000	50,102,000
Acquired Trade Names and Other Intangible Assets [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life (in years)	2 years
Acquired Trade Names and Other Intangible Assets [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life (in years)	15 years
Acquired Customer Base [Member]
Summary of Significant Accounting Policies [Line Items]
Number of distinct intangible asset	1
Finite-lived intangible assets, net	97,383,000	56,226,000
Building Photography [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible assets, net	2,278,000	1,738,000
Building Photography [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life (in years)	3 years
Building Photography [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life (in years)	5 years
Capitalized Product Development Costs [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible assets, net	0	111,000
Amortization expense	$111,000	$191,000	$191,000
Capitalized Product Development Costs [Member] | Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life (in years)	3 years
Capitalized Product Development Costs [Member] | Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Finite-lived intangible asset, useful life (in years)	5 years

SUMMARY_OF_SIGNIFICANT_ACCOUNT4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), NET OF TAX (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Accumulated Other Comprehensive Income (Loss) Net of Tax [Abstract]
Foreign currency translation adjustment, net of tax	($5,693)	($4,003)
Accumulated net unrealized loss on investments, net of tax	-691	-1,527
Total accumulated other comprehensive loss	($6,384)	($5,530)

SUMMARY_OF_SIGNIFICANT_ACCOUNT5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, STOCK BASED COMPENSATION EXPENSE (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock-Based Compensation Expense [Abstract]
Compensation expense	$28,267,000	$41,549,000	$12,282,000
Performance service period	3 years
Net cash proceeds from the exercise of stock options and ESPP	5,740,000	18,133,000	9,868,000
Excess tax benefit from stock-based compensation	28,406,000	19,585,000	198,000
Cost of revenues
Stock-Based Compensation Expense [Abstract]
Compensation expense	4,759,000	4,553,000	2,556,000
Selling and marketing
Stock-Based Compensation Expense [Abstract]
Compensation expense	3,776,000	4,954,000	1,966,000
Software development
Stock-Based Compensation Expense [Abstract]
Compensation expense	5,095,000	7,244,000	2,241,000
General and administrative
Stock-Based Compensation Expense [Abstract]
Compensation expense	14,637,000	24,798,000	5,519,000
Performance-Based Restricted Common Stock [Member]
Stock-Based Compensation Expense [Abstract]
Cumulative EBITDA required for award shares to vest	90,000,000
Number of consecutive quarters to mantain cumulative EBITDA required for award shares to vest (in quarters)	4
Compensation expense	2,200,000	21,800,000	0
Performance-based RSAs - with Market Condition [Member]
Stock-Based Compensation Expense [Abstract]
Compensation expense	1,100,000	0	0
Unrecognized compensation cost expected to be recognized in future years	$2,700,000

ACQUISITION_Details

ACQUISITION (Details) (USD $)	0 Months Ended	12 Months Ended
	Apr. 03, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 01, 2014
Business Acquisition [Line Items]
Payments to acquire businesses	$587,104,000
Closing purchase price adjustments	2,104,000
Post closing purchase price adjustments	2,886,000
Goodwill		1,138,805,000	718,587,000	718,078,000
Apartments.com [Member]
Business Acquisition [Line Items]
Goodwill					421,724,000
Goodwill tax deductible amount					421,724,000
Consolidated revenue from acquisition		76,800,000	0	0
Consolidated loss before income taxes from acquisition		-23,900,000	0	0
Effective income tax rate (in percent)	38.00%
Business combination, acquisition related costs		$1,400,000	$0	$5,200,000
Customer Contracts [Member]
Business Acquisition [Line Items]
Number of distinct intangible asset		1
Estimated useful life of acquired assets		10 years
Customer Contracts [Member] | Apartments.com [Member]
Business Acquisition [Line Items]
Number of distinct intangible asset					1
Estimated useful life of acquired assets	10 years
Acquired Database Technology [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		4 years
Acquired Database Technology [Member] | Apartments.com [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	1 year
Trade Names and Other Intangible Assets [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		13 years
Trade Names and Other Intangible Assets [Member] | Apartments.com [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	13 years
Building Photography [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		5 years
Building Photography [Member] | Apartments.com [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	3 years

ACQUISITION_Details_Schedule_o

ACQUISITION (Details - Schedule of Assets Acquired and Liabilities Assumed) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 01, 2014
In Thousands, unless otherwise specified
Business Acquisition [Line Items]
Goodwill	$1,138,805	$718,587	$718,078
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] | Trade Names and Other Intangible Assets [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill				71,779
Apartments.com [Member] | Customer Contracts [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] | Building Photography [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill				$1,006

ACQUISITION_ACQUISITION_Detail

ACQUISITION ACQUISITION (Details - Schedule of Supplemental Pro Forma Information) (Apartments.com [Member], USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Apartments.com [Member]
Business Acquisition [Line Items]
Revenue	$598,340	$526,811
Net income	$51,649	$14,432
Net income per share - basic (in dollars per share)	$1.71	$0.52
Net income per share - diluted (in dollars per share)	$1.69	$0.51

INVESTMENTS_Details

INVESTMENTS (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Debt Maturities Fair Value [Abstract]
2015	$0
2016 — 2019	822
2020 — 2024	0
2025 and thereafter	16,329
Available-for-sale investments	$17,151	$21,990

INVESTMENTS_AVAILABLEFORSALE_S

INVESTMENTS, AVAILABLE-FOR-SALE SECURITIES (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, gross realized gains	$0	$0	$0
Available-for-sale securities, gross realized losses	0	0	0
Available-for-sale securities, continuous unrealized loss position, less than 12 months, Accumulated Loss	0	0
Available-for-sale Securities Reconciliation [Abstract]
Amortized cost	17,842,000	23,517,000
Gross unrealized gains	380,000	411,000
Gross unrealized losses	-1,071,000	-1,938,000
Available-for-sale investments	17,151,000	21,990,000
Available-for-sale securities, unrealized loss positions
Aggregate Fair Value	16,329,000	21,137,000
Auction Rate Securities [Member]
Available-for-sale Securities Reconciliation [Abstract]
Amortized cost	17,842,000	23,517,000
Gross unrealized gains	380,000	411,000
Gross unrealized losses	-1,071,000	-1,938,000
Available-for-sale investments	17,151,000	21,990,000
Available-for-sale securities, unrealized loss positions
Aggregate Fair Value	$16,329,000	$21,137,000

FAIR_VALUE_Details

FAIR VALUE (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Mar. 31, 2014	Mar. 28, 2013
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	($691,000)	($1,527,000)
Unobservable inputs liabilities (level 3) [Roll forward]
Beginning balance	1,344,000	2,304,000
Accretion		384,000
Payments	-1,344,000	-1,344,000
Ending balance	0	1,344,000
Fair Value, Measurements, Recurring [Member]
Assets:
Total assets measured at fair value	544,163,000	277,943,000
Liabilities:
Deferred consideration		1,344,000
Fair Value, Measurements, Recurring [Member] | Cash [Member]
Assets:
Total assets measured at fair value	160,275,000	134,989,000
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	310,482,000	50,593,000
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Assets:
Total assets measured at fair value	56,255,000	70,371,000
Fair Value, Measurements, Recurring [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	17,151,000	21,990,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Total assets measured at fair value	527,012,000	255,953,000
Liabilities:
Deferred consideration		0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Cash [Member]
Assets:
Total assets measured at fair value	160,275,000	134,989,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	310,482,000	50,593,000
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Assets:
Total assets measured at fair value	56,255,000	70,371,000
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
Liabilities:
Deferred consideration		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	17,151,000	21,990,000
Liabilities:
Deferred consideration	0	1,344,000
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	17,151,000	21,990,000
Auction Rate Securities [Member]
Unobservable inputs assets (level 3) [Roll forward]
Beginning balance	21,990,000	21,662,000	24,584,000	29,189,000	29,724,000	29,340,000
Auction rate securities upon acquisition			442,000
Change in unrealized gain (loss) included in accumulated other comprehensive loss	836,000	378,000	836,000	245,000	40,000	684,000
Settlements	-5,675,000	-50,000	-4,200,000	-4,850,000	-575,000	-300,000
Ending balance	17,151,000	21,990,000	21,662,000	24,584,000	29,189,000	29,724,000
Par value of company held auction rate securities	18,700,000
Discount rate (in percent)	4.10%	4.90%
Temporary impairment of the auction rates security investments	-691,000
LandsofAmerica [Member]
Liabilities:
Business acquisition deferred consideration potential cash payment (undiscounted)		1,000,000
Business acquisition deferred consideration cash payment							1,000,000	1,000,000
CoStar Private Sale Network [Member]
Liabilities:
Business acquisition deferred consideration potential cash payment (undiscounted)		344,000
Business acquisition deferred consideration cash payment							$344,000	$344,000

PROPERTY_AND_EQUIPMENT_Details

PROPERTY AND EQUIPMENT (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$122,697	$100,719
Accumulated depreciation and amortization	-48,944	-43,000
Property and equipment, net	73,753	57,719
Depreciation expense for property and equipment	15,111	12,495	10,053
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	42,026	36,933
Furniture, Office Equipment and Research Vehicles [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	31,016	27,395
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$49,655	$36,391

GOODWILL_Details

GOODWILL (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Goodwill [Roll Forward]
Goodwill, beginning balance	$718,587	$718,078
Acquisitions	421,724
Effect of foreign currency translation	-1,506	509
Goodwill, ending balance	1,138,805	718,587
LoopNet [Member]
Goodwill [Line Items]
Goodwill, period increase (decrease)	421,724
International [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	25,948	25,439
Acquisitions	0
Effect of foreign currency translation	-1,506	509
Goodwill, ending balance	24,442	25,948
North America [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	692,639	692,639
Acquisitions	421,724
Effect of foreign currency translation	0	0
Goodwill, ending balance	$1,114,363	$692,639

INTANGIBLE_ASSETS_Details

INTANGIBLE ASSETS (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 30, 2012
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	$241,622,000	$144,472,000
Amortization of intangible assets	55,261,000	27,563,000	22,699,000
Impairment of intangible assets, finite-lived	1,799,000	0	0
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Amortization expense for 2015	48,500,000
Amortization expense for 2016	36,200,000
Amortization expense for 2017	22,400,000
Amortization expense for 2018	14,900,000
Amortization expense for 2019	12,900,000
Capitalized Product Development Costs [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	2,140,000	2,140,000
Finite-lived intangible assets, accumulated amortization	-2,140,000	-2,029,000
Finite-lived intangible assets, net	0	111,000
Weighted-average amortization period (in years}	4 years
Building Photography [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	14,943,000	13,743,000
Finite-lived intangible assets, accumulated amortization	-12,665,000	-12,005,000
Finite-lived intangible assets, net	2,278,000	1,738,000
Weighted-average amortization period (in years}	5 years
Acquired Database Technology [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	88,739,000	77,368,000
Finite-lived intangible assets, accumulated amortization	-60,498,000	-41,073,000
Finite-lived intangible assets, net	28,241,000	36,295,000
Weighted-average amortization period (in years}	4 years
Acquired Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	199,826,000	130,960,000
Finite-lived intangible assets, accumulated amortization	-102,443,000	-74,734,000
Finite-lived intangible assets, net	97,383,000	56,226,000
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	128,171,000	59,336,000
Finite-lived intangible assets, accumulated amortization	-14,451,000	-9,234,000
Finite-lived intangible assets, net	113,720,000	50,102,000
Weighted-average amortization period (in years}	13 years
LoopNet [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived acquired trade names and other intangible assets				$48,700,000

LONGTERM_DEBT_Details

LONG-TERM DEBT (Details) (USD $)	0 Months Ended	3 Months Ended	1 Months Ended
	Apr. 03, 2014	Apr. 04, 2014	Jun. 30, 2014	Dec. 31, 2014	Apr. 01, 2014	Dec. 31, 2013
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				13,199,000	10,000,000	6,542,000
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	150,000,000	0
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%

LONGTERM_DEBT_LONGTERM_DEBT_De

LONG-TERM DEBT LONG-TERM DEBT (Details - Covenant Calculations) (Details)	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%

LONGTERM_DEBT_LONGTERM_DEBT_De1

LONG-TERM DEBT LONG-TERM DEBT (Details - Interest Expense and Amortization of Debt Issuance Costs) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 01, 2014
Debt Instrument [Line Items]
Interest expense, debt	$10,481,000	$6,943,000	$4,832,000
Amortization of debt issuance costs	3,312,000	3,014,000	1,989,000
Interest paid	7,007,000	4,291,000	2,480,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	$0	$0		$150,000,000

LONGTERM_DEBT_Schedule_of_Matu

LONG-TERM DEBT (Schedule of Maturities of Long-Term Debt) (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Long-term Debt, Current and Noncurrent [Abstract]
2015	$20,000
2016	20,000
2017	35,000
2018	55,000
2019	255,000
Long-term debt, including current maturities	$385,000

INCOME_TAXES_Details

INCOME TAXES (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current:
Federal	$24,741,000	$26,516,000	($2,260,000)
State	2,761,000	3,996,000	1,974,000
Foreign	53,000	31,000	55,000
Total current	27,555,000	30,543,000	-231,000
Deferred:
Federal	-698,000	-10,919,000	15,512,000
State	-813,000	-1,849,000	-2,067,000
Foreign	0	28,000	5,000
Total deferred	-1,511,000	-12,740,000	13,450,000
Total provision for income taxes	26,044,000	17,803,000	13,219,000
Deferred tax assets:
Reserve for bad debts	1,825,000	1,274,000
Accrued compensation	7,287,000	6,725,000
Stock compensation	8,758,000	13,381,000
Net operating losses	15,665,000	17,457,000
Accrued reserve and other	3,360,000	4,284,000
Unrealized loss on securities	491,000	786,000
Deferred rent	5,902,000	4,329,000
Deferred revenue	1,879,000	1,538,000
Deferred gain on the sale of building	10,690,000	11,499,000
Total deferred tax assets	55,857,000	61,273,000
Deferred tax liabilities:
Prepaids	-1,258,000	-1,096,000
Depreciation	-9,806,000	-6,033,000
Intangibles	-47,720,000	-55,284,000
Total deferred tax liabilities	-58,784,000	-62,413,000
Net deferred tax liabilities, prior to valuation allowance	-2,927,000	-1,140,000
Valuation allowance	-7,783,000	-10,936,000
Net deferred tax liabilities	-10,710,000	-12,076,000
Income from U.S. sources	70,600,000	53,200,000	36,100,000
Income (loss) from foreign sources	273,000	-5,600,000	-13,000,000
Effective tax rate reconciliation [Abstract]
Expected federal income tax provision at statutory rate	24,820,000	16,638,000	8,097,000
State income taxes, net of federal benefit	1,965,000	885,000	-1,360,000
Foreign income taxes, net effect	336,000	-724,000	-2,971,000
Stock compensation	35,000	-116,000	-313,000
Increase (decrease) in valuation allowance	-2,397,000	588,000	2,978,000
Nondeductible compensation	554,000	431,000	656,000
Nondeductible transaction costs	0	0	5,829,000
Other adjustments	731,000	101,000	303,000
Total provision for income taxes	26,044,000	17,803,000	13,219,000
Income taxes paid	3,000,000	6,500,000	2,600,000
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	3,153,000	-446,000
Cash tax benefits resulting in net operating loss carryforward	1,200,000	4,200,000
Tax adjustment from IRS audit	0
Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits beginning balance	5,957,000	3,909,000	3,347,000
Increase for current year tax positions	51,000	66,000	792,000
Decrease for prior period tax positions	-189,000		-161,000
Increase for prior year tax positions		2,037,000
Expiration of the statute of limitation for assessment of taxes	-70,000	-55,000	-69,000
Unrecognized tax benefits ending balance	5,749,000	5,957,000	3,909,000
Unrecognized tax benefits that would favorably affect the annual effective tax rate if recognized in future periods	1,500,000	1,600,000
Interest and penalties on income taxes recognized	62,000	62,000	58,000
Interest and penalties accrued on income taxes	466,000	404,000	342,000
Foreign Country [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	835,000
Net operating loss carryforward	27,900,000
Domestic Country [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	11,000,000
Operating loss carryforwards, first expiration date	31-Dec-20
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	2,318,000
Net operating loss carryforward	4,400,000
Operating loss carryforwards, first expiration date	31-Dec-20
Income tax credit carryforward	1,900,000
LoopNet [Member]
Operating Loss Carryforwards [Line Items]
Tax adjustment from IRS audit	$0

COMMITMENTS_AND_CONTINGENCIES_2

COMMITMENTS AND CONTINGENCIES (Details) (USD $)	1 Months Ended	12 Months Ended			0 Months Ended
	31-May-11	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 03, 2014	Apr. 01, 2014
	class_action_lawsuits	current_litigation_matters
Operating Leases Rent Expense [Abstract]
Rent expense for operating leases		$19,200,000	$18,300,000	$16,700,000
Future Minimum Lease Payments [Abstract]
2015		19,442,000
2016		18,783,000
2017		18,607,000
2018		17,680,000
2019		15,907,000
2020 and thereafter		75,878,000
Total future minimum lease payments		166,297,000
Business Acquisition [Line Items]
Number of class action lawsuits brought by alleged LoopNet stockholders	3
Number of minimum of current litigation matters (in claims)		1
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
LoopNet Stockholders [Member]
Business Acquisition [Line Items]
Loss contingency accrual		200,000
Loss contingency, estimated damages sought		300,000
Civix [Member]
Business Acquisition [Line Items]
Loss contingency accrual		2,900,000
Loss contingency, estimated damages sought		$3,700,000

SEGMENT_REPORTING_Details

SEGMENT REPORTING (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	operating_segments
Segment Reporting Information [Line Items]
Number of business segments (in segments)	2
Summarized information by operating segment [Abstract]
Revenues	$575,936,000	$440,943,000	$349,936,000
Property and equipment, net	73,753,000	57,719,000
Goodwill	1,138,805,000	718,587,000	718,078,000
Reconciliation of EBITDA to net income [Abstract]
EBITDA	151,250,000	94,212,000	60,192,000
Purchase amortization in cost of revenues	-26,290,000	-11,883,000	-8,634,000
Purchase amortization in operating expenses	-28,432,000	-15,183,000	-13,607,000
Depreciation and other amortization	-15,650,000	-12,992,000	-10,511,000
Interest income	516,000	326,000	526,000
Interest expense	-10,481,000	-6,943,000	-4,832,000
Income tax expense, net	-26,044,000	-17,803,000	-13,219,000
Net income	44,869,000	29,734,000	9,915,000
Reconciliation of operating segment assets to total assets [Abstract]
Total operating segment assets	2,180,664,000	1,354,756,000
Investment in subsidiaries	-18,344,000	-18,344,000
Intersegment receivables	-78,638,000	-79,430,000
Total assets	2,083,682,000	1,256,982,000
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total operating segment liabilities	640,416,000	403,892,000
Intersegment payables	-70,280,000	-74,772,000
Total liabilities	570,136,000	329,120,000
North America [Member]
Segment Reporting Information [Line Items]
Corporate allocation North America	1,100,000	800,000	0
Summarized information by operating segment [Abstract]
Revenues	552,141,000	420,817,000	330,805,000
Property and equipment, net	71,209,000	53,733,000
Goodwill	1,114,363,000	692,639,000	692,639,000
Reconciliation of EBITDA to net income [Abstract]
EBITDA	148,913,000	97,348,000	70,199,000
Reconciliation of operating segment assets to total assets [Abstract]
Total operating segment assets	2,138,768,000	1,311,292,000
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total operating segment liabilities	564,832,000	324,626,000
International [Member]
Segment Reporting Information [Line Items]
Corporate allocation International	300,000	400,000	5,300,000
Summarized information by operating segment [Abstract]
Revenues	23,852,000	20,465,000	20,645,000
Property and equipment, net	2,544,000	3,986,000
Goodwill	24,442,000	25,948,000	25,439,000
Reconciliation of EBITDA to net income [Abstract]
EBITDA	2,337,000	-3,136,000	-10,007,000
Reconciliation of operating segment assets to total assets [Abstract]
Total operating segment assets	41,896,000	43,464,000
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total operating segment liabilities	75,584,000	79,266,000
External Customers [Member]
Summarized information by operating segment [Abstract]
Revenues	23,795,000	20,126,000	19,131,000
Intersegment Revenue [Member]
Summarized information by operating segment [Abstract]
Revenues	57,000	339,000	1,514,000
Intersegment Eliminator [Member]
Summarized information by operating segment [Abstract]
Revenues	($57,000)	($339,000)	($1,514,000)

STOCKHOLDERS_EQUITY_Details

STOCKHOLDERS' EQUITY (Details) (USD $)	1 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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	$529,360,000	$0	$0
Payments for underwriting expense	22,100,000
Payments of equity offering costs	$500,000

NET_INCOME_PER_SHARE_Details

NET INCOME PER SHARE (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net income	$44,869	$29,734	$9,915
Denominator:
Denominator for basic net income per share — weighted-average outstanding shares	30,215,000	27,670,000	26,533,000
Effect of dilutive securities:
Stock options and restricted stock (in shares)	426,000	542,000	416,000
Denominator for diluted net income per share — weighted-average outstanding shares	30,641,000	28,212,000	26,949,000
Net income per share — basic (in dollars per share)	$1.48	$1.07	$0.37
Net income per share — diluted (in dollars per share)	$1.46	$1.05	$0.37
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	104,000	379,000	399,000
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	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)	23,000	379,000	399,000
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	0	0

EMPLOYEE_BENEFIT_PLANS_Details

EMPLOYEE BENEFIT PLANS (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 05, 2012	Jun. 02, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$28,267,000	$41,549,000	$12,282,000
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%	37.00%	40.00%
Risk-free interest rate (in hundredths)	1.50%	0.90%	0.90%
Expected life (in years)	5 years	5 years	5 years
Employee 401(k) Plan [Member]
Employee Plans [Abstract]
Percentage of company match of employee contribution (in hundredths)	100.00%	100.00%	50.00%
Maximum percentage of employee total compensation matched by employer (in hundredths)	4.00%	4.00%	6.00%
Company match to employee contributions	6,100,000	5,100,000	2,700,000
Administrative expense	0	0	0
Employee Pension Plan, London Office [Member]
Employee Plans [Abstract]
Percentage of company match of employee contribution (in hundredths)			50.00%
Maximum percentage of employee total compensation matched by employer (in hundredths)	6.00%	6.00%	6.00%
Company match to employee contributions	390,000	280,000	180,000
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)	58,603
Weighted-average remaining contractual life (in years)	5 years 0 months 19 days
Weighted- average exercise price (in dollars per share)	$41.20
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)	$55.83
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	19,666
Weighted-average remaining contractual life (in years)	3 years 5 months 18 days
Weighted- average exercise price (in dollars per share)	$49.24
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)	$55.84
Range of exercise price, maximum, (in dollars per share)	$57.61
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	43,549
Weighted-average remaining contractual life (in years)	6 years 2 months 1 day
Weighted- average exercise price (in dollars per share)	$57.16
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)	$57.62
Range of exercise price, maximum, (in dollars per share)	$58.51
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	473
Weighted-average remaining contractual life (in years)	6 years 1 month 1 day
Weighted- average exercise price (in dollars per share)	$58.06
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)	$58.52
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)	54,068
Weighted-average remaining contractual life (in years)	7 years 1 month 20 days
Weighted- average exercise price (in dollars per share)	$58.95
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)	$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)	2,320
Weighted-average remaining contractual life (in years)	6 years 5 months 0 days
Weighted- average exercise price (in dollars per share)	$60.23
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)	$81.20
Range of exercise price, maximum, (in dollars per share)	$151.60
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	111,600
Weighted-average remaining contractual life (in years)	8 years 2 months 8 days
Weighted- average exercise price (in dollars per share)	$102.16
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)	$151.61
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)	79,900
Weighted-average remaining contractual life (in years)	9 years 1 month 26 days
Weighted- average exercise price (in dollars per share)	$201.04
Options Outstanding 9 [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)	370,179
Weighted-average remaining contractual life (in years)	7 years 3 months 0 days
Weighted- average exercise price (in dollars per share)	$99.12
Options Exercisable 1 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	58,603
Weighted-average exercise price (in dollars per share)	$41.20
Options Exercisable 2 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	19,666
Weighted-average exercise price (in dollars per share)	$49.24
Options Exercisable 3 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	43,549
Weighted-average exercise price (in dollars per share)	$57.16
Options Exercisable 4 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	100
Weighted-average exercise price (in dollars per share)	$58.06
Options Exercisable 5 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	27,800
Weighted-average exercise price (in dollars per share)	$58.95
Options Exercisable 6 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	1,740
Weighted-average exercise price (in dollars per share)	$60.23
Options Exercisable 7 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	37,198
Weighted-average exercise price (in dollars per share)	$102.16
Options Exercisable 8 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	0
Weighted-average exercise price (in dollars per share)	$0
Options Exercisable 9 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	188,656
Weighted-average exercise price (in dollars per share)	$60.54
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)	0
Shares of common stock authorized for issuance under the plan (in 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)	1,200,000	1,200,000
Increase in shares of common stock pursuant to amendment to the plan (in shares)					1,300,000
Increase in shares of common stock issued pursuant to stock plan (in shares)				900,000
Shares of common stock authorized for issuance under the plan (in shares)	3,200,000
Employee Stock Purchase Plan [Member]
Employee Plans [Abstract]
Percentage of purchase price of Company's common stock to the market price (in hundredths)	90.00%
Shares available for purchase (in shares)	21,774	34,895
Shares of Company's common stock purchased during the period (in shares)	13,121	11,291
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)	0
Granted (in shares)	887
Vested (in shares)	0
Canceled (in shares)	0
Unvested restricted stock at end of period (in shares)	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)	$0
Granted (in dollars per share)	$169.16
Vested (in dollars per share)	$0
Canceled (in dollars per share)	$0
Unvested restricted stock at end of period (in dollars per share)	$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)	374,340	673,719	847,102
Granted (in shares)	87,700	126,800	102,000
Exercised (in shares)	-68,126	-409,799	-274,842
Canceled or expired (in shares)	-23,735	-16,380	-541
Outstanding at end of period (in shares)	370,179	374,340	673,719
Exercisable at end of period (in shares)	188,656	146,161	432,196
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)	$68.94	$45.20	$39.93
Weighted-average exercise price, granted (in dollars per share)	$201.04	$102.16	$58.95
Weighted-average exercise price, exercised (in dollars per share)	$55.81	$41.05	$34.04
Weighted-average exercise price, canceled or expired (in dollars per share)	$124.09	$47.54	$54.51
Weighted-average exercise price, outstanding at end of period (in dollars per share)	$99.12	$68.94	$45.20
Weighted-average exercise price, exercisable at end of period (in dollars per share)	$60.54	$47.72	$40.22
Weighted-average remaining contract life of options outstanding at end of period	7 years 3 months 0 days
Weighted-average remaining contract life of options exercisable at end of period	6 years 1 month 3 days
Aggregate intrinsic value of options outstanding at end of period	32,677,000
Aggregate intrinsic value of options exercisable at end of period	23,221,000
Aggregate intrinsic value of options exercised	8,900,000	39,000,000	11,900,000
Unrecognized compensation cost expected to be recognized in future years	51,300,000
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)	2 years 5 months 24 days
Weighted-average grant date fair value of each option granted during the period (in dollars per share)	$58.12	$34.10	$20.99
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)	968,270
Granted (in shares)	234,282
Vested (in shares)	-586,842
Canceled (in shares)	-58,917
Unvested restricted stock at end of period (in shares)	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)	$80.52
Granted (in dollars per share)	$167.49
Vested (in dollars per share)	$68.34
Canceled (in dollars per share)	$117.70
Unvested restricted stock at end of period (in dollars per share)	$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	90,000,000
Number of consecutive quarters to mantain cumulative EBITDA required for award shares to vest (in quarters)	4
Compensation expense	2,200,000	21,800,000	0
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted (in shares)	0	0	399,413
Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	1,100,000	0	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Unrecognized compensation cost expected to be recognized in future years	$2,700,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (in hundredths)	0.00%
Expected volatility (in hundredths)	30.00%
Risk-free interest rate (in hundredths)	0.60%
Expected life (in 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)	0
Granted (in shares)	24,720
Vested (in shares)	0
Canceled (in shares)	-2,160
Unvested restricted stock at end of period (in shares)	22,560	0
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)	$0
Granted (in dollars per share)	$216.20
Vested (in dollars per share)	$0
Canceled (in dollars per share)	$216.20
Unvested restricted stock at end of period (in dollars per share)	$216.20	$0
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	$25	$17.34
Granted (in dollars per share)	$201.04	$102.16	$58.95
Exercised (in dollars per share)	$39	$25	$17.34
Canceled or expired (in dollars per share)	$58.95	$36.48	$54.51
Outstanding at end of period, (in dollars per share)	$36.48	$36.48	$25
Exercisable at end of period (in dollars per share)	$36.48	$36.48	$25
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)	$102.16	$60.23	$60.23
Granted (in dollars per share)	$201.04	$102.16	$58.95
Exercised (in dollars per share)	$102.16	$58.95	$57.16
Canceled or expired (in dollars per share)	$201.04	$58.95	$54.51
Outstanding at end of period, (in dollars per share)	$201.04	$102.16	$60.23
Exercisable at end of period (in dollars per share)	$102.16	$60.23	$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%

Schedule_II_Valuation_and_Qual2

Schedule II Valuation and Qualifying Accounts (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Valuation and Qualifying Accounts [Abstract]
Balance at beginning of year	$3,397	[1]	$2,935	[1]	$2,524	[1]
Charged to expense	4,822	[1]	2,317	[1]	1,456	[1]
Charged to other accounts	881	[1],[2]	0	[1],[2]	475	[1],[2]
Write-offs, net of recoveries	4,285	[1]	1,855	[1]	1,520	[1]
Balance at end of year	$4,815	[1]	$3,397	[1]	$2,935	[1]
[1]	Additions to the allowance for doubtful accounts are charged to bad debt expense.
[2]	Amounts represent opening balances from acquired businesses.