CoStar (CSGP) 10-K 28 Feb 19 2018 FY Annual report Financial data

DOCUMENT AND ENTITY INFORMATION

DOCUMENT AND ENTITY INFORMATION - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2018	Feb. 22, 2019	Jun. 29, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	COSTAR GROUP INC
Entity Central Index Key	1,057,352
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 15
Entity Common Stock, Shares Outstanding (in shares)		36,451,829
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenues	$ 1,191,832	$ 965,230	$ 837,630
Cost of Goods and Services Sold	269,933	220,403	173,814
Gross profit	921,899	744,827	663,816
Operating expenses:
Selling and marketing (excluding customer base amortization)	359,858	318,362	296,483
Software development	100,937	88,850	76,400
General and administrative	156,659	146,128	123,297
Customer base amortization	30,881	17,671	22,731
Total operating expenses	648,335	571,011	518,911
Income from operations	273,564	173,816	144,905
Interest and other income	13,281	4,044	1,773
Interest and other expense	(2,830)	(9,014)	(10,016)
Loss on debt extinguishment	0	(3,788)	0
Income before income taxes	284,015	165,058	136,662
Income tax expense	45,681	42,363	51,591
Net income	$ 238,334	$ 122,695	$ 85,071
Net income (loss) per share — basic (in dollars per share)	$ 6.61	$ 3.70	$ 2.64
Net income (loss) per share — diluted (in dollars per share)	$ 6.54	$ 3.66	$ 2.62
Weighted average outstanding shares — basic (in shares)	36,058	33,200	32,167
Weighted average outstanding shares — diluted (in shares)	36,448	33,559	32,436

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 238,334	$ 122,695	$ 85,071
Other comprehensive (loss) income, net of tax
Foreign currency translation adjustment	(2,668)	3,901	(5,032)
Net decrease in unrealized loss on investments	0	118	395
Reclassification adjustment for realized gains on investments included in net income	0	0	(808)
Total other comprehensive (loss) income	(2,668)	4,019	(5,445)
Total comprehensive income	$ 235,666	$ 126,714	$ 79,626

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 1,100,416	$ 1,211,463
Accounts receivable, less allowance of $5,709 and $6,469 as of December 31, 2018 and December 31, 2017, respectively	89,192	60,900
Prepaid expenses and other current assets	23,690	15,572
Total current assets	1,213,298	1,287,935
Long-term investments	10,070	10,070
Deferred income taxes, net	7,469	5,431
Property and equipment, net	83,303	84,496
Goodwill	1,611,535	1,283,457
Intangible assets, net	288,911	182,892
Deferred commission costs, net	76,031	0
Deposits and other assets	7,432	6,179
Income tax receivable	14,908	12,981
Total assets	3,312,957	2,873,441
Current liabilities:
Accounts payable	6,327	9,262
Accrued wages and commissions	45,588	54,104
Accrued expenses	29,821	22,193
Deferred gain on the sale of building	2,523	2,523
Income taxes payable	14,288	8,166
Deferred rent	4,153	4,732
Deferred revenue	51,459	45,686
Total current liabilities	154,159	146,666
Deferred gain on the sale of building	13,669	16,192
Deferred rent	31,944	33,909
Deferred income taxes, net	69,857	12,070
Income taxes payable	17,386	13,354
Other long-term liabilities	4,000	0
Total liabilities	291,015	222,191
Commitments and contingencies (Note 12)
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; 36,446 and 36,107 issued and outstanding as of December 31, 2018 and 2017, respectively	364	361
Additional paid-in capital	2,419,812	2,339,253
Accumulated other comprehensive loss	(11,688)	(9,020)
Retained earnings	613,454	320,656
Total stockholders’ equity	3,021,942	2,651,250
Total liabilities and stockholders’ equity	$ 3,312,957	$ 2,873,441

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts	$ 5,709	$ 6,469
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)	36,446,000	36,107,000
Common stock, shares outstanding (in shares)	36,446,000	36,107,000

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings
Balance (in shares) at Dec. 31, 2015		32,509,000
Balance at Dec. 31, 2015	$ 1,543,780	$ 325	$ 1,440,321	$ (7,594)	$ 110,728
Net income (loss)	85,071				85,071
Other comprehensive loss	(5,445)			(5,445)
Exercise of stock options (in shares)		29,000
Exercise of stock options	3,303	$ 0	3,303
Restricted stock grants (in shares)		195,000
Restricted stock grants	0	$ 2	(2)
Restricted stock grants surrendered (in shares)		(142,000)
Restricted stock grants surrendered	(16,424)	$ (1)	(16,423)
Stock compensation expense, net of forfeitures	36,388		36,388
Employee stock purchase plan (in shares)		15,000
Employee stock purchase plan	2,842	$ 0	2,842
Excess tax benefit from stock-based compensation	4,698		4,698
Balance (in shares) at Dec. 31, 2016		32,606,000
Balance at Dec. 31, 2016	1,654,213	$ 326	1,471,127	(13,039)	195,799
Net income (loss)	122,695				122,695
Other comprehensive loss	4,019			4,019
Exercise of stock options (in shares)		82,000
Exercise of stock options	6,797	$ 1	6,796
Restricted stock grants (in shares)		187,000
Restricted stock grants	0	$ 2
Restricted stock grants adjustments			(2)
Restricted stock grants surrendered (in shares)		(99,000)
Restricted stock grants surrendered	(14,902)	$ (1)	(14,901)
Stock compensation expense, net of forfeitures	38,921		38,921
Stock issued for equity offering (in shares)		3,317,000
Stock issued for equity offering	833,911	$ 33	833,878
Employee stock purchase plan (in shares)		14,000
Employee stock purchase plan	3,434	$ 0	3,434
Balance (in shares) at Dec. 31, 2017		36,107,000
Balance at Dec. 31, 2017	2,651,250	$ 361	2,339,253	(9,020)	320,656
Balance at January 1, 2018	2,705,714	$ 361	2,339,253	(9,020)	375,120
Net income (loss)	238,334				238,334
Other comprehensive loss	(2,668)			(2,668)
Exercise of stock options (in shares)		177,000
Exercise of stock options	21,993	$ 2	21,991
Restricted stock grants (in shares)		160,000
Restricted stock grants	0	$ 1	(1)
Restricted stock grants surrendered (in shares)		(116,000)
Restricted stock grants surrendered	(24,327)	$ (1)	(24,326)
Stock compensation expense, net of forfeitures	40,889		40,889
Employee stock purchase plan (in shares)		15,000
Employee stock purchase plan	5,641	$ 0	5,641
Stock issued for acquisitions (in shares)		103,000
Stock issued for acquisitions	36,366	$ 1	36,365
Balance (in shares) at Dec. 31, 2018		36,446,000
Balance at Dec. 31, 2018	$ 3,021,942	$ 364	$ 2,419,812	$ (11,688)	$ 613,454

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities:
Net income	$ 238,334	$ 122,695	$ 85,071
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	77,743	63,643	70,165
Amortization of deferred commissions costs	48,313	0	0
Amortization of debt issuance costs	876	2,303	3,227
Loss on extinguishment of debt	0	3,788	0
Impairment loss	0	0	23
Loss on disposal of property and equipment	73	129	839
Realized gain on investments	0	0	(808)
Stock-based compensation expense	41,214	39,030	36,349
Deferred income taxes, net	3,666	(2,903)	15,635
Bad debt expense	6,542	5,690	7,358
Changes in operating assets and liabilities, net of acquisitions:
Accounts receivable	(27,819)	(17,524)	(16,044)
Prepaid expenses and other current assets	(1,651)	(3,672)	(1,157)
Deferred commissions	(53,497)	0	0
Income tax receivable	(1,927)	(12,981)	0
Accounts payable and other liabilities	(14,132)	11,525	(1,520)
Income taxes payable	9,632	16,937	2,816
Deferred revenue	7,879	6,004
Deferred revenue			(2,070)
Deposits and other assets	212	39	758
Net cash provided by operating activities	335,458	234,703	200,642
Investing activities:
Proceeds from sale and settlement of investments	0	0	5,950
Purchases of property and equipment and other assets	(29,632)	(24,499)	(18,766)
Cash paid for acquisitions, net of cash acquired	(418,369)	(47,768)	(10,443)
Net cash used in investing activities	(448,001)	(72,267)	(23,259)
Financing activities:
Payments of long-term debt	0	(345,000)	(20,000)
Payments of debt issuance costs	0	(3,467)	0
Repurchase of restricted stock to satisfy tax withholding obligations	(24,327)	(14,902)	(16,424)
Proceeds from equity offering, net of transaction costs	0	833,911	0
Proceeds from exercise of stock options and employee stock purchase plan	27,071	9,888	5,861
Net cash provided by (used in) financing activities	2,744	480,430	(30,563)
Effect of foreign currency exchange rates on cash and cash equivalents	(1,248)	1,374	(1,415)
Net (decrease) increase in cash and cash equivalents	(111,047)	644,240	145,405
Cash and cash equivalents at beginning of year	1,211,463	567,223	421,818
Cash and cash equivalents at end of year	1,100,416	1,211,463	567,223
Supplemental cash flow disclosures:
Interest paid	1,421	6,445	6,712
Income taxes paid	35,980	41,283	34,132
Supplemental non-cash investing and financing activities:
Stock issued in connection with acquisition - ForRent	36,366	0	0
Consideration owed for acquisitions	$ 1,534	$ 0	$ 0

ORGANIZATION

ORGANIZATION	12 Months Ended
	Dec. 31, 2018
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.”), and parts of Canada, Spain, Germany and France. The Company provides online marketplaces for commercial real estate, apartment rentals, lands for sale and businesses for sale. The Company operates within two operating segments, North America and International, and its services are typically distributed to its clients under subscription-based license agreements that renew automatically, a majority of which have a term of one year .

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Accounting policies are consistent for each operating segment. Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company evaluates its estimates and assumptions, including those related to revenue recognition, allowance for doubtful accounts, useful lives of property and equipment and intangible assets, recoverability of long-lived assets and intangible assets with definite lives, goodwill, income taxes, fair value of equity instruments, fair value of auction rate securities (“ARS”), accounting for business combinations, stock-based compensation and contingencies, among others. The Company bases these estimates on historical and anticipated results, trends, and various other assumptions that it believes are reasonable, including assumptions as to future events. These estimates form the basis for making judgments about the carrying values of assets and liabilities and recorded revenues and expenses. Actual results could differ from these estimates. Revenue Recognition Subsequent to the Adoption of Accounting Standards Update (“ASU") 2014-09, Revenue from Contracts with Customers, later codified as Accounting Standards Codification 606 ("ASC 606"), on January 1, 2018 The Company derives revenues primarily by (i) providing access to its proprietary database of commercial real estate information and (ii) providing online marketplaces for professional property management companies, property owners, brokers, and landlords, in each case typically through a fixed monthly fee for its subscription-based services. The Company's subscription-based services consist primarily of information, analytics and online marketplace services offered over the Internet to commercial real estate industry and related professionals. Subscription contract rates are based on the number of sites, number of users, organization size, the client’s business focus, geography, the number and types of services to which a client subscribes, the number of properties a client advertises and the prominence and placement of a client's advertised properties in the search results. A majority of the subscription-based license agreements have a term of one year and renew automatically. The Company analyzes contracts to determine the appropriate revenue recognition using the following steps: (i) identification of contracts with customers, (ii) identification of distinct performance obligations in the contract, (iii) determination of contract transaction price, (iv) allocation of contract transaction price to the performance obligations and (v) determination of revenue recognition based on timing of satisfaction of the performance obligation(s). The Company recognizes revenues upon the satisfaction of its performance obligation(s) (upon transfer of control of promised services to its customers) in an amount that reflects the consideration to which it expects to be entitled to in exchange for those services. Revenues from subscription-based services are recognized on a straight-line basis over the term of the agreement. The Company's contracts with customers often include promises to transfer multiple services. For these contracts, the Company accounts for individual performance obligations separately if they are distinct. Determining whether services are considered distinct performance obligations may require significant judgment. Judgment is required to determine the standalone selling price (“SSP”) for each distinct performance obligation. In instances where SSP is not directly observable, such as when the Company does not sell the services separately, the Company determines the SSP using available information, including market conditions and other observable inputs. Deferred revenue results from amounts billed in advance to customers or cash received from customers in advance of the sale of subscription licenses and is recognized over the term of the license agreement. Contract assets represent a conditional right to consideration for satisfied performance obligations that become a receivable when the conditions are satisfied. Contract assets are generated when contractual billing schedules differ from revenue recognition timing. Certain sales commissions are considered incremental and recoverable costs of obtaining a contract with a customer. Sales commissions incurred for obtaining new contracts are deferred and then amortized as selling and marketing expenses on a straight-line basis over a period of benefit that the Company has determined to be three years. The three-year amortization period was determined based on several factors, including the nature of the technology and proprietary data underlying the services being purchased, customer contract renewal rates and industry competition. Certain commission costs are not capitalized as they do not represent incremental costs of obtaining a contract. See Note 3 for further discussion on the impact of the adoption of ASC 606 . For details about the Company’s revenue recognition policy prior to the adoption of ASC 606 , refer to the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, filed with the Securities and Exchange Commission on February 23, 2018. Cost of Revenues Cost of revenues principally consists of salaries, benefits, bonuses, and stock-based compensation 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, technology and other intangible assets. Foreign Currency Translation The Company’s functional currency in its foreign locations is the local currency. Assets and liabilities are translated into U.S. dollars using the exchange rates as of the balance sheet dates. Revenues, expenses, gains and losses are translated at the average exchange rates in effect during each period. Gains and losses resulting from translation are included in accumulated other comprehensive loss. Currency g ains and losses on the translation of intercompany loans made to foreign subsidiaries that are of a long-term investment nature are also 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, 2018 , 2017 , and 2016 . Accumulated Other Comprehensive Loss The components of accumulated other comprehensive loss were as follows (in thousands): As of December 31, 2018 2017 Foreign currency translation adjustment $ (10,958 ) $ (8,290 ) Net unrealized loss on investments, net of tax (730 ) (730 ) Total accumulated other comprehensive loss $ (11,688 ) $ (9,020 ) There were no amounts reclassified out of accumulated other comprehensive loss to the consolidated statements of operations for the years ended December 31, 2018 and December 31, 2017 . The amount of realized gain from the redemption of available-for-sale securities reclassified out of accumulated other comprehensive loss to the consolidated statement of operations for the year ended December 31, 2016 was approximately $0.8 million . Advertising Costs The Company expenses advertising costs as incurred. Advertising costs include e-commerce, television, radio, print and other media advertising. Advertising costs were approximately $124 million , $104 million and $109 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. Income Taxes Deferred income taxes result from temporary differences between the tax basis of assets and liabilities and the basis reported in the Company’s consolidated financial statements. Deferred tax liabilities and assets are determined based on the difference between the financial statement and the tax basis of assets and liabilities using enacted rates in effect during the year in which the Company expects differences to reverse. Valuation allowances are provided against assets, including net operating losses, if the Company determines it is more likely than not that some portion or all of an asset may not be realized. Interest and penalties related to income tax matters are recognized in income tax expense. See Note 11 for additional information regarding income taxes. 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 awards. 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. See Note 15 for additional information on the Company's calculation of net income per share. Stock-Based Compensation Equity instruments issued in exchange for services performed by officers, employees, and directors of the Company 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 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 will 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. Stock-based compensation expense for stock options and restricted stock awards issued under equity incentive plans and stock purchases under the Employee Stock Purchase Plan ("ESPP") included in the Company’s results of operations were as follows (in thousands): Year Ended December 31, 2018 2017 2016 Cost of revenues (1) $ 7,688 $ 4,971 $ 5,495 Selling and marketing (excluding customer base amortization) 6,881 7,086 6,634 Software development 7,454 7,071 6,546 General and administrative 20,695 19,902 17,674 Total stock-based compensation $ 42,718 $ 39,030 $ 36,349 __________________________ (1) Includes $1.5 million of expense related to the cash settlement of stock options in connection with the acquisition of Cozy Services, Ltd. See Note 4 for details of the acquisition. 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 as of December 31, 2018 and 2017 consisted of money market funds. 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's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities, and are classified as available-for-sale. The Company's auction rate security investments are carried at fair value and any changes in unrealized holding gains and losses, net of the related tax effect, 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 investment 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. Concentration of Credit Risk and Financial Instruments The Company performs ongoing assessments of its customers’ financial conditions and generally does not require that its customers’ obligations to the Company be secured. The Company maintains reserves for doubtful accounts, which have historically been immaterial to the Company's consolidated financial statements. Further, the large size of the Company’s customer base creates a lack of dependence on any individual customer that 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, 2018 , 2017 , and 2016 . The carrying amount of the accounts receivable approximates the net realizable value. The Company holds cash at major financial institutions that often exceed Federal Deposit Insurance Corporation insured limits. The Company manages its credit risk associated with cash concentrations by concentrating its cash deposits in high quality financial institutions and by periodically evaluating the credit quality of the primary financial institutions holding such deposits. The carrying value of cash approximates fair value. Historically, the Company has not experienced any losses due to such cash concentrations. Accounts Receivable, Net of Allowance for Doubtful Accounts Accounts receivable are recorded at the invoiced amount net of credits due. Accounts receivable payment terms vary and amounts due from customers are stated in the financial statements net of an allowance for doubtful accounts. When evaluating the adequacy of the allowance for doubtful accounts, the Company analyzes historical collection experience, changes in customer payment profiles and the aging of receivable balances, as well as current economic conditions, all of which 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 Vehicles Five to ten years Computer hardware and software Three to five years Qualifying internal-use software costs incurred during the application development stage, which consist primarily of internal product development costs, outside services and purchased software license costs are capitalized and amortized over the estimated useful life of the asset. All other costs are expensed as incurred. Goodwill and Intangible Assets Goodwill represents the future economic benefits arising from a business combination and is calculated as the excess of the purchase consideration paid in a business combination over the fair value of assets acquired. Goodwill is not amortized, but instead is assigned to each of the Company's reporting units and tested for impairment at least annually or when events and circumstances indicate that the fair value of a reporting unit may be below its carrying value. 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 or elect to bypass such assessment. If it is determined that it is more likely than not that the fair value of a reporting unit is less than its carrying value, or the Company elects to bypass such assessment, the Company performs a quantitative test that requires the determination of 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 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 an impairment loss is recognized for the difference. Acquired technology, trade names and other intangible assets, and customer base assets are related to the Company’s acquisitions (see Notes 8 and 9 ). Acquired technology is amortized on a straight-line basis over periods ranging from two years to eight years . Acquired trade names and other intangible assets are amortized on a straight-line basis over periods ranging from one year to fifteen years . Acquired intangible assets characterized as customer base assets consist of acquired customer contracts and the related customer relationships and are amortized over periods ranging from five years to thirteen years . Acquired customer bases are amortized on an accelerated or straight-line basis depending on the expected economic benefit of the intangible asset. 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 are separately presented in the balance sheet and reported at the lower of the carrying amount or fair value less costs to sell, and are no longer depreciated. The assets and liabilities of a disposal group classified as held for sale are presented separately in the appropriate asset and liability sections of the balance sheet. Debt Issuance Costs Costs incurred in connection with the issuance of long-term debt are deferred and amortized as interest expense over the term of the related debt using the effective interest method for term debt and on a straight-line basis for revolving debt. To the extent that debt is outstanding, these amounts are reflected in the consolidated balance sheets as direct deductions from a combination of the current and long-term portions of debt for term debt and as current and long-term assets for costs related to revolving debt. Upon a refinancing or amendment, 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. See Note 10 for additional information on the Company's long-term debt and related debt issuance costs. Business Combinations The Company allocates the purchase consideration to the tangible assets acquired, liabilities assumed and intangible assets acquired based on their estimated fair values. The purchase price is determined based on the fair value of the assets transferred, liabilities incurred and equity interests issued, after considering any transactions that are separate from the business combination. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is recorded as goodwill. Such valuations require management to make significant estimates and assumptions, especially with respect to intangible assets and contingent liabilities. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired technology and acquired trade names, useful lives, royalty rates and discount rates. Any adjustments to provisional amounts that are identified during the measurement period are recorded in the reporting period in which the adjustment amounts are determined. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings. For a given acquisition, the Company may identify certain pre-acquisition contingencies as of the acquisition date and may extend its review and evaluation of these pre-acquisition contingencies throughout the measurement period in order to obtain sufficient information to assess whether the Company includes these contingencies as a part of the fair value estimates of assets acquired and liabilities assumed and, if so, to determine their estimated amounts. If the Company cannot reasonably determine the fair value of a pre-acquisition contingency (non-income tax related) by the end of the measurement period, which is generally the case given the nature of such matters, the Company will recognize an asset or a liability for such pre-acquisition contingency if: (i) it is probable that an asset existed or a liability had been assumed at the acquisition date and (ii) the amount of the asset or liability can be reasonably estimated. Subsequent to the measurement period, changes in the Company's estimates of such contingencies will affect earnings and could have a material effect on its results of operations and financial position. In addition, uncertain tax positions and tax related valuation allowances assumed in connection with a business combination are initially estimated as of the acquisition date. The Company reevaluates these items based upon facts and circumstances that existed as of the acquisition date, with any adjustments to its preliminary estimates being recorded to goodwill provided that the Company is within the measurement period. Subsequent to the measurement period, changes to these uncertain tax positions and tax related valuation allowances will affect the Company's provision for income taxes in its consolidated statements of operations and comprehensive income and could have a material impact on its results of operations and financial position. Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) and International Accounting Standards Board (“IASB”) jointly issued a new revenue recognition standard, Accounting Standards Update (“ASU") 2014-09, Revenue from Contracts with Customers , later codified as Accounting Standards Codification ("ASC") 606 (" ASC 606" ), that is designed to improve financial reporting by creating common recognition guidance for GAAP and International Financial Reporting Standards (“IFRS”). This guidance provides a robust framework for addressing revenue issues, improves the comparability of revenue recognition practices across industries, provides 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 services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those services. On January 1, 2018, the Company adopted ASC 606, using the modified retrospective method. Results for reporting periods beginning subsequent to December 31, 2017 are presented under ASC 606 , while prior period amounts are not adjusted and continue to be reported in accordance with the Company’s historical accounting policies prior to adoption. In adopting the guidance, the Company applied the guidance to all customer contracts and used several available practical expedients including assessing contracts with similar terms and conditions on a “portfolio” basis and not including contracts with a duration of one year or less in the unsatisfied performance obligations disclosure. The Company recorded a net cumulative increase to beginning retained earnings of $54 million . The Company adjusted the condensed consolidated financial statements from amounts previously reported due to the adoption of ASC 606 . Select condensed consolidated balance sheet line items which were adjusted upon adoption were as follows (in thousands): As of December 31, 2017 ASC 606 Adjustments As of January 1, 2018 Assets Accounts receivable, less allowance for doubtful accounts $ 60,900 $ (1,867 ) $ 59,033 Prepaid expenses and other current assets 15,572 1,867 17,439 Deferred commissions costs, net — 71,118 71,118 Liabilities Deferred revenue $ 45,686 $ (1,716 ) $ 43,970 Deferred income taxes, net 12,070 18,370 30,440 Retained earnings 320,656 54,464 375,120 The impact of the adoption of ASC 606 on the condensed financial statements for the period ended December 31, 2018 was as follows (in thousands): As of December 31, 2018 without adoption of ASC 606 ASC 606 Adjustments As Reported as of December 31, 2018 Assets Accounts receivable, less allowance for doubtful accounts $ 91,122 $ (1,930 ) $ 89,192 Prepaid expenses and other current assets 21,760 1,930 23,690 Deferred commissions costs, net — 76,031 76,031 Liabilities Deferred revenue $ 57,284 $ (5,825 ) $ 51,459 Deferred income taxes, net 49,147 20,710 69,857 Retained earnings 552,308 61,146 613,454 If the Company had not adopted ASC 606 , revenue recognized would have been $4 million lower and selling and marketing expense would have been $5 million higher for the year ended December 31, 2018 . The impact on net income and basic and diluted earnings per share for the year ended December 31, 2018 would have been a decrease of approximately $7 million or $0.19 per share, respectively. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments , which is designed to reduce the existing diversity in how certain cash receipts and cash payments are presented and classified in the consolidated statements of cash flows. This guidance is effective on a retrospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company's consolidated financial statements and related disclosures. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business , which is designed to clarify the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance indicates that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set of assets is not a business. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company's consolidated financial statements and related disclosures. In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting , which is designed to reduce the existing diversity and complexity in the accounting for changes to terms or conditions of a share-based payment award. This guidance clarifies that an entity will not apply modification accounting to a share-based payment award if all of the following are the same immediately before and after the change: (i) the fair value of the award, (ii) the vesting conditions of the award, and (iii) the classification of the award as an equity instrument or liability instrument. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company's consolidated financial statements and related disclosures. Recent Accounting Pronouncements Not Yet Adopted In February 2016, the FASB issued ASU 2016-02, Leases ("ASC 842") , to increase transparency and comparability among organizations' accounting for leases. The guidance requires a company to recognize right-of-use assets and lease liabilities on the balance sheet, as well as to disclose key quantitative and qualitative information about leasing arrangements. This guidance is effective on a modified retrospective basis for reporting periods beginning after December 15, 2018, with early adoption permitted. As permitted by the guidance, the Company will elect to retain the original lease classification and historical accounting for initial direct costs for leases existing prior to the adoption date. Furthermore, the Company will not have to reassess contracts entered into prior to the adoption date for the existence of a lease. The Company will also elect not to restate prior periods for the impact of the adoption of the new standard and will instead recognize a cumulative-effect adjustment to beginning retained earnings as of January 1, 2019 for any prior period income statement effects identified. The Company assessed the changes required to support the adoption of the new standard, as well as the quantitative impact this guidance will have on its financial statements and related disclosures. As a result, the Company expects that the adoption of this standard will result in the recognition of Right of Use Assets between $100 million and $120 million , as well as Lease Liabilities between $140 million and $160 million on its consolidated balance sheet, primarily as a result of recognizing assets and liabilities associated with existing office leases . Lastly, the Company expects to recognize a cumulative-effect adjustment to beginning retained earnings of $12 million , net of tax, as of January 1, 2019 to recognize the remaining gain on the Company's outstanding deferred gain on the sale of building, pursuant to the guidance in ASC 842 . Beginning in 2019, the Company expects significant changes to its disclosed lease recognition policies and practices, as well as to other related financial statement disclosures due to the adoption of this standard. These revised disclosures will be made in the Company’s first quarterly report in 2019. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurements (subsequent to adoption of ASU 2018-13, Fair Value Measurement. The ASU was issued to eliminate certain disclosure requirements for fair value measurements, and add and modify other disclosure requirements, as part of its disclosure framework project, including additional requ

REVENUE FROM CONTRACTS WITH CUS

REVENUE FROM CONTRACTS WITH CUSTOMERS	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
REVENUE FROM CONTRACTS WITH CUSTOMERS	REVENUE FROM CONTRACTS WITH CUSTOMERS Disaggregated Revenue The Company provides information, analytics and online marketplaces to the commercial real estate industry and related professionals. The revenues by operating segment and type of service consist of the following (in thousands): Year Ended December 31, 2018 2017 North America International Total North America International Total Information and analytics CoStar Suite $ 519,661 $ 25,534 $ 545,195 $ 440,534 $ 22,651 $ 463,185 Information services 58,708 8,916 67,624 64,503 8,115 72,618 Online marketplaces Multifamily 405,795 — 405,795 279,855 — 279,855 Commercial property and land 173,137 81 173,218 149,572 — 149,572 Total revenues $ 1,157,301 $ 34,531 $ 1,191,832 $ 934,464 $ 30,766 $ 965,230 Deferred Revenue Changes in deferred revenue for the period were as follows (in thousands): Balance at December 31, 2017 $ 45,686 Cumulative effect of adoption of ASC 606 (1,716 ) Balance at January 1, 2018 43,970 Revenue recognized in the current period from the amounts in the beginning balance (43,121 ) New deferrals, net of amounts recognized in the current period 51,000 Effects of foreign currency (390 ) Balance at December 31, 2018 $ 51,459 Contract Assets The Company had contract assets of $2 million as of December 31, 2018 and January 1, 2018 , which are generated when contractual billing schedules differ from revenue recognition timing. Contract assets represent a conditional right to consideration for satisfied performance obligations that becomes a receivable when the conditions are satisfied. Commissions The Company recognized $48 million of amortization of deferred commissions included in selling and marketing expense during the year ended months ended December 31, 2018 . The Company determined that no deferred commissions were impaired as of December 31, 2018 . Commissi ons expense activity for the year ended December 31, 2018 was as follows (in thousands): Year Ended December 31, 2018 Commissions incurred $ 72,899 Commissions capitalized in the current period (53,497 ) Amortization of deferred commissions costs 48,313 Total commissions expense $ 67,715 Refer to Note 2 for the Company's policy on accounting for commissions. Unsatisfied Performance Obligations Remaining contract consideration for which revenue has not been recognized due to unsatisfied performance obligations was approximately $193 million at December 31, 2018 , which the Company expects to recognize over the next three years . This amount does not include contract consideration for contracts with a duration of one year or less.

ACQUISITIONS

ACQUISITIONS	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
ACQUISITIONS	ACQUISITIONS ForRent On February 21, 2018 (the "Acquisition Date"), the Company acquired all of the issued and outstanding capital stock of DE Holdings, Inc., including its ForRent division ("ForRent"), a wholly owned subsidiary of Dominion Enterprises ("Seller"), for a purchase price of approximately $376 million . The purchase price was comprised of approximately $340 million in cash and 103,280 shares of Company common stock, valued at approximately $36 million . ForRent's primary service is digital advertising provided through a network of four multifamily websites. The acquisition is expected to yield increased revenue, significant cost synergies and an improved competitive position in the industry. The Company applied the acquisition method to account for the ForRent transaction, which requires that assets acquired and liabilities assumed be recorded at their fair values as of the acquisition date. The following table summarizes the amounts recorded for acquired assets and assumed liabilities recorded at their fair values as of the Acquisition Date (in thousands): Preliminary: February 21, 2018 Measurement Period Adjustments Final: February 21, 2018 Cash and cash equivalents $ 59 $ — $ 59 Accounts receivable 8,769 — 8,769 Indemnification asset 5,443 — 5,443 Goodwill 266,720 (125 ) 266,595 Intangible assets 141,300 — 141,300 Deferred tax liabilities (34,032 ) — (34,032 ) Contingent sales tax liability (6,260 ) — (6,260 ) State uncertain income tax position liability (2,047 ) — (2,047 ) Other assets and liabilities (3,453 ) (82 ) (3,535 ) Fair value of identifiable net assets acquired $ 376,499 $ (207 ) $ 376,292 The net assets of ForRent were recorded at their estimated fair values. In valuing acquired assets and assumed liabilities, fair value estimates were based primarily on future expected cash flows, market rate assumptions for contractual obligations and appropriate discount rates. Measurement period adjustments relate to the determination of working capital as of the Acquisition Date. The acquired customer base for the acquisition is composed of acquired customer contracts and the related customer relationships, and has a weighted average estimated useful life of ten years . The acquired technology has an estimated useful life of three years . The acquired trade name has a weighted average estimated useful life of ten years . The acquired building photography has an estimated useful life of one year . 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 technology, acquired building photography and acquired trade names and other intangible assets is recognized on a straight-line basis over their respective estimated useful lives. Goodwill recorded in connection with this acquisition is not amortized, but is subject to an annual impairment test. The $267 million of goodwill recorded as part of the acquisition is associated with the Company's North America operating segment. $8 million of the goodwill recognized 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 ForRent acquisition includes but is not limited to: (i) the expected synergies and other benefits that the Company believes will result from combining its operations with ForRent's operations; and (ii) any intangible assets that do not qualify for separate recognition, such as the assembled workforce. Upon acquisition, the Company assessed the (i) probability of a contingent sales tax liability and (ii) a state uncertain income tax position liability due to apportionment factors, and recorded accruals of $6 million and $2 million , respectively. The Company could not determine the fair value for the pre-acquisition state sales tax liability and therefore estimated a liability in accordance with ASC 450 , using a state-by-state assessment. The uncertain income tax position was determined in accordance with the provisions of ASC 740 and was recorded as part of the purchase price allocation. The Seller has provided an indemnity for tax liabilities related to periods prior to the acquisition. The Seller's indemnification for sales taxes in the state of Texas is limited to approximately $2 million . The total indemnification asset established as of the acquisition date is $5 million . $1 million of the uncertain income tax position liability and related indemnification asset recognized as of the acquisition date were reversed during the year, upon expiration of the statute of limitations applicable to the uncertain income tax position. As part of the ForRent acquisition, the Company incurred $3 million of transaction costs. Additionally, the Company paid $12 million cash into a cash escrow account for retention compensation for certain ForRent employees, payable if they remained employed by the Company for a defined six-month period following the acquisition or were earlier terminated without cause or resigned for good reason. In the event some or all of those employees were not entitled to their retention bonus, those funds would have been remitted to the Seller. The Company expensed the retention compensation as the services were performed in the post-combination period. Other Acquisitions On October 12, 2018, the Company acquired Realla Ltd. ("Realla"), the operator of a commercial property listings and data management platform in the U.K. for £12 million ( $15 million ). The purchase agreement required an initial payment of £10 million ( $13 million ), net of cash acquired, at the time of closing, and the remainder is due one year following the acquisition date. In connection with the acquisition, the Company recorded goodwill and intangible assets of £8 million ( $10 million ) and £4 million ( $5 million ), respectively. The net assets of Realla were recorded at their estimated fair value. The estimated fair values are preliminary, subject to the finalization of the Company's assessment of the fair value of certain acquired intangible assets, the final determination of net working capital as of the acquisition date and completion of the Company's assessment of certain tax matters. On November 8, 2018, the Company acquired Cozy Services, Ltd. ("Cozy"), a leading provider of online rental solutions that provides a broad spectrum of services to both landlords and tenants, for $65 million , net of cash acquired. As part of the acquisition, the Company recorded goodwill and intangible assets of $53 million and $11 million , respectively. The net assets of Cozy were recorded at their estimated fair value. The estimated fair values are preliminary, subject to the finalization of the Company's assessment of the fair value of certain acquired intangible assets, the final determination of net working capital as of the acquisition date and completion of the Company's assessment of certain tax matters. Pro Forma Financial Information The unaudited pro forma financial information presented below summarizes the combined results of operations for the Company and ForRent as though the companies were combined as of January 1, 2017. The impact of Realla and Cozy on the pro forma financial information was not material and therefore those acquisitions were not included. The unaudited pro forma financial information for all periods presented includes amortization charges from acquired intangible assets, retention compensation, as referenced above, and the related tax effects, along with certain other accounting effects, but excludes the impacts of any expected operational synergies. The unaudited pro forma financial information as presented below is for informational purposes only and is not necessarily indicative of the results of operations that would have been achieved if the acquisition had taken place on January 1, 2017. The unaudited pro forma financial information for the year ended months ended December 31, 2018 and 2017 combine the historical results of the Company for the year ended months ended December 31, 2018 and 2017 , the historical results of ForRent for the period prior to the Acquisition Date, and the effects of the pro forma adjustments listed above. The unaudited pro forma financial information was as follows (in thousands, except per share data): Year Ended 2018 2017 Revenue $ 1,205,584 $ 1,067,742 Net income $ 251,196 $ 103,000 Net income per share - basic $ 6.96 $ 3.09 Net income per share - diluted $ 6.89 $ 3.06 The Company began integrating the ForRent sales force and operations after the closing of the acquisition as part of its efforts to create operating synergies. As a result of these integration activities, it is impracticable to disclose revenue and earnings from ForRent from the Acquisition Date through December 31, 2018 .

INVESTMENTS

INVESTMENTS	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
INVESTMENTS	INVESTMENTS The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities ("ARS"), and are classified as available-for-sale and are carried at fair value. Scheduled maturities of investments classified as available-for-sale as of December 31, 2018 are as follows (in thousands): Maturity Fair Value Due in: 2019 $ — 2020 — 2023 — 2024 — 2028 — 2029 and thereafter 10,070 Available-for-sale investments $ 10,070 The Company had no realized gains or losses on its investments during the years ended December 31, 2018 , 2017 and 2016 . Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis. As of December 31, 2018 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 10,800 $ — $ (730 ) $ 10,070 Available-for-sale investments $ 10,800 $ — $ (730 ) $ 10,070 As of December 31, 2017 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 10,800 $ — $ (730 ) $ 10,070 Available-for-sale investments $ 10,800 $ — $ (730 ) $ 10,070 The unrealized losses on the Company’s investments as of December 31, 2018 and 2017 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, 2018 and 2017 . See Note 6 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, 2018 2017 Aggregate Fair Value Gross Unrealized Losses Aggregate Fair Value Gross Unrealized Losses Auction rate securities $ 10,070 $ (730 ) $ 10,070 $ (730 ) Investments in an unrealized loss position $ 10,070 $ (730 ) $ 10,070 $ (730 ) The Company did not have any investments in an unrealized loss position for less than twelve months as of December 31, 2018 and 2017 , respectively.

FAIR VALUE

FAIR VALUE	12 Months Ended
	Dec. 31, 2018
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 equivalents and investments) measured at fair value on a recurring basis as of December 31, 2018 (in thousands): Level 1 Level 2 Level 3 Total Assets: Money market funds $ 590,567 $ — $ — $ 590,567 Auction rate securities — — 10,070 10,070 Total assets measured at fair value $ 590,567 $ — $ 10,070 $ 600,637 The following table represents the Company's fair value hierarchy for its financial assets (cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2017 (in thousands): Level 1 Level 2 Level 3 Total Assets: Money market funds $ 586,084 $ — $ — $ 586,084 Auction rate securities — — 10,070 10,070 Total assets measured at fair value $ 586,084 $ — $ 10,070 $ 596,154 The carrying value of accounts receivable, accounts payable and accrued expenses approximates fair value. The Company’s Level 3 assets consist of ARS, whose underlying assets are primarily student loan securities supported by guarantees from the Federal Family Education Loan Program (“FFELP”) of the U.S. Department of Education. The following table summarizes changes in fair value of the Company’s Level 3 assets from December 31, 2016 to December 31, 2018 (in thousands): Auction Rate Securities Balance at December 31, 2016 $ 9,952 Decrease in unrealized loss included in accumulated other comprehensive loss 118 Balance at December 31, 2017 10,070 Decrease in unrealized loss included in accumulated other comprehensive loss — Settlements — Balance at December 31, 2018 $ 10,070 ARS are variable rate debt instruments whose interest rates are reset approximately every 28 days . The underlying securities have contractual maturities greater than twenty years . The ARS are recorded at fair value. As of December 31, 2018 , the Company held ARS with $11 million par value, all of which failed to settle at auction. The majority of these investments are of high credit quality 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, 2018 . See Note 5 for further discussion of the scheduled maturities of investments classified as available-for-sale. 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, 2018 . 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 models as of December 31, 2018 and 2017 was approximately 6% . 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. Based on this assessment of fair value, as of December 31, 2018 , the Company determined there was no decline in the fair value of its ARS investments. In addition, the ARS are of high credit quality, if the issuers are unable to successfully close future auctions and/or their credit ratings deteriorate, the Company may be required to record additional unrealized losses in accumulated other comprehensive loss or an other-than-temporary impairment charge to earnings on these investments.

PROPERTY AND EQUIPMENT

PROPERTY AND EQUIPMENT	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
PROPERTY AND EQUIPMENT	PROPERTY AND EQUIPMENT Property and equipment consists of the following (in thousands): December 31, 2018 2017 Leasehold improvements $ 65,332 $ 59,447 Furniture, office equipment and vehicles 53,020 52,163 Computer hardware and software 74,742 71,281 Property and equipment, gross 193,094 182,891 Accumulated depreciation and amortization (109,791 ) (98,395 ) Property and equipment, net $ 83,303 $ 84,496 Depreciation expense for property and equipment was approximately $26 million , $26 million and $24 million , for the years ended December 31, 2018 , 2017 and 2016 , respectively.

GOODWILL

GOODWILL	12 Months Ended
	Dec. 31, 2018
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, 2016 $ 1,227,777 $ 27,089 $ 1,254,866 Acquisition 25,717 — 25,717 Effect of foreign currency translation — 2,874 2,874 Goodwill, December 31, 2017 1,253,494 29,963 1,283,457 Acquisitions 319,594 10,344 329,938 Effect of foreign currency translation — (1,860 ) (1,860 ) Goodwill, December 31, 2018 $ 1,573,088 $ 38,447 $ 1,611,535 The Company recorded goodwill of approximately $8 million in connection with the January 31, 2017 acquisition of Koa Lei, Inc. (doing business as Westside Rentals ® and now known as Westside Rentals, LLC), an online marketplace specializing in Southern California real estate rentals, and its affiliated entity Westside Credit Services, LLC, a provider of credit checks and tenant screening for landlords in the Southern California real estate rental market. The Company recorded goodwill of approximately $15 million in connection with the May 10, 2017 acquisition of certain assets and assumption of certain liabilities from Datasphere Technologies, Inc., in each case, related to the LandWatch.com ® business (collectively referred to as “LandWatch”), a leading listing site dedicated to land and rural properties. The Company recorded goodwill of approximately $2 million in connection with the July 18, 2017 acquisition of The Screening Pros, LLC, an online apartment leasing platform that includes tenant screening services, rental applications and payments processing and lease renewals. The Company recorded goodwill of approximately $267 million in connection with the February 21, 2018 acquisition of ForRent, a digital advertising service provided through a network of four multifamily websites. The Company recorded goodwill of approximately $10 million in connection with the October 12, 2018 acquisition of Realla, the operator of a commercial property listings and data management platform in the U.K., including a free-to-list search engine for commercial property listings. The company recorded goodwill of approximately $53 million in connection with the November 8, 2018 acquisition of Cozy, a leading provider of online rental solutions that provides a broad spectrum of services to both landlords and tenants, including property listings, rent estimates, rental applications, tenant screening, online rent payments and expense tracking. The total amount of goodwill that is expected to be deductible for tax purposes is approximately $16 million as of December 31, 2018 . No impairments of the Company's goodwill were recognized during the years ended December 31, 2018 , 2017 and 2016 .

INTANGIBLE ASSETS

INTANGIBLE ASSETS	12 Months Ended
	Dec. 31, 2018
Intangible Assets, Net (Excluding Goodwill) [Abstract]
INTANGIBLE ASSETS	INTANGIBLE ASSETS Intangible assets consist of the following (in thousands, except amortization period data): December 31, Weighted- Average Amortization Period (in years) 2018 2017 Capitalized product development cost $ 2,173 $ 2,275 4 Accumulated amortization (2,173 ) (2,262 ) Capitalized product development cost, net — 13 Building photography 9,035 18,739 2 Accumulated amortization (8,809 ) (18,212 ) Building photography, net 226 527 Acquired technology 103,128 83,469 4 Accumulated amortization (85,344 ) (79,188 ) Acquired technology, net 17,784 4,281 Acquired customer base 339,574 225,879 10 Accumulated amortization (199,405 ) (169,157 ) Acquired customer base, net 140,169 56,722 Acquired trade names and other intangible assets 190,717 167,718 12 Accumulated amortization (59,985 ) (46,369 ) Acquired trade names and other intangible assets, net 130,732 121,349 Intangible assets, net $ 288,911 $ 182,892 Amortization expense for intangible assets was approximately $52 million , $37 million and $46 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. In the aggregate, the Company expects the future amortization expense for intangible assets existing as of December 31, 2018 to be approximately $48 million , $42 million , $34 million , $29 million and $26 million for the years ending December 31, 2019, 2020, 2021, 2022 and 2023, respectively. Intangible assets are reviewed for impairment at least annually and more frequently whenever events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. No impairments of the Company's intangible assets were recognized during the years ended December 31, 2018 , 2017 and 2016 .

LONG-TERM DEBT

LONG-TERM DEBT	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
LONG-TERM DEBT	LONG-TERM DEBT On October 19, 2017, the Company entered into an amended and restated credit agreement (the ‘‘2017 Credit Agreement’’), which amended and restated in its entirety the then-existing credit agreement dated April 1, 2014 (the "2014 Credit Agreement"). The 2017 Credit Agreement provides for a $750 million revolving credit facility with a term of five years from a syndicate of financial institutions as lenders and issuing banks. The 2017 facility may be used for working capital and other general corporate purposes of the Company and its subsidiaries. In connection with the transaction, the Company incurred $4 million of issuance costs. Those costs along with the $4 million of unamortized costs related to the prior agreement were allocated between the extinguishment of the 2014 Credit Agreement and the 2017 Credit Agreement. This allocation resulted in the Company recognizing a loss of $4 million on the extinguishment with the remaining $4 million being deferred and amortized on a straight-line basis as interest expense over the term of the 2017 Credit Agreement. Up to $20 million of the revolving credit facility is available for the issuance of letters of credit. The Company had an irrevocable standby letter of credit outstanding totaling $0.2 million as of December 31, 2018 and December 31, 2017 , which was required to secure its San Francisco office lease. The letter of credit was established in 2014 and automatically renews through January 31, 2025. The loans under the 2017 Credit Agreement bear interest during any interest period selected by the Company, at either (i) 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 1.25% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio (as defined in the 2017 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 half of 1% and (z) LIBOR for a one-month interest period plus 1.00% , plus an initial spread of 0.25% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio of the Company. If an event of default occurs under the 2017 Credit Agreement, the interest rate on overdue amounts will increase by 2.00% per annum. The obligations under the 2017 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 its material subsidiaries, in each case subject to certain exceptions, pursuant to security and guarantee agreements entered into on the closing date of the 2017 Credit Agreement. The 2017 Credit Agreement requires the Company to maintain (i) a First Lien Secured Leverage Ratio not exceeding 3.50 to 1.00 and (ii) after the incurrence of additional indebtedness under certain specified exceptions in the 2017 Credit Agreement, a Total Leverage Ratio (as defined in the 2017 Credit Agreement) not exceeding 4.50 to 1.00 . The 2017 Credit Agreement also includes other covenants, including ones 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 2017 Credit Agreement as of December 31, 2018 . The Company had no outstanding long-term debt at December 31, 2018 and December 31, 2017 . For the years ended December 31, 2018 , 2017 and 2016 , the Company recognized interest expense, including amortization of debt issuance costs and commitment fees, on its revolving credit facility and term loan of approximately $3 million , $9 million and $10 million , respectively. Interest expense included amortized debt issuance costs of approximately $1 million , $2 million and $3 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. The Company had $3 million and $4 million of deferred debt issuance costs included in deposits and other assets at December 31, 2018 and December 31, 2017 , respectively.

INCOME TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The components of the provision for income taxes attributable to operations consist of the following (in thousands): Year Ended December 31, 2018 2017 2016 Current: Federal $ 36,167 $ 41,453 $ 32,198 State 5,140 3,518 3,682 Foreign 708 295 76 Total current 42,015 45,266 35,956 Deferred: Federal 6,576 (7,917 ) 12,586 State (2,582 ) 4,695 3,014 Foreign (328 ) 319 35 Total deferred 3,666 (2,903 ) 15,635 Total provision for income taxes $ 45,681 $ 42,363 $ 51,591 The components of deferred tax assets and liabilities consist of the following (in thousands): December 31, 2018 2017 Deferred tax assets: Reserve for bad debts $ 1,457 $ 1,636 Accrued compensation 4,803 6,706 Stock compensation 10,041 10,568 Net operating losses 26,349 25,899 Accrued reserve and other 1,773 1,578 Deferred rent 5,928 6,533 Deferred gain on the sale of building 4,140 4,741 Research and development credits 6,331 — Total deferred tax assets, prior to valuation allowance 60,822 57,661 Valuation allowance (14,246 ) (13,032 ) Total deferred tax assets, net of valuation allowance 46,576 44,629 Deferred tax liabilities: Deferred commission costs, net (19,314 ) — Prepaid expenses (2,204 ) (1,239 ) Property and equipment, net (5,367 ) (6,229 ) Intangible assets, net (82,079 ) (43,800 ) Total deferred tax liabilities (108,964 ) (51,268 ) Net deferred tax assets (liabilities) $ (62,388 ) $ (6,639 ) As of December 31, 2018 and 2017 , a valuation allowance has been established for certain deferred tax assets due to the uncertainty of realization. The valuation allowance as of December 31, 2018 and 2017 includes an allowance for unrealized losses on ARS investments, foreign deferred tax assets and state net operating losses and tax credits. The valuation allowance for the deferred tax asset for unrealized losses on ARS has been recorded as an adjustment to accumulated other comprehensive loss. The Company established the valuation allowance because it is more likely than not that a portion of the deferred tax asset for certain items will not be realized based on the weight of available evidence. A valuation allowance was established for the unrealized losses on securities as the Company has not historically generated capital gains, and it is uncertain whether the Company will generate sufficient capital gains in the future to absorb the capital losses. A valuation allowance was established for the foreign deferred tax assets due to the cumulative loss in recent years in those jurisdictions. The Company has not had sufficient taxable income historically to utilize the foreign deferred tax assets, and it is uncertain whether the Company will generate sufficient taxable income in the future to utilize the deferred tax assets. Similarly, the Company has established a valuation allowance for net operating losses and tax credits in certain states where it is uncertain whether the Company will generate sufficient taxable income to utilize the net operating losses and tax credits before they expire. The Company’s change in valuation allowance was an increase of approximately $1 million for the year ended December 31, 2018 and a decrease of approximately $4 million for the year ended December 31, 2017 . The increase for the year ended December 31, 2018 is due to an increase in the valuation allowance for state tax credits related to the D.C. qualified high technology company credit of approximately $1 million . The increase for the year ended December 31, 2017 is due to an increase in the valuation allowance for foreign deferred tax assets related to foreign net operating losses of approximately $4 million . The Company had U.S. income before income taxes of approximately $294 million , $167 million and $135 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. The Company had foreign losses before income taxes of approximately $10 million and $2 million for the years ended December 31, 2018 and December 31, 2017 , respectively. The Company had foreign income before income taxes of approximately $2 million for the year ended December 31, 2016 . 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, 2018 2017 2016 Expected federal income tax provision at statutory rate $ 59,643 $ 57,770 $ 47,832 State income taxes, net of federal benefit 10,312 4,776 3,638 Foreign income taxes, net effect (315 ) (3,540 ) (31 ) Increase (decrease) in valuation allowance 1,214 3,624 (103 ) Tax rate changes 141 (7,340 ) 283 Research credits (15,373 ) (20,547 ) (920 ) Excess tax benefit (14,227 ) (7,010 ) — Tax reserves 1,870 12,646 (150 ) Other adjustments 2,416 1,984 1,042 Income tax expense $ 45,681 $ 42,363 $ 51,591 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 recognized an income tax benefit during the year ended December 31, 2018 for state research credits of $14 million for tax years December 31, 2013 through December 31, 2018. These research credits relate to eligible activities including the development of new products, product enhancements and new or improved processes. The Company has net operating loss carryforwards for international income tax purposes of approximately $53 million , which do not expire. The Company has federal net operating loss carryforwards of approximately $44 million that begin to expire in 2020 , state net operating loss carryforwards with a tax value of approximately $4 million that begin to expire in 2020 and state income tax credit carryforwards with a tax value of approximately $11 million primarily relating to state research and development credits and the D.C. qualified high technology company tax credit that begin to expire in 2020 . The Company realized a cash benefit relating to the use of its tax loss carryforwards of approximately $6 million , $7 million and $5 million in December 31, 2018 , 2017 and 2016 , respectively. The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands): Unrecognized tax benefit as of December 31, 2015 $ 7,664 Increase for current year tax positions 368 Decrease for prior year tax positions (6,115 ) Expiration of the statute of limitation for assessment of taxes (74 ) Unrecognized tax benefit as of December 31, 2016 1,843 Increase for current year tax positions 12,620 Decrease for prior year tax positions (34 ) Expiration of the statute of limitation for assessment of taxes (66 ) Unrecognized tax benefit as of December 31, 2017 14,363 Increase for current year tax positions 9,561 Decrease for prior year tax positions (70 ) Expiration of the statute of limitation for assessment of taxes (1,482 ) Unrecognized tax benefit as of December 31, 2018 $ 22,372 Approximately $22 million and $14 million of the unrecognized tax benefits as of December 31, 2018 and 2017 , respectively, would favorably affect the annual effective tax rate, if recognized in future periods. The increase for current year tax positions of $9 million for the year ended December 31, 2018 is primarily attributable to research credits and state apportionment methodology reserve related to the ForRent acquisition. The decrease of $1 million for the year ended December 31, 2018 is primarily attributable to the expiration of the statute of limitation on the state apportionment methodology reserve. The Company recognized $224,000 for interest and penalties in its consolidated statement of operations for the year ended December 31, 2018 . The Company recognized $72,000 for interest and penalties in its consolidated statement of operations for the year ended December 31, 2017 . The Company reversed interest and penalties of $416,000 in its consolidated statement of operations for the year ended December 31, 2016 . The Company had liabilities of $430,000 , $205,000 and $133,000 for interest and penalties in its consolidated balance sheets as of December 31, 2018 , 2017 , 2016 respectively. The Company does not anticipate the amount of the unrecognized tax benefits will change significantly over the next twelve months. The Company is subject to taxation in the U.S. federal jurisdiction and various states and foreign jurisdictions. The Company’s federal income tax returns for tax years 2013 through 2017 remain open to examination. The Company is under Internal Revenue Service examination for tax year 2013 related to the research and development credit. Most of the Company’s state income tax returns for tax years 2015 through 2017 remain open to examination. For states that have a four-year statute of limitations, the state income tax returns for tax years 2014 through 2017 remain open to examination. The Company’s U.K. income tax returns for tax years 2013 through 2017 remain open to examination. The Company believes that an adequate provision has been made for any adjustments that may result from tax examinations. On December 22, 2017, the U.S. enacted the Tax Cuts and Jobs Act (the “Tax Act”), which significantly changed U.S. tax law. The Tax Act lowered the Company’s U.S. statutory federal income tax rate from 35% to 21% effective January 1, 2018, while also imposing a deemed repatriation tax on deferred foreign income. The Tax Act also created a new minimum tax on certain future foreign earnings under section 951(a) and allows foreign-derived intangible income deduction under section 250(a). The Securities and Exchange Commission staff issued Staff Accounting Bulletin 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act, ("SAB 118"), which provides guidance on accounting for the tax effects of the Tax Act. SAB 118 provided a measurement period of up to one year from the Tax Act enactment date for companies to complete the accounting under ASC 740. As of December 31, 2018, the Company's accounting for income tax effects of the Tax Act is complete and the Company has reflected all income tax effects of the Tax Act in the financial statements and related disclosures. Any additional guidance issued by the U.S. Treasury Department, the IRS, and other standard-setting bodies, may result in the Company deciding to make adjustments. Those adjustments may materially impact the provision for income taxes in the period in which the adjustments are made. As of December 31, 2018 the Company has evaluated the global intangible low taxed income inclusion under section 951(a)("GILTI"). Under U.S. GAAP, the Company is allowed to make an accounting policy choice of either (1) treating GILTI as a current-period expense when incurred (the “period cost method”) or (2) factoring such amounts into the measurement of deferred taxes (the “deferred method”). The Company elected to record the GILTI income inclusion under the current-period cost method.

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2018
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, 2018 , 2017 and 2016 , was approximately $29 million , $26 million and $22 million , respectively. Future minimum lease payments as of December 31, 2018 are as follows (in thousands): 2019 $ 30,485 2020 29,255 2021 27,421 2022 25,634 2023 24,515 Thereafter 31,768 Total future minimum lease payments $ 169,078 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. 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, at this time management has concluded that the resolutions of these matters are not expected to have a material adverse effect on the Company's consolidated financial position, future results of operations or liquidity. Legal defense costs are expensed as incurred.

SEGMENT REPORTING

SEGMENT REPORTING	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
SEGMENT REPORTING	SEGMENT REPORTING Segment Information 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., Spain, Germany and France. Management relies on an internal management reporting process that provides revenue and operating segment net income before interest and other income (expense), loss on debt extinguishment, income taxes, depreciation and amortization (“EBITDA”). 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, 2018 2017 2016 EBITDA North America $ 358,036 $ 236,906 $ 210,901 International (6,729 ) 553 4,169 Total EBITDA $ 351,307 $ 237,459 $ 215,070 The reconciliation of net income to EBITDA consists of the following (in thousands): Year Ended December 31, 2018 2017 2016 Net income $ 238,334 $ 122,695 $ 85,071 Amortization of acquired intangible assets in cost of revenues 20,586 19,707 22,819 Amortization of acquired intangible assets in operating expenses 30,881 17,684 22,731 Depreciation and other amortization 26,276 26,252 24,615 Interest and other income (13,281 ) (4,044 ) (1,773 ) Interest and other expense 2,830 9,014 10,016 Loss on debt extinguishment — 3,788 — Income tax expense 45,681 42,363 51,591 EBITDA $ 351,307 $ 237,459 $ 215,070 Summarized information by operating segment consists of the following (in thousands): December 31, 2018 2017 Property and equipment, net North America $ 79,493 $ 79,736 International 3,810 4,760 Total property and equipment, net $ 83,303 $ 84,496 Goodwill North America $ 1,573,088 $ 1,253,494 International 38,447 29,963 Total goodwill $ 1,611,535 $ 1,283,457 Assets North America $ 3,253,035 $ 2,816,156 International 59,922 57,285 Total assets $ 3,312,957 $ 2,873,441 Liabilities North America $ 272,776 $ 201,831 International 18,239 20,360 Total liabilities $ 291,015 $ 222,191

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	12 Months Ended
	Dec. 31, 2018
Stockholders' Equity Note [Abstract]
STOCKHOLDERS' EQUITY	STOCKHOLDERS' EQUITY Preferred Stock The Company has 2 million shares of preferred stock, $0.01 par value, authorized for issuance as of December 31, 2018 . 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 million 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 In October 2017, the Company completed a public equity offering of 3.3 million shares of common stock for $260 per share. Net proceeds from the public equity offering were approximately $834 million , after deducting approximately $29 million of underwriting discounts and other fees. The Company used net proceeds from the public equity offering to fund the costs of strategic acquisitions, to finance business growth and for working capital and other general corporate purposes. The Company expects to use any remaining net proceeds from the equity offering to fund all or a portion of the costs of any additional strategic acquisitions the Company determines to pursue, to finance the growth of its business and for working capital and other 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, 2018
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, 2018 2017 2016 Numerator: Net income $ 238,334 $ 122,695 $ 85,071 Denominator: Denominator for basic net income per share — weighted-average outstanding shares 36,058 33,200 32,167 Effect of dilutive securities: Stock options and restricted stock awards 390 359 269 Denominator for diluted net income per share — weighted-average outstanding shares 36,448 33,559 32,436 Net income per share — basic $ 6.61 $ 3.70 $ 2.64 Net income per share — diluted $ 6.54 $ 3.66 $ 2.62 Stock options to purchase approximately 64,000 , 87,000 and 194,000 shares that were outstanding for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 , respectively, were not included in the computation of diluted net income per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Shares underlying restricted common stock awards 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. Shares underlying restricted 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 shares underlying the performance-based restricted stock awards and service-based restricted stock units excluded from the basic and diluted calculation (in thousands): Year Ended December 31, 2018 2017 2016 Performance-based restricted stock awards 53 58 59 Service-based restricted stock units 1 1 1 Total shares excluded from computation 54 59 60

EMPLOYEE BENEFIT PLANS

EMPLOYEE BENEFIT PLANS	12 Months Ended
	Dec. 31, 2018
Compensation Related Costs [Abstract]
EMPLOYEE BENEFIT PLANS	EMPLOYEE BENEFIT PLANS Stock Incentive Plans 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. In April 2016, the Company’s Board of Directors adopted the CoStar Group, Inc. 2016 Stock Incentive Plan (as amended, the “2016 Plan”), subject to stockholder approval, which was obtained on June 9, 2016. All shares of common stock that were authorized for issuance under the 2007 Plan that, as of June 9, 2016, remained available for issuance under the 2007 Plan (excluding shares subject to outstanding awards) were rolled into the 2016 Plan and, as of that date, no shares of common stock were available for new awards under the 2007 Plan. The 2007 Plan continues to govern unexercised and unexpired awards issued under the 2007 Plan prior to June 9, 2016. The 2007 Plan provided for the grant of stock options, restricted stock, restricted stock units and stock appreciation rights to officers, directors and employees of the Company and its subsidiaries. Stock options granted under the 2007 Plan could be incentive or non-qualified, and except in limited circumstances related to a merger or other acquisition, the exercise price for a 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, restricted stock and restricted stock unit grants under the 2007 Plan was determined by the Board of Directors or a committee thereof and was generally three to four years . In some cases, vesting of restricted stock awards under the 2007 Plan is subject to performance conditions. Upon the occurrence of a Change of Control, as defined in the 2007 Plan, all outstanding unexercisable options and restricted stock grants under the 2007 Plan immediately become exercisable. The 2016 Plan provides for the grant of stock options, restricted stock, restricted stock units, and stock appreciation rights to officers, directors and employees of the Company and its subsidiaries. Stock options granted under the 2016 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 2016 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 2016 Plan may be based on performance conditions. The Company has issued and/or reserved the following shares of common stock for issuance under the 2016 Plan: (a) 1,450,000 shares of common stock, plus (b) 815,464 shares of common stock that were authorized for issuance under the 2007 Plan that, as of June 9, 2016, remained available for issuance under the 2007 Plan (not including any Shares that were subject as of such date to outstanding awards under the 2007 Plan), and (c) any shares of common stock subject to outstanding awards under the 2007 Plan as of June 9, 2016, 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 2016 Plan will terminate in June 2026, but will continue to govern unexercised and unexpired awards issued under the 2016 Plan prior to that date. Approximately 2 million shares were available for future grant under the 2016 Plan as of December 31, 2018 . At December 31, 2018 , there was approximately $66 million of unrecognized compensation cost related to stock incentive plans, net of estimated forfeitures, which the Company expects to recognize over a weighted-average-period of 2.4 years . Stock Options Option activity was as follows: Number of Shares Range of Weighted- Average Exercise Price Weighted- Average Remaining Contract Life (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2015 400,077 $36.48 - $201.04 $ 122.30 Granted 82,400 $182.75 - $182.75 $ 182.75 Exercised (29,285 ) $36.48 - $201.04 $ 112.78 Canceled or expired (13,034 ) $193.69 - $201.04 $ 195.78 Outstanding at December 31, 2016 440,158 $36.48 - $201.04 $ 132.08 Granted 95,500 $204.91 $ 204.91 Exercised (81,815 ) $36.48 - $201.04 $ 83.07 Outstanding at December 31, 2017 453,843 $36.73 - $204.91 $ 156.24 Granted 82,500 342.13 $ 342.13 Exercised (177,299 ) $36.73 - $204.91 $ 125.16 Canceled or expired (14,768 ) $182.75 - $342.13 $ 261.20 Outstanding at December 31, 2018 344,276 $36.73 - $342.13 $ 212.28 7.03 $ 43,418 Exercisable at December 31, 2016 284,489 $36.48 - $201.04 $ 100.94 Exercisable at December 31, 2017 278,239 $36.73 - $201.04 $ 130.91 Exercisable at December 31, 2018 185,405 $54.51 - $204.91 $ 165.31 5.79 $ 31,895 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 177,299 , 81,815 , and 29,285 , shares were exercised during the years ended 2018 , 2017 and 2016 , respectively. The aggregate intrinsic value of options exercised, determined as of the date of option exercise, was approximately $45 million , $13 million and $3 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. The weighted-average grant date fair value of each option granted during the years ended December 31, 2018 , 2017 and 2016 using the Black-Scholes option-pricing model was $101.02 , $59.06 and $54.34 , respectively. The Company estimated the fair value of each option granted on the date of grant using the Black-Scholes option-pricing model, using the assumptions in the following table: Year Ended December 31, 2018 2017 2016 Dividend yield 0 % 0 % 0 % Expected volatility 28 % 28 % 31 % Risk-free interest rate 3 % 2 % 1 % Expected life (in years) 5 5 5 The expected dividend yield is determined based on the Company's past cash dividend history and anticipated future cash dividend payments. The Company has never declared or paid any dividends on its common stock and does not anticipate paying any dividends on its common stock during the foreseeable future, but intends to retain any earnings for future growth of its business. Expected volatility is calculated based on historical volatility of the daily closing price of the Company's common stock over a period consistent with the expected life of the options granted. The risk-free interest rate is based on the U.S. Treasury rate with terms similar to the expected life of the options granted. The expected life for the options is determined based on multiple factors, including historical employee behavior patterns of exercising options and post-employment termination behavior as well as expected future employee option exercise patterns. The following table summarizes information regarding options outstanding at December 31, 2018 : Options Outstanding Options Exercisable Range of Exercise Price Number of Shares Weighted-Average Remaining Contractual Life (in years) Weighted- Average Exercise Price Number of Shares Weighted- Average Exercise Price $54.51 - $78.33 300 1.92 $ 54.51 300 $ 54.51 $78.34 - $142.45 58,844 4.19 $ 102.16 58,844 $ 102.16 $142.46 - $188.22 57,768 7.19 $ 182.75 33,033 $ 182.75 $188.23 - $197.37 34,264 6.17 $ 193.69 34,264 $ 193.69 $197.38 - $202.98 34,600 5.16 $ 201.04 34,600 $ 201.04 $202.99 - $273.52 82,500 8.16 $ 204.91 24,364 $ 204.91 $273.53 - $342.13 76,000 9.16 $ 342.13 — $ — $54.51 - $342.13 344,276 7.03 $ 212.28 185,405 $ 165.31 Restricted Stock Awards In February 2018, March 2017 and March 2016, the Compensation Committee of the Board of Directors of the Company approved grants of restricted common stock to the executive officers that vest based on the Company’s achievement of a three-year cumulative revenue goal established at the grant date, and are subject to forfeiture in the event the foregoing performance condition is not met by the end of each respective three-year period. The number of shares that may be earned ranges between 0% (if the specified threshold performance level is not attained) and 200% (if performance meets or exceeds the maximum achievement level) of the target award. If actual performance exceeds the pre-established threshold, the number of shares earned is calculated based on the relative performance between specified levels of achievement. These awards support the Company’s goals of aligning executive incentives with long-term stockholder value and ensuring that executive officers have a continuing stake in the long-term success of the Company. These grants of restricted common stock are subject to continuing employment requirements and to a market condition. The actual number of shares that vest at the end of the respective three-year period is determined based on the Company’s achievement of the three -year performance goals described above, as well as its TSR relative to the Russell 1000 Index over the same three-year performance period. At the end of the three-year performance period, if the performance condition is achieved at or above the pre-established threshold, the number of shares earned is further adjusted by a TSR payout percentage, which ranges between 80% and 120% , 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 26,160 , 32,160 and 25,680 shares of performance-based restricted common stock during the years ended December 31, 2018 , 2017 and 2016 , respectively. The Company estimates the fair value of its performance-based restricted common stock awards with a market condition on the date of grant using a Monte-Carlo simulation valuation model. This pricing model uses multiple simulations to evaluate the probability of the Company achieving various stock price levels to determine the expected TSR performance ranking. Expense is only recorded for awards that are expected to vest, net of estimated forfeitures. The assumptions used to estimate the fair value of performance-based restricted common stock awards with a market condition granted were as follows: Year Ended December 31, 2018 2017 2016 Dividend yield 0 % 0 % 0 % Expected volatility 28 % 28 % 28 % Risk-free interest rate 2 % 2 % 1 % Expected life (in years) 3 3 3 Weighted-average grant date fair value $ 342.13 $ 218.59 $ 184.97 The expected dividend yield is determined based on the Company's past cash dividend history and anticipated future cash dividend payments. The Company has never declared or paid any dividends on its common stock and does not anticipate paying any dividends on its common stock during the foreseeable future, but intends to retain any earnings for future growth of its business. Expected volatility is calculated based on historical volatility of the daily closing price of the common stock of the companies within the Russell 1000 Index over a period consistent with the expected life of the performance-based restricted common stock awards with a market condition. The risk-free interest rate is based on the U.S. Treasury rate with terms similar to the expected life of the performance-based restricted common stock awards with a market condition. The expected life is consistent with the performance measurement period of the performance-based restricted common stock awards with a market condition. As of December 31, 2018 , the Company reassessed the probability of achieving the performance and market conditions and determined that it was probable that the performance and market conditions for the 2018, 2017 and 2016 performance-based restricted common stock awards would be met by their forfeiture dates. As a result, the Company recorded a total of approximately $5 million , $5 million and $3 million of stock-based compensation expense related to the performance-based restricted common stock awards with a market condition for the years ended December 31, 2018 , 2017 and 2016 , respectively. The Company expects to record an aggregate amount of stock-based compensation expense related to the performance-based restricted common stock awards of approximately $7 million over the periods 2019 , 2020 and 2021 . The following table presents unvested restricted stock awards activity for the year ended December 31, 2018 : Restricted Stock Awards — without Market Condition Restricted Stock Awards — with Market Condition Number of Shares Weighted-Average Grant Date Fair Value per Share Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock awards at December 31, 2017 363,883 $ 206.59 85,200 $ 205.08 Granted 134,209 $ 358.51 26,160 $ 380.24 Vested (151,995 ) $ 193.70 (27,360 ) $ 361.20 Canceled (41,936 ) $ 258.19 (7,680 ) $ 361.20 Unvested restricted stock awards at December 31, 2018 304,161 $ 272.95 76,320 $ 193.44 Restricted Stock Units The following table presents unvested restricted stock units activity for the year ended December 31, 2018 : Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock units at December 31, 2017 962 $ 190.27 Granted 214 $ 342.13 Vested (324 ) $ 189.08 Canceled — $ — Unvested restricted stock units at December 31, 2018 852 $ 228.86 Management Stock Purchase Plan The Board of Directors adopted the Company’s Management Stock Purchase Plan (the “MSPP”) in December 2017 with the intent of providing selected key employees of the Company and its subsidiaries, including the Company's executive officers, the opportunity to defer a portion of their cash incentive compensation and to align management and stockholder interests through awards of Deferred Stock Units (“DSUs”) under the MSPP and awards of matching restricted stock units (“Matching RSUs”) issued under the Company 2016 Plan. Commencing with cash incentive compensation for calendar year 2018, participants were permitted to elect to defer up to 100% of their annual incentive bonus earned during the calendar year or commissions earned from March through December of 2018 by submitting an irrevocable election in accordance with Section 409A of the Internal Revenue Code, as amended. On the date the incentive bonus or commission would otherwise be paid in cash (typically during the following calendar year), the Company will award to the participant DSUs representing a number of shares of common stock having an aggregate fair market value on that date equal to the amount of compensation elected to be deferred under the MSPP. On the same date the DSUs are awarded, a participant will receive a grant of Matching RSUs covering a number of shares of common stock equal to 100% of the DSUs granted. The expense related to the DSUs will be recognized at the grant date and the expense related to the Matching RSUs will be recognized over the four year vesting period following the grant date. There were no grants of DSUs or Matching RSUs and no expense recognized under the MSPP in 2018. Employee 401(k) Plan The Company maintains a 401(k) Plan (the “401(k)”) as a defined contribution retirement plan for all eligible employees. The 401(k) provides for tax-deferred contributions of employees’ salaries, limited to a maximum annual amount as established by the IRS. In addition to the traditional 401(k), effective January 1, 2015, eligible employees have the option of making an after-tax contribution to a Roth 401(k) plan or a combination of both. In 2018 , 2017 and 2016 , 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, 2018 , 2017 and 2016 were approximately $12 million , $10 million and $9 million , respectively. The Company had no administrative expenses in connection with the 401(k) plan for the years ended December 31, 2018 , 2017 and 2016 , respectively. 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 2018 , 2017 and 2016 , 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, 2018 , 2017 and 2016 , were approximately $0.5 million, $0.4 million and $0.4 million respectively. Registered Retirement Savings Plan As of January 1, 2015, the Company introduced a registered retirement savings plan (“RRSP”) for all eligible employees in the Company’s Canadian offices. In 2017, 2016 and 2015, the Company matched 100% of employee contributions up to a maximum of 4% of total compensation. Amounts contributed to the RRSP by the Company to match employee contributions for the years ended December 31, 2018 , 2017 and 2016 were approximately $ 58,000 , $43,000 and $10,000 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. On June 3, 2015, the Company’s stockholders approved an amendment to the ESPP to increase the number of shares available for purchase under the ESPP by 100,000 shares. On September 14, 2015, the Company registered the issuance of these additional shares under the ESPP pursuant to the registration statement filed September 14, 2015. There were 65,174 and 80,022 shares available for purchase under the ESPP as of December 31, 2018 and 2017 , respectively, and approximately 14,848 and 13,790 shares of the Company’s common stock were purchased under the ESPP during 2018 and 2017 , respectively.

QUARTERLY RESULTS OF OPERATIONS

QUARTERLY RESULTS OF OPERATIONS	12 Months Ended
	Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]
QUARTERLY RESULTS OF OPERATIONS	QUARTERLY RESULTS OF OPERATIONS The following is a summary of the unaudited quarterly results of operations for the years ended December 31, 2018 and 2017 . Information about prior period acquisitions and the adoption of recent accounting pronouncements that may affect the comparability of the quarterly financial information presented below are included in Note 2 and Note 4 . 2018 Mar. 31 Jun. 30 Sep. 30 Dec. 31 Revenues $ 273,718 $ 297,018 $ 305,525 $ 315,571 Cost of revenues 62,477 67,136 72,072 68,248 Gross profit 211,241 229,882 233,453 247,323 Operating expenses 157,796 186,108 162,765 141,666 Income from operations 53,445 43,774 70,688 105,657 Interest and other income 2,987 2,652 3,035 4,607 Interest and other expense (690 ) (728 ) (717 ) (695 ) Income before income taxes 55,742 45,698 73,006 109,569 Income tax expense 3,511 1,863 14,247 26,060 Net income $ 52,231 $ 43,835 $ 58,759 $ 83,509 Net income per share — basic $ 1.46 $ 1.22 $ 1.63 $ 2.31 Net income per share — diluted $ 1.44 $ 1.20 $ 1.61 $ 2.29 2017 Mar. 31 Jun. 30 Sep. 30 Dec. 31 Revenues $ 226,553 $ 237,153 $ 247,533 $ 253,991 Cost of revenues 51,346 55,273 55,483 58,301 Gross profit 175,207 181,880 192,050 195,690 Operating expenses 137,545 153,997 134,537 144,932 Income from operations 37,662 27,883 57,513 50,758 Interest and other income 429 605 555 2,455 Interest and other expense (2,686 ) (2,693 ) (2,901 ) (734 ) Loss on debt extinguishment — — — (3,788 ) Income before income taxes 35,405 25,795 55,167 48,691 Income tax expense 13,275 3,611 20,990 4,487 Net income $ 22,130 $ 22,184 $ 34,177 $ 44,204 Net income per share — basic $ 0.69 $ 0.68 $ 1.05 $ 1.24 Net income per share — diluted $ 0.68 $ 0.68 $ 1.04 $ 1.22

Schedule II Valuation and Quali

Schedule II Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2018
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]
Schedule II - Valuation and Qualifying Accounts	Schedule II – Valuation and Qualifying Accounts The table below details the activity of the allowance for doubtful accounts and sales credits (1) for the years ended December 31, 2018 , 2017 , and 2016 (in thousands): Balance at Beginning of Year Charged to Expense Reductions Balance at End of Year Year ended December 31, 2016 $ 7,478 $ 7,358 $ 8,492 $ 6,344 Year ended December 31, 2017 $ 6,344 $ 5,690 $ 5,565 $ 6,469 Year ended December 31, 2018 $ 6,469 $ 6,542 $ 7,302 $ 5,709 __________________________ (1) Additions to the allowance for doubtful accounts are charged to bad debt expense. Additions to the allowance for sales credits are charged against revenues.

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Accounting policies are consistent for each operating segment.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company evaluates its estimates and assumptions, including those related to revenue recognition, allowance for doubtful accounts, useful lives of property and equipment and intangible assets, recoverability of long-lived assets and intangible assets with definite lives, goodwill, income taxes, fair value of equity instruments, fair value of auction rate securities (“ARS”), accounting for business combinations, stock-based compensation and contingencies, among others. The Company bases these estimates on historical and anticipated results, trends, and various other assumptions that it believes are reasonable, including assumptions as to future events. These estimates form the basis for making judgments about the carrying values of assets and liabilities and recorded revenues and expenses. Actual results could differ from these estimates.
Revenue Recognition	Revenue Recognition Subsequent to the Adoption of Accounting Standards Update (“ASU") 2014-09, Revenue from Contracts with Customers, later codified as Accounting Standards Codification 606 ("ASC 606"), on January 1, 2018 The Company derives revenues primarily by (i) providing access to its proprietary database of commercial real estate information and (ii) providing online marketplaces for professional property management companies, property owners, brokers, and landlords, in each case typically through a fixed monthly fee for its subscription-based services. The Company's subscription-based services consist primarily of information, analytics and online marketplace services offered over the Internet to commercial real estate industry and related professionals. Subscription contract rates are based on the number of sites, number of users, organization size, the client’s business focus, geography, the number and types of services to which a client subscribes, the number of properties a client advertises and the prominence and placement of a client's advertised properties in the search results. A majority of the subscription-based license agreements have a term of one year and renew automatically. The Company analyzes contracts to determine the appropriate revenue recognition using the following steps: (i) identification of contracts with customers, (ii) identification of distinct performance obligations in the contract, (iii) determination of contract transaction price, (iv) allocation of contract transaction price to the performance obligations and (v) determination of revenue recognition based on timing of satisfaction of the performance obligation(s). The Company recognizes revenues upon the satisfaction of its performance obligation(s) (upon transfer of control of promised services to its customers) in an amount that reflects the consideration to which it expects to be entitled to in exchange for those services. Revenues from subscription-based services are recognized on a straight-line basis over the term of the agreement. The Company's contracts with customers often include promises to transfer multiple services. For these contracts, the Company accounts for individual performance obligations separately if they are distinct. Determining whether services are considered distinct performance obligations may require significant judgment. Judgment is required to determine the standalone selling price (“SSP”) for each distinct performance obligation. In instances where SSP is not directly observable, such as when the Company does not sell the services separately, the Company determines the SSP using available information, including market conditions and other observable inputs. Deferred revenue results from amounts billed in advance to customers or cash received from customers in advance of the sale of subscription licenses and is recognized over the term of the license agreement. Contract assets represent a conditional right to consideration for satisfied performance obligations that become a receivable when the conditions are satisfied. Contract assets are generated when contractual billing schedules differ from revenue recognition timing. Certain sales commissions are considered incremental and recoverable costs of obtaining a contract with a customer. Sales commissions incurred for obtaining new contracts are deferred and then amortized as selling and marketing expenses on a straight-line basis over a period of benefit that the Company has determined to be three years. The three-year amortization period was determined based on several factors, including the nature of the technology and proprietary data underlying the services being purchased, customer contract renewal rates and industry competition. Certain commission costs are not capitalized as they do not represent incremental costs of obtaining a contract. See Note 3 for further discussion on the impact of the adoption of ASC 606 .
Foreign Currency Translation	Foreign Currency Translation The Company’s functional currency in its foreign locations is the local currency. Assets and liabilities are translated into U.S. dollars using the exchange rates as of the balance sheet dates. Revenues, expenses, gains and losses are translated at the average exchange rates in effect during each period. Gains and losses resulting from translation are included in accumulated other comprehensive loss. Currency g ains and losses on the translation of intercompany loans made to foreign subsidiaries that are of a long-term investment nature are also included in accumulated other comprehensive loss. Net gains or losses resulting from foreign currency exchange transactions are included in the consolidated statements of operations.
Advertising Costs	Advertising Costs The Company expenses advertising costs as incurred. Advertising costs include e-commerce, television, radio, print and other media advertising.
Income Taxes	Income Taxes Deferred income taxes result from temporary differences between the tax basis of assets and liabilities and the basis reported in the Company’s consolidated financial statements. Deferred tax liabilities and assets are determined based on the difference between the financial statement and the tax basis of assets and liabilities using enacted rates in effect during the year in which the Company expects differences to reverse. Valuation allowances are provided against assets, including net operating losses, if the Company determines it is more likely than not that some portion or all of an asset may not be realized. 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 awards. 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 options to purchase approximately 64,000 , 87,000 and 194,000 shares that were outstanding for the years ended December 31, 2018 , December 31, 2017 and December 31, 2016 , respectively, were not included in the computation of diluted net income per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Shares underlying restricted common stock awards 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. Shares underlying restricted 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 services performed by officers, employees, and directors of the Company 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 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 will 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 as of December 31, 2018 and 2017 consisted of money market funds.
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's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities, and are classified as available-for-sale. The Company's auction rate security investments are carried at fair value and any changes in unrealized holding gains and losses, net of the related tax effect, 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 investment 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. Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis. and are carried at fair value.
Concentration of Credit Risk and Financial Instruments	Concentration of Credit Risk and Financial Instruments The Company performs ongoing assessments of its customers’ financial conditions and generally does not require that its customers’ obligations to the Company be secured. The Company maintains reserves for doubtful accounts, which have historically been immaterial to the Company's consolidated financial statements. Further, the large size of the Company’s customer base creates a lack of dependence on any individual customer that 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, 2018 , 2017 , and 2016 . The carrying amount of the accounts receivable approximates the net realizable value. The Company holds cash at major financial institutions that often exceed Federal Deposit Insurance Corporation insured limits. The Company manages its credit risk associated with cash concentrations by concentrating its cash deposits in high quality financial institutions and by periodically evaluating the credit quality of the primary financial institutions holding such deposits. The carrying value of cash approximates fair value. Historically, the Company has not experienced any losses due to such cash concentrations.
Accounts Receivable, Net of Allowance for Doubtful Accounts	Accounts Receivable, Net of Allowance for Doubtful Accounts Accounts receivable are recorded at the invoiced amount net of credits due. Accounts receivable payment terms vary and amounts due from customers are stated in the financial statements net of an allowance for doubtful accounts. When evaluating the adequacy of the allowance for doubtful accounts, the Company analyzes historical collection experience, changes in customer payment profiles and the aging of receivable balances, as well as current economic conditions, all of which 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 Vehicles Five to ten years Computer hardware and software Three 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 future economic benefits arising from a business combination and is calculated as the excess of the purchase consideration paid in a business combination over the fair value of assets acquired. Goodwill is not amortized, but instead is assigned to each of the Company's reporting units and tested for impairment at least annually or when events and circumstances indicate that the fair value of a reporting unit may be below its carrying value. 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 or elect to bypass such assessment. If it is determined that it is more likely than not that the fair value of a reporting unit is less than its carrying value, or the Company elects to bypass such assessment, the Company performs a quantitative test that requires the determination of 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 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 an impairment loss is recognized for the difference. Acquired technology, trade names and other intangible assets, and customer base assets are related to the Company’s acquisitions (see Notes 8 and 9 ). Acquired technology is amortized on a straight-line basis over periods ranging from two years to eight years . Acquired trade names and other intangible assets are amortized on a straight-line basis over periods ranging from one year to fifteen years . Acquired intangible assets characterized as customer base assets consist of acquired customer contracts and the related customer relationships and are amortized over periods ranging from five years to thirteen years . Acquired customer bases are amortized on an accelerated or straight-line basis depending on the expected economic benefit of the intangible asset.
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 are separately presented in the balance sheet and reported at the lower of the carrying amount or fair value less costs to sell, and are no longer depreciated. The assets and liabilities of a disposal group classified as held for sale are presented separately in the appropriate asset and liability sections of the balance sheet.
Debt Issuance Costs	Debt Issuance Costs Costs incurred in connection with the issuance of long-term debt are deferred and amortized as interest expense over the term of the related debt using the effective interest method for term debt and on a straight-line basis for revolving debt. To the extent that debt is outstanding, these amounts are reflected in the consolidated balance sheets as direct deductions from a combination of the current and long-term portions of debt for term debt and as current and long-term assets for costs related to revolving debt. Upon a refinancing or amendment, 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.
Business Combinations	Business Combinations The Company allocates the purchase consideration to the tangible assets acquired, liabilities assumed and intangible assets acquired based on their estimated fair values. The purchase price is determined based on the fair value of the assets transferred, liabilities incurred and equity interests issued, after considering any transactions that are separate from the business combination. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is recorded as goodwill. Such valuations require management to make significant estimates and assumptions, especially with respect to intangible assets and contingent liabilities. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired technology and acquired trade names, useful lives, royalty rates and discount rates. Any adjustments to provisional amounts that are identified during the measurement period are recorded in the reporting period in which the adjustment amounts are determined. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings. For a given acquisition, the Company may identify certain pre-acquisition contingencies as of the acquisition date and may extend its review and evaluation of these pre-acquisition contingencies throughout the measurement period in order to obtain sufficient information to assess whether the Company includes these contingencies as a part of the fair value estimates of assets acquired and liabilities assumed and, if so, to determine their estimated amounts. If the Company cannot reasonably determine the fair value of a pre-acquisition contingency (non-income tax related) by the end of the measurement period, which is generally the case given the nature of such matters, the Company will recognize an asset or a liability for such pre-acquisition contingency if: (i) it is probable that an asset existed or a liability had been assumed at the acquisition date and (ii) the amount of the asset or liability can be reasonably estimated. Subsequent to the measurement period, changes in the Company's estimates of such contingencies will affect earnings and could have a material effect on its results of operations and financial position. In addition, uncertain tax positions and tax related valuation allowances assumed in connection with a business combination are initially estimated as of the acquisition date. The Company reevaluates these items based upon facts and circumstances that existed as of the acquisition date, with any adjustments to its preliminary estimates being recorded to goodwill provided that the Company is within the measurement period. Subsequent to the measurement period, changes to these uncertain tax positions and tax related valuation allowances will affect the Company's provision for income taxes in its consolidated statements of operations and comprehensive income and could have a material impact on its results of operations and financial position.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) and International Accounting Standards Board (“IASB”) jointly issued a new revenue recognition standard, Accounting Standards Update (“ASU") 2014-09, Revenue from Contracts with Customers , later codified as Accounting Standards Codification ("ASC") 606 (" ASC 606" ), that is designed to improve financial reporting by creating common recognition guidance for GAAP and International Financial Reporting Standards (“IFRS”). This guidance provides a robust framework for addressing revenue issues, improves the comparability of revenue recognition practices across industries, provides 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 services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those services. On January 1, 2018, the Company adopted ASC 606, using the modified retrospective method. Results for reporting periods beginning subsequent to December 31, 2017 are presented under ASC 606 , while prior period amounts are not adjusted and continue to be reported in accordance with the Company’s historical accounting policies prior to adoption. In adopting the guidance, the Company applied the guidance to all customer contracts and used several available practical expedients including assessing contracts with similar terms and conditions on a “portfolio” basis and not including contracts with a duration of one year or less in the unsatisfied performance obligations disclosure. The Company recorded a net cumulative increase to beginning retained earnings of $54 million . The Company adjusted the condensed consolidated financial statements from amounts previously reported due to the adoption of ASC 606 . Select condensed consolidated balance sheet line items which were adjusted upon adoption were as follows (in thousands): As of December 31, 2017 ASC 606 Adjustments As of January 1, 2018 Assets Accounts receivable, less allowance for doubtful accounts $ 60,900 $ (1,867 ) $ 59,033 Prepaid expenses and other current assets 15,572 1,867 17,439 Deferred commissions costs, net — 71,118 71,118 Liabilities Deferred revenue $ 45,686 $ (1,716 ) $ 43,970 Deferred income taxes, net 12,070 18,370 30,440 Retained earnings 320,656 54,464 375,120 The impact of the adoption of ASC 606 on the condensed financial statements for the period ended December 31, 2018 was as follows (in thousands): As of December 31, 2018 without adoption of ASC 606 ASC 606 Adjustments As Reported as of December 31, 2018 Assets Accounts receivable, less allowance for doubtful accounts $ 91,122 $ (1,930 ) $ 89,192 Prepaid expenses and other current assets 21,760 1,930 23,690 Deferred commissions costs, net — 76,031 76,031 Liabilities Deferred revenue $ 57,284 $ (5,825 ) $ 51,459 Deferred income taxes, net 49,147 20,710 69,857 Retained earnings 552,308 61,146 613,454 If the Company had not adopted ASC 606 , revenue recognized would have been $4 million lower and selling and marketing expense would have been $5 million higher for the year ended December 31, 2018 . The impact on net income and basic and diluted earnings per share for the year ended December 31, 2018 would have been a decrease of approximately $7 million or $0.19 per share, respectively. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments , which is designed to reduce the existing diversity in how certain cash receipts and cash payments are presented and classified in the consolidated statements of cash flows. This guidance is effective on a retrospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company's consolidated financial statements and related disclosures. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business , which is designed to clarify the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance indicates that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set of assets is not a business. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company's consolidated financial statements and related disclosures. In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting , which is designed to reduce the existing diversity and complexity in the accounting for changes to terms or conditions of a share-based payment award. This guidance clarifies that an entity will not apply modification accounting to a share-based payment award if all of the following are the same immediately before and after the change: (i) the fair value of the award, (ii) the vesting conditions of the award, and (iii) the classification of the award as an equity instrument or liability instrument. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. On January 1, 2018, the Company adopted this guidance and the adoption did not have a material impact on the Company's consolidated financial statements and related disclosures. Recent Accounting Pronouncements Not Yet Adopted In February 2016, the FASB issued ASU 2016-02, Leases ("ASC 842") , to increase transparency and comparability among organizations' accounting for leases. The guidance requires a company to recognize right-of-use assets and lease liabilities on the balance sheet, as well as to disclose key quantitative and qualitative information about leasing arrangements. This guidance is effective on a modified retrospective basis for reporting periods beginning after December 15, 2018, with early adoption permitted. As permitted by the guidance, the Company will elect to retain the original lease classification and historical accounting for initial direct costs for leases existing prior to the adoption date. Furthermore, the Company will not have to reassess contracts entered into prior to the adoption date for the existence of a lease. The Company will also elect not to restate prior periods for the impact of the adoption of the new standard and will instead recognize a cumulative-effect adjustment to beginning retained earnings as of January 1, 2019 for any prior period income statement effects identified. The Company assessed the changes required to support the adoption of the new standard, as well as the quantitative impact this guidance will have on its financial statements and related disclosures. As a result, the Company expects that the adoption of this standard will result in the recognition of Right of Use Assets between $100 million and $120 million , as well as Lease Liabilities between $140 million and $160 million on its consolidated balance sheet, primarily as a result of recognizing assets and liabilities associated with existing office leases . Lastly, the Company expects to recognize a cumulative-effect adjustment to beginning retained earnings of $12 million , net of tax, as of January 1, 2019 to recognize the remaining gain on the Company's outstanding deferred gain on the sale of building, pursuant to the guidance in ASC 842 . Beginning in 2019, the Company expects significant changes to its disclosed lease recognition policies and practices, as well as to other related financial statement disclosures due to the adoption of this standard. These revised disclosures will be made in the Company’s first quarterly report in 2019. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurements (subsequent to adoption of ASU 2018-13, Fair Value Measurement. The ASU was issued to eliminate certain disclosure requirements for fair value measurements, and add and modify other disclosure requirements, as part of its disclosure framework project, including additional requirements for public companies to disclose certain information about the significant unobservable inputs for Level 3 fair value measurements. This guidance is effective for fiscal years beginning after December 15, 2019 and for interim periods within those fiscal years. The Company is currently evaluating the impact this guidance will have on its financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 requires a customer in a cloud computing arrangement that is a service contract to follow the internal-use software guidance in Accounting Standards Codification 350-40 to determine which implementation costs to defer and recognize as an asset. For public business entities, the guidance is effective for annual and interim periods beginning after December 15, 2019. The Company is currently evaluating the impact this guidance will have on its consolidated financial statements and related disclosures.
Liability Reserve Estimate	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.

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of accumulated other comprehensive loss	The components of accumulated other comprehensive loss were as follows (in thousands): As of December 31, 2018 2017 Foreign currency translation adjustment $ (10,958 ) $ (8,290 ) Net unrealized loss on investments, net of tax (730 ) (730 ) Total accumulated other comprehensive loss $ (11,688 ) $ (9,020 )
Stock-based compensation expense for stock options and restricted stock	Stock-based compensation expense for stock options and restricted stock awards issued under equity incentive plans and stock purchases under the Employee Stock Purchase Plan ("ESPP") included in the Company’s results of operations were as follows (in thousands): Year Ended December 31, 2018 2017 2016 Cost of revenues (1) $ 7,688 $ 4,971 $ 5,495 Selling and marketing (excluding customer base amortization) 6,881 7,086 6,634 Software development 7,454 7,071 6,546 General and administrative 20,695 19,902 17,674 Total stock-based compensation $ 42,718 $ 39,030 $ 36,349 __________________________ (1) Includes $1.5 million of expense related to the cash settlement of stock options in connection with the acquisition of Cozy Services, Ltd. See Note 4 for details of the acquisition.
Schedule of property and equipment	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 Vehicles Five to ten years Computer hardware and software Three to five years Property and equipment consists of the following (in thousands): December 31, 2018 2017 Leasehold improvements $ 65,332 $ 59,447 Furniture, office equipment and vehicles 53,020 52,163 Computer hardware and software 74,742 71,281 Property and equipment, gross 193,094 182,891 Accumulated depreciation and amortization (109,791 ) (98,395 ) Property and equipment, net $ 83,303 $ 84,496
Schedule of new accounting pronouncements and changes in accounting principles	elect condensed consolidated balance sheet line items which were adjusted upon adoption were as follows (in thousands): As of December 31, 2017 ASC 606 Adjustments As of January 1, 2018 Assets Accounts receivable, less allowance for doubtful accounts $ 60,900 $ (1,867 ) $ 59,033 Prepaid expenses and other current assets 15,572 1,867 17,439 Deferred commissions costs, net — 71,118 71,118 Liabilities Deferred revenue $ 45,686 $ (1,716 ) $ 43,970 Deferred income taxes, net 12,070 18,370 30,440 Retained earnings 320,656 54,464 375,120 The impact of the adoption of ASC 606 on the condensed financial statements for the period ended December 31, 2018 was as follows (in thousands): As of December 31, 2018 without adoption of ASC 606 ASC 606 Adjustments As Reported as of December 31, 2018 Assets Accounts receivable, less allowance for doubtful accounts $ 91,122 $ (1,930 ) $ 89,192 Prepaid expenses and other current assets 21,760 1,930 23,690 Deferred commissions costs, net — 76,031 76,031 Liabilities Deferred revenue $ 57,284 $ (5,825 ) $ 51,459 Deferred income taxes, net 49,147 20,710 69,857 Retained earnings 552,308 61,146 613,454

REVENUE FROM CONTRACTS WITH C_2

REVENUE FROM CONTRACTS WITH CUSTOMERS (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Disaggregation of Revenue	The Company provides information, analytics and online marketplaces to the commercial real estate industry and related professionals. The revenues by operating segment and type of service consist of the following (in thousands): Year Ended December 31, 2018 2017 North America International Total North America International Total Information and analytics CoStar Suite $ 519,661 $ 25,534 $ 545,195 $ 440,534 $ 22,651 $ 463,185 Information services 58,708 8,916 67,624 64,503 8,115 72,618 Online marketplaces Multifamily 405,795 — 405,795 279,855 — 279,855 Commercial property and land 173,137 81 173,218 149,572 — 149,572 Total revenues $ 1,157,301 $ 34,531 $ 1,191,832 $ 934,464 $ 30,766 $ 965,230
Contract with Customer, Asset and Liability	Changes in deferred revenue for the period were as follows (in thousands): Balance at December 31, 2017 $ 45,686 Cumulative effect of adoption of ASC 606 (1,716 ) Balance at January 1, 2018 43,970 Revenue recognized in the current period from the amounts in the beginning balance (43,121 ) New deferrals, net of amounts recognized in the current period 51,000 Effects of foreign currency (390 ) Balance at December 31, 2018 $ 51,459
Schedule of Commissions Expense	Commissi ons expense activity for the year ended December 31, 2018 was as follows (in thousands): Year Ended December 31, 2018 Commissions incurred $ 72,899 Commissions capitalized in the current period (53,497 ) Amortization of deferred commissions costs 48,313 Total commissions expense $ 67,715

ACQUISITIONS (Tables)

ACQUISITIONS (Tables)	12 Months Ended
	Dec. 31, 2018
Business Combinations [Abstract]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	The following table summarizes the amounts recorded for acquired assets and assumed liabilities recorded at their fair values as of the Acquisition Date (in thousands): Preliminary: February 21, 2018 Measurement Period Adjustments Final: February 21, 2018 Cash and cash equivalents $ 59 $ — $ 59 Accounts receivable 8,769 — 8,769 Indemnification asset 5,443 — 5,443 Goodwill 266,720 (125 ) 266,595 Intangible assets 141,300 — 141,300 Deferred tax liabilities (34,032 ) — (34,032 ) Contingent sales tax liability (6,260 ) — (6,260 ) State uncertain income tax position liability (2,047 ) — (2,047 ) Other assets and liabilities (3,453 ) (82 ) (3,535 ) Fair value of identifiable net assets acquired $ 376,499 $ (207 ) $ 376,292
Business Acquisition, Pro Forma Information	The unaudited pro forma financial information was as follows (in thousands, except per share data): Year Ended 2018 2017 Revenue $ 1,205,584 $ 1,067,742 Net income $ 251,196 $ 103,000 Net income per share - basic $ 6.96 $ 3.09 Net income per share - diluted $ 6.89 $ 3.06

INVESTMENTS (Tables)

INVESTMENTS (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 are as follows (in thousands): Maturity Fair Value Due in: 2019 $ — 2020 — 2023 — 2024 — 2028 — 2029 and thereafter 10,070 Available-for-sale investments $ 10,070
Schedule of available for sale securities reconciliation	As of December 31, 2018 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 10,800 $ — $ (730 ) $ 10,070 Available-for-sale investments $ 10,800 $ — $ (730 ) $ 10,070 As of December 31, 2017 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 10,800 $ — $ (730 ) $ 10,070 Available-for-sale investments $ 10,800 $ — $ (730 ) $ 10,070
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, 2018 2017 Aggregate Fair Value Gross Unrealized Losses Aggregate Fair Value Gross Unrealized Losses Auction rate securities $ 10,070 $ (730 ) $ 10,070 $ (730 ) Investments in an unrealized loss position $ 10,070 $ (730 ) $ 10,070 $ (730 )

FAIR VALUE (Tables)

FAIR VALUE (Tables)	12 Months Ended
	Dec. 31, 2018
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 equivalents and investments) measured at fair value on a recurring basis as of December 31, 2018 (in thousands): Level 1 Level 2 Level 3 Total Assets: Money market funds $ 590,567 $ — $ — $ 590,567 Auction rate securities — — 10,070 10,070 Total assets measured at fair value $ 590,567 $ — $ 10,070 $ 600,637 The following table represents the Company's fair value hierarchy for its financial assets (cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2017 (in thousands): Level 1 Level 2 Level 3 Total Assets: Money market funds $ 586,084 $ — $ — $ 586,084 Auction rate securities — — 10,070 10,070 Total assets measured at fair value $ 586,084 $ — $ 10,070 $ 596,154
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, 2016 to December 31, 2018 (in thousands): Auction Rate Securities Balance at December 31, 2016 $ 9,952 Decrease in unrealized loss included in accumulated other comprehensive loss 118 Balance at December 31, 2017 10,070 Decrease in unrealized loss included in accumulated other comprehensive loss — Settlements — Balance at December 31, 2018 $ 10,070

PROPERTY AND EQUIPMENT (Tables)

PROPERTY AND EQUIPMENT (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of property and equipment	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 Vehicles Five to ten years Computer hardware and software Three to five years Property and equipment consists of the following (in thousands): December 31, 2018 2017 Leasehold improvements $ 65,332 $ 59,447 Furniture, office equipment and vehicles 53,020 52,163 Computer hardware and software 74,742 71,281 Property and equipment, gross 193,094 182,891 Accumulated depreciation and amortization (109,791 ) (98,395 ) Property and equipment, net $ 83,303 $ 84,496

GOODWILL (Tables)

GOODWILL (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2016 $ 1,227,777 $ 27,089 $ 1,254,866 Acquisition 25,717 — 25,717 Effect of foreign currency translation — 2,874 2,874 Goodwill, December 31, 2017 1,253,494 29,963 1,283,457 Acquisitions 319,594 10,344 329,938 Effect of foreign currency translation — (1,860 ) (1,860 ) Goodwill, December 31, 2018 $ 1,573,088 $ 38,447 $ 1,611,535

INTANGIBLE ASSETS (Tables)

INTANGIBLE ASSETS (Tables)	12 Months Ended
	Dec. 31, 2018
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Schedule of acquired finite-lived intangible assets by major class	Intangible assets consist of the following (in thousands, except amortization period data): December 31, Weighted- Average Amortization Period (in years) 2018 2017 Capitalized product development cost $ 2,173 $ 2,275 4 Accumulated amortization (2,173 ) (2,262 ) Capitalized product development cost, net — 13 Building photography 9,035 18,739 2 Accumulated amortization (8,809 ) (18,212 ) Building photography, net 226 527 Acquired technology 103,128 83,469 4 Accumulated amortization (85,344 ) (79,188 ) Acquired technology, net 17,784 4,281 Acquired customer base 339,574 225,879 10 Accumulated amortization (199,405 ) (169,157 ) Acquired customer base, net 140,169 56,722 Acquired trade names and other intangible assets 190,717 167,718 12 Accumulated amortization (59,985 ) (46,369 ) Acquired trade names and other intangible assets, net 130,732 121,349 Intangible assets, net $ 288,911 $ 182,892

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of components of income tax expense	The components of the provision for income taxes attributable to operations consist of the following (in thousands): Year Ended December 31, 2018 2017 2016 Current: Federal $ 36,167 $ 41,453 $ 32,198 State 5,140 3,518 3,682 Foreign 708 295 76 Total current 42,015 45,266 35,956 Deferred: Federal 6,576 (7,917 ) 12,586 State (2,582 ) 4,695 3,014 Foreign (328 ) 319 35 Total deferred 3,666 (2,903 ) 15,635 Total provision for income taxes $ 45,681 $ 42,363 $ 51,591
Schedule of deferred tax assets and liabilities	The components of deferred tax assets and liabilities consist of the following (in thousands): December 31, 2018 2017 Deferred tax assets: Reserve for bad debts $ 1,457 $ 1,636 Accrued compensation 4,803 6,706 Stock compensation 10,041 10,568 Net operating losses 26,349 25,899 Accrued reserve and other 1,773 1,578 Deferred rent 5,928 6,533 Deferred gain on the sale of building 4,140 4,741 Research and development credits 6,331 — Total deferred tax assets, prior to valuation allowance 60,822 57,661 Valuation allowance (14,246 ) (13,032 ) Total deferred tax assets, net of valuation allowance 46,576 44,629 Deferred tax liabilities: Deferred commission costs, net (19,314 ) — Prepaid expenses (2,204 ) (1,239 ) Property and equipment, net (5,367 ) (6,229 ) Intangible assets, net (82,079 ) (43,800 ) Total deferred tax liabilities (108,964 ) (51,268 ) Net deferred tax assets (liabilities) $ (62,388 ) $ (6,639 )
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, 2018 2017 2016 Expected federal income tax provision at statutory rate $ 59,643 $ 57,770 $ 47,832 State income taxes, net of federal benefit 10,312 4,776 3,638 Foreign income taxes, net effect (315 ) (3,540 ) (31 ) Increase (decrease) in valuation allowance 1,214 3,624 (103 ) Tax rate changes 141 (7,340 ) 283 Research credits (15,373 ) (20,547 ) (920 ) Excess tax benefit (14,227 ) (7,010 ) — Tax reserves 1,870 12,646 (150 ) Other adjustments 2,416 1,984 1,042 Income tax expense $ 45,681 $ 42,363 $ 51,591
Schedule of unrecognized tax benefits	The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands): Unrecognized tax benefit as of December 31, 2015 $ 7,664 Increase for current year tax positions 368 Decrease for prior year tax positions (6,115 ) Expiration of the statute of limitation for assessment of taxes (74 ) Unrecognized tax benefit as of December 31, 2016 1,843 Increase for current year tax positions 12,620 Decrease for prior year tax positions (34 ) Expiration of the statute of limitation for assessment of taxes (66 ) Unrecognized tax benefit as of December 31, 2017 14,363 Increase for current year tax positions 9,561 Decrease for prior year tax positions (70 ) Expiration of the statute of limitation for assessment of taxes (1,482 ) Unrecognized tax benefit as of December 31, 2018 $ 22,372

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Future minimum lease payments	Future minimum lease payments as of December 31, 2018 are as follows (in thousands): 2019 $ 30,485 2020 29,255 2021 27,421 2022 25,634 2023 24,515 Thereafter 31,768 Total future minimum lease payments $ 169,078

SEGMENT REPORTING (Tables)

SEGMENT REPORTING (Tables)	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Summarized information by operating segment	Summarized information by operating segment consists of the following (in thousands): Year Ended December 31, 2018 2017 2016 EBITDA North America $ 358,036 $ 236,906 $ 210,901 International (6,729 ) 553 4,169 Total EBITDA $ 351,307 $ 237,459 $ 215,070
Reconciliation of net income to EBITDA	The reconciliation of net income to EBITDA consists of the following (in thousands): Year Ended December 31, 2018 2017 2016 Net income $ 238,334 $ 122,695 $ 85,071 Amortization of acquired intangible assets in cost of revenues 20,586 19,707 22,819 Amortization of acquired intangible assets in operating expenses 30,881 17,684 22,731 Depreciation and other amortization 26,276 26,252 24,615 Interest and other income (13,281 ) (4,044 ) (1,773 ) Interest and other expense 2,830 9,014 10,016 Loss on debt extinguishment — 3,788 — Income tax expense 45,681 42,363 51,591 EBITDA $ 351,307 $ 237,459 $ 215,070
Summarized information by operating segment, assets and liabilities	Summarized information by operating segment consists of the following (in thousands): December 31, 2018 2017 Property and equipment, net North America $ 79,493 $ 79,736 International 3,810 4,760 Total property and equipment, net $ 83,303 $ 84,496 Goodwill North America $ 1,573,088 $ 1,253,494 International 38,447 29,963 Total goodwill $ 1,611,535 $ 1,283,457 Assets North America $ 3,253,035 $ 2,816,156 International 59,922 57,285 Total assets $ 3,312,957 $ 2,873,441 Liabilities North America $ 272,776 $ 201,831 International 18,239 20,360 Total liabilities $ 291,015 $ 222,191

NET INCOME (LOSS) PER SHARE (Ta

NET INCOME (LOSS) PER SHARE (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Calculation of basic and diluted net income (loss) per share	The following table sets forth the calculation of basic and diluted net income per share (in thousands except per share data): Year Ended December 31, 2018 2017 2016 Numerator: Net income $ 238,334 $ 122,695 $ 85,071 Denominator: Denominator for basic net income per share — weighted-average outstanding shares 36,058 33,200 32,167 Effect of dilutive securities: Stock options and restricted stock awards 390 359 269 Denominator for diluted net income per share — weighted-average outstanding shares 36,448 33,559 32,436 Net income per share — basic $ 6.61 $ 3.70 $ 2.64 Net income per share — diluted $ 6.54 $ 3.66 $ 2.62
Schedule of anti-dilutive securities excluded from computation of earnings per share	The following table summarizes the shares underlying the performance-based restricted stock awards and service-based restricted stock units excluded from the basic and diluted calculation (in thousands): Year Ended December 31, 2018 2017 2016 Performance-based restricted stock awards 53 58 59 Service-based restricted stock units 1 1 1 Total shares excluded from computation 54 59 60

EMPLOYEE BENEFIT PLANS (Tables)

EMPLOYEE BENEFIT PLANS (Tables)	12 Months Ended
	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of option activity	Option activity was as follows: Number of Shares Range of Weighted- Average Exercise Price Weighted- Average Remaining Contract Life (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2015 400,077 $36.48 - $201.04 $ 122.30 Granted 82,400 $182.75 - $182.75 $ 182.75 Exercised (29,285 ) $36.48 - $201.04 $ 112.78 Canceled or expired (13,034 ) $193.69 - $201.04 $ 195.78 Outstanding at December 31, 2016 440,158 $36.48 - $201.04 $ 132.08 Granted 95,500 $204.91 $ 204.91 Exercised (81,815 ) $36.48 - $201.04 $ 83.07 Outstanding at December 31, 2017 453,843 $36.73 - $204.91 $ 156.24 Granted 82,500 342.13 $ 342.13 Exercised (177,299 ) $36.73 - $204.91 $ 125.16 Canceled or expired (14,768 ) $182.75 - $342.13 $ 261.20 Outstanding at December 31, 2018 344,276 $36.73 - $342.13 $ 212.28 7.03 $ 43,418 Exercisable at December 31, 2016 284,489 $36.48 - $201.04 $ 100.94 Exercisable at December 31, 2017 278,239 $36.73 - $201.04 $ 130.91 Exercisable at December 31, 2018 185,405 $54.51 - $204.91 $ 165.31 5.79 $ 31,895
Summarized information regarding options outstanding	The following table summarizes information regarding options outstanding at December 31, 2018 : Options Outstanding Options Exercisable Range of Exercise Price Number of Shares Weighted-Average Remaining Contractual Life (in years) Weighted- Average Exercise Price Number of Shares Weighted- Average Exercise Price $54.51 - $78.33 300 1.92 $ 54.51 300 $ 54.51 $78.34 - $142.45 58,844 4.19 $ 102.16 58,844 $ 102.16 $142.46 - $188.22 57,768 7.19 $ 182.75 33,033 $ 182.75 $188.23 - $197.37 34,264 6.17 $ 193.69 34,264 $ 193.69 $197.38 - $202.98 34,600 5.16 $ 201.04 34,600 $ 201.04 $202.99 - $273.52 82,500 8.16 $ 204.91 24,364 $ 204.91 $273.53 - $342.13 76,000 9.16 $ 342.13 — $ — $54.51 - $342.13 344,276 7.03 $ 212.28 185,405 $ 165.31
Unvested restricted stock awards activity	The following table presents unvested restricted stock awards activity for the year ended December 31, 2018 : Restricted Stock Awards — without Market Condition Restricted Stock Awards — with Market Condition Number of Shares Weighted-Average Grant Date Fair Value per Share Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock awards at December 31, 2017 363,883 $ 206.59 85,200 $ 205.08 Granted 134,209 $ 358.51 26,160 $ 380.24 Vested (151,995 ) $ 193.70 (27,360 ) $ 361.20 Canceled (41,936 ) $ 258.19 (7,680 ) $ 361.20 Unvested restricted stock awards at December 31, 2018 304,161 $ 272.95 76,320 $ 193.44
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, 2018 2017 2016 Dividend yield 0 % 0 % 0 % Expected volatility 28 % 28 % 31 % Risk-free interest rate 3 % 2 % 1 % Expected life (in years) 5 5 5
Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value assumption for options granted	The assumptions used to estimate the fair value of performance-based restricted common stock awards with a market condition granted were as follows: Year Ended December 31, 2018 2017 2016 Dividend yield 0 % 0 % 0 % Expected volatility 28 % 28 % 28 % Risk-free interest rate 2 % 2 % 1 % Expected life (in years) 3 3 3 Weighted-average grant date fair value $ 342.13 $ 218.59 $ 184.97
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, 2018 : Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock units at December 31, 2017 962 $ 190.27 Granted 214 $ 342.13 Vested (324 ) $ 189.08 Canceled — $ — Unvested restricted stock units at December 31, 2018 852 $ 228.86

QUARTERLY RESULTS OF OPERATIO_2

QUARTERLY RESULTS OF OPERATIONS (Tables)	12 Months Ended
	Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]
Quarterly Financial Information	The following is a summary of the unaudited quarterly results of operations for the years ended December 31, 2018 and 2017 . Information about prior period acquisitions and the adoption of recent accounting pronouncements that may affect the comparability of the quarterly financial information presented below are included in Note 2 and Note 4 . 2018 Mar. 31 Jun. 30 Sep. 30 Dec. 31 Revenues $ 273,718 $ 297,018 $ 305,525 $ 315,571 Cost of revenues 62,477 67,136 72,072 68,248 Gross profit 211,241 229,882 233,453 247,323 Operating expenses 157,796 186,108 162,765 141,666 Income from operations 53,445 43,774 70,688 105,657 Interest and other income 2,987 2,652 3,035 4,607 Interest and other expense (690 ) (728 ) (717 ) (695 ) Income before income taxes 55,742 45,698 73,006 109,569 Income tax expense 3,511 1,863 14,247 26,060 Net income $ 52,231 $ 43,835 $ 58,759 $ 83,509 Net income per share — basic $ 1.46 $ 1.22 $ 1.63 $ 2.31 Net income per share — diluted $ 1.44 $ 1.20 $ 1.61 $ 2.29 2017 Mar. 31 Jun. 30 Sep. 30 Dec. 31 Revenues $ 226,553 $ 237,153 $ 247,533 $ 253,991 Cost of revenues 51,346 55,273 55,483 58,301 Gross profit 175,207 181,880 192,050 195,690 Operating expenses 137,545 153,997 134,537 144,932 Income from operations 37,662 27,883 57,513 50,758 Interest and other income 429 605 555 2,455 Interest and other expense (2,686 ) (2,693 ) (2,901 ) (734 ) Loss on debt extinguishment — — — (3,788 ) Income before income taxes 35,405 25,795 55,167 48,691 Income tax expense 13,275 3,611 20,990 4,487 Net income $ 22,130 $ 22,184 $ 34,177 $ 44,204 Net income per share — basic $ 0.69 $ 0.68 $ 1.05 $ 1.24 Net income per share — diluted $ 0.68 $ 0.68 $ 1.04 $ 1.22

ORGANIZATION (Details)

ORGANIZATION (Details)	12 Months Ended
	Dec. 31, 2018operating_segments
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of business segments	2
Term of subscription-based license agreements (in years)	1 year

SUMMARY OF SIGNIFICANT ACCOUN_4

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Revenue Recognition and Advertising Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Term of subscription-based license agreements (in years)	1 year
Advertising costs	$ 124	$ 104	$ 109

SUMMARY OF SIGNIFICANT ACCOUN_5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Foreign Currency Translation and Accumulated Other Comprehensive Loss) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) Net of Tax [Abstract]
Material gains or losses from foreign currency exchange transactions	$ 0	$ 0	$ 0
Foreign currency translation adjustment	(10,958,000)	(8,290,000)
Net unrealized loss on investments, net of tax	(730,000)	(730,000)
Total accumulated other comprehensive loss	(11,688,000)	(9,020,000)
Reclassifications out of accumulated other comprehensive loss	0	0
Reclassification adjustment for realized gains on investments included in net income	$ 0	$ 0	$ (808,000)

SUMMARY OF SIGNIFICANT ACCOUN_6

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Stock-Based Compensation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation Expense [Abstract]
Compensation expense	$ 42,718	$ 39,030	$ 36,349
Expense related to stock options settled	1,500
Cost of revenues
Stock-Based Compensation Expense [Abstract]
Compensation expense	7,688	4,971	5,495
Selling and marketing (excluding customer base amortization)
Stock-Based Compensation Expense [Abstract]
Compensation expense	6,881	7,086	6,634
Software development
Stock-Based Compensation Expense [Abstract]
Compensation expense	7,454	7,071	6,546
General and administrative
Stock-Based Compensation Expense [Abstract]
Compensation expense	$ 20,695	$ 19,902	$ 17,674

SUMMARY OF SIGNIFICANT ACCOUN_7

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Property and Equipment) (Details)	12 Months Ended
	Dec. 31, 2018
Furniture and Office Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Furniture and Office Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	10 years
Vehicles [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Vehicles [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	10 years
Computer Hardware and Software [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	3 years
Computer Hardware and Software [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years

SUMMARY OF SIGNIFICANT ACCOUN_8

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Goodwill and Intangible Assets) (Details)	12 Months Ended
	Dec. 31, 2018
Acquired Database Technology [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	2 years
Acquired Database Technology [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	8 years
Acquired Trade Names and Other Intangible Assets [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	1 year
Acquired Trade Names and Other Intangible Assets [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	15 years
Acquired Customer Base [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	5 years
Acquired Customer Base [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	13 years

SUMMARY OF SIGNIFICANT ACCOUN_9

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Debt Issuance Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Debt issuance costs, net	$ 3	$ 4
Amortization of debt issuance costs	$ 1	$ 2	$ 3

SUMMARY OF SIGNIFICANT ACCOU_10

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Recent Accounting Pronouncements) (Details) - USD ($) $ in Thousands	Jan. 01, 2019	Jan. 01, 2018	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of new accounting standard, net of tax		$ 54,464
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of new accounting standard, net of tax		54,464	$ 2,162
Retained Earnings [Member] | Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of new accounting standard, net of tax		$ 54,000
Subsequent Event [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of new accounting standard, net of tax	$ 12,000
Subsequent Event [Member] | Minimum [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-Use asset	100,000
Lease liability	140,000
Subsequent Event [Member] | Maximum [Member] | Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Right-of-Use asset	120,000
Lease liability	$ 160,000

SUMMARY OF SIGNIFICANT ACCOU_11

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Effect of New Accounting Change) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, less allowance for doubtful accounts	$ 89,192	$ 59,033	$ 60,900
Prepaid expenses and other current assets	23,690	17,439	15,572
Deferred commission costs, net	76,031	71,118	0
Deferred revenue	51,459	43,970	45,686
Deferred income taxes, net	69,857	30,440	12,070
Retained earnings	613,454	375,120	320,656
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, less allowance for doubtful accounts	91,122		60,900
Prepaid expenses and other current assets	21,760		15,572
Deferred commission costs, net	0		0
Deferred revenue	57,284		45,686
Deferred income taxes, net	49,147		12,070
Retained earnings	552,308		$ 320,656
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, less allowance for doubtful accounts	(1,930)	(1,867)
Prepaid expenses and other current assets	1,930	1,867
Deferred commission costs, net	76,031	71,118
Deferred revenue	(5,825)	(1,716)
Deferred income taxes, net	20,710	18,370
Retained earnings	$ 61,146	$ 54,464

SUMMARY OF SIGNIFICANT ACCOU_12

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Impact of Adoptions) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Selling and marketing expense									$ 359,858	$ 318,362	$ 296,483
Net income	$ 83,509	$ 58,759	$ 43,835	$ 52,231	$ 44,204	$ 34,177	$ 22,184	$ 22,130	238,334	$ 122,695	$ 85,071
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenues									4,000
Accounting Standards Update 2016-09 [Member] | Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Selling and marketing expense									5,000
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income									$ (7,000)
Earnings per share basic and diluted (usd per share)									$ (0.19)

REVENUE FROM CONTRACTS WITH C_3

REVENUE FROM CONTRACTS WITH CUSTOMERS (Disaggregated Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Total revenues	$ 315,571	$ 305,525	$ 297,018	$ 273,718	$ 253,991	$ 247,533	$ 237,153	$ 226,553	$ 1,191,832	$ 965,230	$ 837,630
Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Total revenues									545,195	463,185
Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Total revenues									67,624	72,618
Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Total revenues									405,795	279,855
Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Total revenues									173,218	149,572
North America [Member]
Disaggregation of Revenue [Line Items]
Total revenues									1,157,301	934,464
North America [Member] | Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Total revenues									519,661	440,534
North America [Member] | Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Total revenues									58,708	64,503
North America [Member] | Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Total revenues									405,795	279,855
North America [Member] | Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Total revenues									173,137	149,572
International [Member]
Disaggregation of Revenue [Line Items]
Total revenues									34,531	30,766
International [Member] | Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Total revenues									25,534	22,651
International [Member] | Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Total revenues									8,916	8,115
International [Member] | Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Total revenues									0	0
International [Member] | Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Total revenues									$ 81	$ 0

REVENUE FROM CONTRACTS WITH C_4

REVENUE FROM CONTRACTS WITH CUSTOMERS (Contract Assets and Liabilities) (Details) - USD ($) $ in Thousands	Jan. 01, 2018	Dec. 31, 2018
Change in Contract with Customer, Liability [Roll Forward]
Beginning balance	$ 45,686	$ 45,686
Cumulative effect of adoption of ASC 606	(1,716)
Revenue recognized in the current period from the amounts in the beginning balance		(43,121)
New deferrals, net of amounts recognized in the current period		51,000
Effects of foreign currency		(390)
Ending balance	$ 43,970	$ 51,459

REVENUE FROM CONTRACTS WITH C_5

REVENUE FROM CONTRACTS WITH CUSTOMERS (Contract Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018
Revenue from Contract with Customer [Abstract]
Contract with customer, asset, gross	$ 2	$ 2

REVENUE FROM CONTRACTS WITH C_6

REVENUE FROM CONTRACTS WITH CUSTOMERS (Commissions) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Commissions	$ 67,715
Amortization of deferred commissions costs	48,313	$ 0	$ 0
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Commissions	72,899
Amortization of deferred commissions costs	48,313
Restatement Adjustment [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Commissions	$ (53,497)

REVENUE FROM CONTRACTS WITH C_7

REVENUE FROM CONTRACTS WITH CUSTOMERS (Performance Obligations) (Details) $ in Millions	Dec. 31, 2018USD ($)
Revenue from Contract with Customer [Abstract]
Revenue, remaining performance obligation	$ 193
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period	3 years

ACQUISITIONS (ForRent Narrative

ACQUISITIONS (ForRent Narrative) (Details) - USD ($)	Feb. 21, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 01, 2015
Business Acquisition [Line Items]
Goodwill		$ 1,611,535,000	$ 1,283,457,000	$ 1,254,866,000
Goodwill, expected tax deductible amount		16,000,000
ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Aggregate purchase price	$ 376,000,000
Cash payment	$ 340,000,000
Investment owned (shares)	103,280
Purchase price, shares issued	$ 36,000,000
Goodwill	266,595,000				$ 267,000,000
Goodwill, expected tax deductible amount		$ 8,000,000
Indemnification asset	6,260,000
State uncertain income tax position liability	2,047,000
Indemnification asset	5,443,000
Indemnification asset	1,000,000
Transaction costs	3,000,000
Customer Relationships [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period		10 years
Acquired Database Technology [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period		3 years
Acquired trade names and other intangible assets [Member]
Business Acquisition [Line Items]
Weighted-average amortization period		12 years
Acquired trade names and other intangible assets [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period		10 years
Building Photography [Member]
Business Acquisition [Line Items]
Weighted-average amortization period		2 years
Building Photography [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period		1 year
Maximum [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
State uncertain income tax position liability	2,000,000
Employee retention bonus	$ 11,627,000

ACQUISITIONS (Schedule of Recog

ACQUISITIONS (Schedule of Recognized Identified Assets Acquired and Liabilities Assumed) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Feb. 21, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 01, 2015
Business Acquisition [Line Items]
Goodwill	$ 1,611,535		$ 1,283,457	$ 1,254,866
ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		$ 59
Accounts receivable		8,769
Indemnification asset		5,443
Goodwill		266,595			$ 267,000
Intangible assets		141,300
Deferred tax liabilities		(34,032)
Contingent sales tax liability		(6,260)
State uncertain income tax position liability		(2,047)
Other assets and liabilities		(3,535)
Fair value of identifiable net assets acquired		376,292
Previously Reported [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		59
Accounts receivable		8,769
Indemnification asset		5,443
Goodwill		266,720
Intangible assets		141,300
Deferred tax liabilities		(34,032)
Contingent sales tax liability		(6,260)
State uncertain income tax position liability		(2,047)
Other assets and liabilities		(3,453)
Fair value of identifiable net assets acquired		376,499
Restatement Adjustment [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		0
Accounts receivable		0
Indemnification asset		0
Goodwill		(125)
Intangible assets		0
Deferred tax liabilities		0
Contingent sales tax liability		0
State uncertain income tax position liability		0
Other assets and liabilities		(82)
Fair value of identifiable net assets acquired		$ (207)

ACQUISITIONS (Other - Narrative

ACQUISITIONS (Other - Narrative) (Details) $ in Thousands, £ in Millions	Nov. 08, 2018USD ($)	Oct. 12, 2018GBP (£)	Oct. 12, 2018USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Business Acquisition [Line Items]
Cash paid for acquisitions, net of cash acquired				$ 418,369	$ 47,768	$ 10,443
Goodwill acquired				$ 329,938	$ 25,717
Realla Ltd [Member]
Business Acquisition [Line Items]
Cash paid for acquisitions, net of cash acquired		£ 12	$ 15,000
Initial payment for acquisiton		10	13,000
Goodwill acquired		8	10,000
Intangible assets acquired		£ 4	$ 5,000
Cozy Services Ltd [Member]
Business Acquisition [Line Items]
Cash paid for acquisitions, net of cash acquired	$ 65,000
Goodwill acquired	53,000
Intangible assets acquired	$ 11,000

ACQUISITIONS (Business Acquisit

ACQUISITIONS (Business Acquisition, Pro Forma Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Business Combinations [Abstract]
Revenue	$ 1,205,584	$ 1,067,742
Net income	$ 251,196	$ 103,000
Net income per share - basic (usd per share)	$ 6.96	$ 3.09
Net income per share - diluted (usd per share)	$ 6.89	$ 3.06

INVESTMENTS (Scheduled Maturiti

INVESTMENTS (Scheduled Maturities of Investments Classified as Available-for-sale) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Realized gain on investments	$ 0	$ 0	$ 0
Realized loss on investments	0	$ 0	$ 0
Debt Maturities Fair Value [Abstract]
2,019	0
2020 — 2023	0
2024 — 2028	0
2029 and thereafter	10,070,000
Available-for-sale investments	$ 10,070,000

INVESTMENTS (Available For Sale

INVESTMENTS (Available For Sale Securities) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Securities, Available-for-sale [Line Items]
Realized gain on investments	$ 0	$ 0	$ 0
Realized loss on investments	0	0	$ 0
Amortized Cost	10,800,000	10,800,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(730,000)	(730,000)
Fair Value	10,070,000	10,070,000
Aggregate Fair Value	10,070,000	10,070,000
Gross Unrealized Losses	(730,000)	(730,000)
Auction Rate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,800,000	10,800,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(730,000)	(730,000)
Fair Value	10,070,000	10,070,000
Aggregate Fair Value	10,070,000	10,070,000
Gross Unrealized Losses	$ (730,000)	$ (730,000)

FAIR VALUE (Details)

FAIR VALUE (Details)	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Assets:
Total assets measured at fair value	$ 600,637,000	$ 596,154,000
Unobservable inputs assets (level 3) [Roll forward]
Temporary impairment of the auction rates security investments	(730,000)	(730,000)
Money Market Funds [Member]
Assets:
Total assets measured at fair value	590,567,000	586,084,000
Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	10,070,000	10,070,000
Fair Value, Inputs, Level 1 [Member]
Assets:
Total assets measured at fair value	590,567,000	586,084,000
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	590,567,000	586,084,000
Fair Value, Inputs, Level 1 [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 2 [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 2 [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3 [Member]
Assets:
Total assets measured at fair value	10,070,000	10,070,000
Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Inputs, Level 3 [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	10,070,000	10,070,000
Auction Rate Securities [Member]
Unobservable inputs assets (level 3) [Roll forward]
Beginning balance	10,070,000	9,952,000
Decrease in unrealized loss included in accumulated other comprehensive loss	0	118,000
Settlements	0
Ending balance	$ 10,070,000	$ 10,070,000
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
Par value of company held auction rate securities	$ 11,000,000
Temporary impairment of the auction rates security investments	$ 0
Weighted Average [Member] | Measurement Input, Discount Rate [Member]
Unobservable inputs assets (level 3) [Roll forward]
Debt securities, measurement input	0.06	0.06

PROPERTY AND EQUIPMENT (Details

PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 193,094	$ 182,891
Accumulated depreciation and amortization	(109,791)	(98,395)
Property and equipment, net	83,303	84,496
Depreciation expense for property and equipment	26,000	26,000	$ 24,000
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	65,332	59,447
Furniture, Office Equipment and Vehicles [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	53,020	52,163
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 74,742	$ 71,281

GOODWILL (Goodwill by Segment)

GOODWILL (Goodwill by Segment) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill, impairment loss	$ 0	$ 0	$ 0
Goodwill [Roll Forward]
Goodwill, beginning balance	1,283,457,000	1,254,866,000
Acquisitions	329,938,000	25,717,000
Effect of foreign currency translation	(1,860,000)	2,874,000
Goodwill, ending balance	1,611,535,000	1,283,457,000	1,254,866,000
North America [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	1,253,494,000	1,227,777,000
Acquisitions	319,594,000	25,717,000
Effect of foreign currency translation	0	0
Goodwill, ending balance	1,573,088,000	1,253,494,000	1,227,777,000
International [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	29,963,000	27,089,000
Acquisitions	10,344,000	0
Effect of foreign currency translation	(1,860,000)	2,874,000
Goodwill, ending balance	$ 38,447,000	$ 29,963,000	$ 27,089,000

GOODWILL (Narrative) (Details)

GOODWILL (Narrative) (Details) £ in Millions	Nov. 08, 2018USD ($)	Oct. 12, 2018GBP (£)	Oct. 12, 2018USD ($)	Jul. 18, 2017USD ($)	May 10, 2017USD ($)	Jan. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Feb. 21, 2018USD ($)	Jun. 01, 2015USD ($)
Goodwill [Line Items]
Acquisitions							$ 329,938,000	$ 25,717,000
Goodwill							1,611,535,000	1,283,457,000	$ 1,254,866,000
Goodwill tax deductible amount							16,000,000
Goodwill, impairment loss							0	$ 0	$ 0
Westside Rentals [Member]
Goodwill [Line Items]
Acquisitions						$ 8,000,000
LandWatch [Member]
Goodwill [Line Items]
Acquisitions					$ 15,000,000
The Screening Pros, LLC [Member]
Goodwill [Line Items]
Acquisitions				$ 2,000,000
ForRent, Division Of DE Holdings, Inc. [Member]
Goodwill [Line Items]
Goodwill										$ 266,595,000	$ 267,000,000
Goodwill tax deductible amount							$ 8,000,000
Realla Ltd [Member]
Goodwill [Line Items]
Acquisitions		£ 8	$ 10,000,000
Cozy Services Ltd [Member]
Goodwill [Line Items]
Acquisitions	$ 53,000,000

INTANGIBLE ASSETS (Details)

INTANGIBLE ASSETS (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	$ 288,911,000	$ 182,892,000
Amortization of intangible assets	52,000,000	37,000,000	$ 46,000,000
Impairment of Intangible Assets (Excluding Goodwill)	0	0	$ 0
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Amortization expense for 2019	48,000,000
Amortization expense for 2020	42,000,000
Amortization expense for 2021	34,000,000
Amortization expense for 2022	29,000,000
Amortization expense for 2023	26,000,000
Capitalized Product Development Costs [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	2,173,000	2,275,000
Finite-lived intangible assets, accumulated amortization	(2,173,000)	(2,262,000)
Finite-lived intangible assets, net	$ 0	13,000
Weighted-average amortization period (in years}	4 years
Building Photography [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 9,035,000	18,739,000
Finite-lived intangible assets, accumulated amortization	(8,809,000)	(18,212,000)
Finite-lived intangible assets, net	$ 226,000	527,000
Weighted-average amortization period (in years}	2 years
Acquired Technology [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 103,128,000	83,469,000
Finite-lived intangible assets, accumulated amortization	(85,344,000)	(79,188,000)
Finite-lived intangible assets, net	$ 17,784,000	4,281,000
Weighted-average amortization period (in years}	4 years
Acquired Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 339,574,000	225,879,000
Finite-lived intangible assets, accumulated amortization	(199,405,000)	(169,157,000)
Finite-lived intangible assets, net	$ 140,169,000	56,722,000
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	$ 190,717,000	167,718,000
Finite-lived intangible assets, accumulated amortization	(59,985,000)	(46,369,000)
Finite-lived intangible assets, net	$ 130,732,000	$ 121,349,000
Weighted-average amortization period (in years}	12 years

LONG-TERM DEBT (Details)

LONG-TERM DEBT (Details)	Oct. 19, 2017USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Debt Instrument [Line Items]
Loss on debt extinguishment		$ (3,788,000)	$ 0	$ 0	$ 0	$ 0	$ (3,788,000)	$ 0
Long-term debt		0					0
Interest expense, debt						3,000,000	9,000,000	10,000,000
Amortization of debt issuance costs						1,000,000	2,000,000	$ 3,000,000
Debt issuance costs, net		4,000,000				3,000,000	4,000,000
Letter of Credit [Member]
Debt Instrument [Line Items]
Long-term debt		$ 200,000				$ 200,000	$ 200,000
Letter of Credit [Member] | 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Revolving credit sub-facility for swing-line loans and issuances of letters of credit	$ 20,000,000
Interest rate increase (in case of default)	2.00%
Revolving Credit Facility [Member] | 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Revolving credit sub-facility for swing-line loans and issuances of letters of credit	$ 750,000,000
Term of facility	5 years
Debt issuance costs, line of credit	$ 4,000,000
Unamortized debt issuance expense	4,000,000
Loss on debt extinguishment	$ (4,000,000)
Line of credit, covenant compliance, Secured Leverage Ratio	3.50
Line of credit, covenant compliance, Total Leverage Ratio	4.50
Federal Funds Rate [Member] | Letter of Credit [Member] | 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Basis spread on variable rate						0.50%
London Interbank Offered Rate (LIBOR) [Member] | Letter of Credit [Member] | 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
London Interbank Offered Rate (LIBOR) [Member] | Initial Basis Spread [Member] | Letter of Credit [Member] | 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.25%
London Interbank Offered Rate (LIBOR) [Member] | Initial Basis Spread One Month LIBOR [Member] | Letter of Credit [Member] | 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.25%

INCOME TAXES (Components for Pr

INCOME TAXES (Components for Provision for Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal									$ 36,167	$ 41,453	$ 32,198
State									5,140	3,518	3,682
Foreign									708	295	76
Total current									42,015	45,266	35,956
Deferred:
Federal									6,576	(7,917)	12,586
State									(2,582)	4,695	3,014
Foreign									(328)	319	35
Total deferred									3,666	(2,903)	15,635
Income tax expense	$ 26,060	$ 14,247	$ 1,863	$ 3,511	$ 4,487	$ 20,990	$ 3,611	$ 13,275	$ 45,681	$ 42,363	$ 51,591

INCOME TAXES (Components of Def

INCOME TAXES (Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Reserve for bad debts	$ 1,457	$ 1,636
Accrued compensation	4,803	6,706
Stock compensation	10,041	10,568
Net operating losses	26,349	25,899
Accrued reserve and other	1,773	1,578
Deferred rent	5,928	6,533
Deferred gain on the sale of building	4,140	4,741
Research and development credits	6,331	0
Total deferred tax assets, prior to valuation allowance	60,822	57,661
Valuation allowance	(14,246)	(13,032)
Total deferred tax assets, net of valuation allowance	46,576	44,629
Deferred tax liabilities:
Deferred commission costs, net	(19,314)	0
Prepaid expenses	(2,204)	(1,239)
Property and equipment, net	(5,367)	(6,229)
Intangible assets, net	(82,079)	(43,800)
Total deferred tax liabilities	(108,964)	(51,268)
Net deferred tax assets (liabilities)	$ (62,388)	$ (6,639)

INCOME TAXES (Reconciliation of

INCOME TAXES (Reconciliation of Provision for Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective tax rate reconciliation [Abstract]
Expected federal income tax provision at statutory rate									$ 59,643	$ 57,770	$ 47,832
State income taxes, net of federal benefit									10,312	4,776	3,638
Foreign income taxes, net effect									(315)	(3,540)	(31)
Increase (decrease) in valuation allowance									1,214	3,624	(103)
Tax rate changes									141	(7,340)	283
Research credits									(15,373)	(20,547)	(920)
Excess tax benefit									(14,227)	(7,010)	0
Tax reserves									1,870	12,646	(150)
Other adjustments									2,416	1,984	1,042
Income tax expense	$ 26,060	$ 14,247	$ 1,863	$ 3,511	$ 4,487	$ 20,990	$ 3,611	$ 13,275	$ 45,681	$ 42,363	$ 51,591

INCOME TAXES (Narrative) (Detai

INCOME TAXES (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	$ 1,000	$ (4,000)
Income from U.S. sources	294,000	167,000	$ 135,000
Income from foreign sources	10,000	2,000	2,000
Research credits	15,373	20,547	920
Cash tax benefits resulting in net operating loss carryforward	6,000	7,000	$ 5,000
Foreign Country [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance		$ 4,000
Net operating loss carryforward	53,000
Domestic Country [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	$ 44,000
Operating loss carryforwards, first expiration date	Dec. 31, 2020
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Research credits	$ 14,000
Net operating loss carryforward	$ 4,000
Operating loss carryforwards, first expiration date	Dec. 31, 2020
Income tax credit carryforward	$ 11,000

INCOME TAXES (Unrecognized Tax

INCOME TAXES (Unrecognized Tax Benefits) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits beginning balance	$ 14,363	$ 1,843	$ 7,664
Increase for current year tax positions	9,561	12,620	368
Decrease for prior period tax positions	(70)	(34)	(6,115)
Expiration of the statute of limitation for assessment of taxes	(1,482)	(66)	(74)
Unrecognized tax benefits ending balance	22,372	14,363	1,843
Unrecognized tax benefits that would favorably affect the annual effective tax rate if recognized in future periods	22,000	14,000
Interest and penalties on income taxes recognized	224	72
Interest and penalties on income taxes reversal			416
Interest and penalties accrued on income taxes	430	$ 205	$ 133
State and Local Jurisdiction [Member]
Unrecognized Tax Benefits [Roll Forward]
Increase for current year tax positions	9,000
Expiration of the statute of limitation for assessment of taxes	$ (1,000)

COMMITMENTS AND CONTINGENCIES_2

COMMITMENTS AND CONTINGENCIES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Leases Rent Expense [Abstract]
Rent expense for operating leases	$ 29,000	$ 26,000	$ 22,000
Future Minimum Lease Payments [Abstract]
2,019	30,485
2,020	29,255
2,021	27,421
2,022	25,634
2,023	24,515
Thereafter	31,768
Total future minimum lease payments	$ 169,078

SEGMENT REPORTING (Reconciliati

SEGMENT REPORTING (Reconciliation of Net Income (Loss) to EBITDA) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)operating_segments	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Segment Reporting Information [Line Items]
Number of business segments | operating_segments									2
Reconciliation of EBITDA to net income (loss) [Abstract]
Net income	$ 83,509	$ 58,759	$ 43,835	$ 52,231	$ 44,204	$ 34,177	$ 22,184	$ 22,130	$ 238,334	$ 122,695	$ 85,071
Amortization of acquired intangible assets in cost of revenues									20,586	19,707	22,819
Amortization of acquired intangible assets in operating expenses									30,881	17,684	22,731
Depreciation and other amortization									26,276	26,252	24,615
Interest and other income	(4,607)	(3,035)	(2,652)	(2,987)	(2,455)	(555)	(605)	(429)	(13,281)	(4,044)	(1,773)
Interest and other expense	695	717	728	690	734	2,901	2,693	2,686	2,830	9,014	10,016
Loss on extinguishment of debt					3,788	0	0	0	0	3,788	0
Income tax expense	$ 26,060	$ 14,247	$ 1,863	$ 3,511	$ 4,487	$ 20,990	$ 3,611	$ 13,275	45,681	42,363	51,591
EBITDA									351,307	237,459	215,070
North America [Member]
Reconciliation of EBITDA to net income (loss) [Abstract]
EBITDA									358,036	236,906	210,901
International [Member]
Reconciliation of EBITDA to net income (loss) [Abstract]
EBITDA									$ (6,729)	$ 553	$ 4,169

SEGMENT REPORTING (Assets and L

SEGMENT REPORTING (Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Property and equipment, net	$ 83,303	$ 84,496
Goodwill	1,611,535	1,283,457	$ 1,254,866
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	3,312,957	2,873,441
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	291,015	222,191
North America [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	79,493	79,736
Goodwill	1,573,088	1,253,494	1,227,777
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	3,253,035	2,816,156
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	272,776	201,831
International [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	3,810	4,760
Goodwill	38,447	29,963	$ 27,089
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	59,922	57,285
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	$ 18,239	$ 20,360

STOCKHOLDERS' EQUITY (Details)

STOCKHOLDERS' EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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)	3,300,000
New shares issued, price per share (in dollars per share)	$ 260
Proceeds from equity offering, net of transaction costs	$ 834,000	$ 0	$ 833,911	$ 0
Payments for underwriting expense	$ 29,000

NET INCOME (LOSS) PER SHARE (De

NET INCOME (LOSS) PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 83,509	$ 58,759	$ 43,835	$ 52,231	$ 44,204	$ 34,177	$ 22,184	$ 22,130	$ 238,334	$ 122,695	$ 85,071
Denominator:
Denominator for basic net income (loss) per share — weighted-average outstanding shares									36,058	33,200	32,167	32,167
Effect of dilutive securities:
Stock options and restricted stock awards (in shares)									390	359	269
Denominator for diluted net income (loss) per share — weighted-average outstanding shares									36,448	33,559	32,436
Net income (loss) per share — basic (in dollars per share)	$ 2.31	$ 1.63	$ 1.22	$ 1.46	$ 1.24	$ 1.05	$ 0.68	$ 0.69	$ 6.61	$ 3.70	$ 2.64
Net income (loss) per share — diluted (in dollars per share)	$ 2.29	$ 1.61	$ 1.20	$ 1.44	$ 1.22	$ 1.04	$ 0.68	$ 0.68	$ 6.54	$ 3.66	$ 2.62
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									54	59	60
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)									64	87	194
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)									53	58	59
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	1	1

EMPLOYEE BENEFIT PLANS (Stock I

EMPLOYEE BENEFIT PLANS (Stock Incentice Plans - Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 09, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 42,718	$ 39,030	$ 36,349
Performance service period	3 years
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)				0
Shares of common stock authorized for issuance under the plan (in shares)	815,464
CoStar Group, Inc. 2007 Stock Incentive Plan [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	3 years
CoStar Group, Inc. 2007 Stock Incentive Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	4 years
CoStar Group, Inc. 2016 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)	2,000,000
Shares of common stock authorized for issuance under the plan (in shares)	1,450,000
CoStar Group, Inc. 2016 Stock Incentive Plan [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	3 years
CoStar Group, Inc. 2016 Stock Incentive Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	4 years
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)	26,160	32,160	25,680
Performance Shares [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	80.00%
Performance Shares [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	120.00%
Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 5,000	$ 5,000	$ 3,000
Compensation cost expected to be recognized in future years	$ 7,000
Granted (in shares)	26,160
Restricted Stock [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	0.00%
Vesting period of options and restricted stock grants	1 year
Restricted Stock [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	200.00%
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise of stock options (in shares)	177,299	81,815	29,285
Compensation cost expected to be recognized in future years	$ 66,000
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)	2 years 4 months 10 days
Aggregate intrinsic value of options exercised	$ 45,000	$ 13,000	$ 3,000
Weighted-average grant date fair value of each option granted during the period (in dollars per share)	$ 101.02	$ 59.06	$ 54.34

EMPLOYEE BENEFIT PLANS (Stock O

EMPLOYEE BENEFIT PLANS (Stock Option Activity) (Details) - Stock Options [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period (in shares)	453,843	440,158	400,077
Granted (in shares)	82,500	95,500	82,400
Exercised (in shares)	(177,299)	(81,815)	(29,285)
Canceled or expired (in shares)	(14,768)		(13,034)
Outstanding at end of period (in shares)	344,276	453,843	440,158
Exercisable at end of period (in shares)	185,405	278,239	284,489
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)	$ 156.24	$ 132.08	$ 122.30
Weighted-average exercise price, granted (in dollars per share)	342.13	204.91	182.75
Weighted-average exercise price, exercised (in dollars per share)	125.16	83.07	112.78
Weighted-average exercise price, canceled or expired (in dollars per share)	261.20		195.78
Weighted-average exercise price, outstanding at end of period (in dollars per share)	212.28	156.24	132.08
Weighted-average exercise price, exercisable at end of period (in dollars per share)	$ 165.31	130.91	100.94
Weighted-average remaining contract life of options outstanding at end of period	7 years 11 days
Weighted-average remaining contract life of options exercisable at end of period	5 years 9 months 14 days
Aggregate intrinsic value of options outstanding at end of period	$ 43,418
Aggregate intrinsic value of options exercisable	$ 31,895
Share Based Compensation Exercisable Range [Abstract]
Granted (dollars per share)	$ 342.13	204.91
Minimum [Member]
Share Based Compensation Exercisable Range [Abstract]
Outstanding (dollars per share)	36.73	36.48	36.48
Granted (dollars per share)			182.75
Exercised (dollars per share)	36.73	36.48	36.48
Canceled or expired (dollars per share)	182.75		193.69
Outstanding (dollars per share)	36.73	36.73	36.48
Exercisable at end of period	54.51	36.73	36.48
Maximum [Member]
Share Based Compensation Exercisable Range [Abstract]
Outstanding (dollars per share)	204.91	201.04	201.04
Granted (dollars per share)			182.75
Exercised (dollars per share)	204.91	201.04	201.04
Canceled or expired (dollars per share)	342.13		201.04
Outstanding (dollars per share)	342.13	204.91	201.04
Exercisable at end of period	$ 204.91	$ 201.04	$ 201.04

EMPLOYEE BENEFIT PLANS (Assumpt

EMPLOYEE BENEFIT PLANS (Assumptions) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%	0.00%	0.00%
Expected volatility	28.00%	28.00%	31.00%
Risk-free interest rate	3.00%	2.00%	1.00%
Expected life (in years)	5 years	5 years	5 years
Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%	0.00%	0.00%
Expected volatility	28.00%	28.00%	28.00%
Risk-free interest rate	2.00%	2.00%	1.00%
Expected life (in years)	3 years	3 years	3 years
Granted (in dollars per share)	$ 342.13	$ 218.59	$ 184.97

EMPLOYEE BENEFIT PLANS (Informa

EMPLOYEE BENEFIT PLANS (Information Regarding Stock Options) (Details)	12 Months Ended
	Dec. 31, 2018$ / sharesshares
$54.51 - $78.33
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of exercise price, minimum, (in dollars per share)	$ 54.51
Range of exercise price, maximum, (in dollars per share)	$ 78.33
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares	300
Weighted-average remaining contractual life (in years)	1 year 11 months 1 day
Weighted- average exercise price (in dollars per share)	$ 54.51
Options Exercisable [Abstract]
Number of shares (in shares) | shares	300
Weighted-average exercise price (in dollars per share)	$ 54.51
$78.34 - $142.45
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of exercise price, minimum, (in dollars per share)	78.34
Range of exercise price, maximum, (in dollars per share)	$ 142.45
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares	58,844
Weighted-average remaining contractual life (in years)	4 years 2 months 8 days
Weighted- average exercise price (in dollars per share)	$ 102.16
Options Exercisable [Abstract]
Number of shares (in shares) | shares	58,844
Weighted-average exercise price (in dollars per share)	$ 102.16
$142.46 - $188.22
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of exercise price, minimum, (in dollars per share)	142.46
Range of exercise price, maximum, (in dollars per share)	$ 188.22
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares	57,768
Weighted-average remaining contractual life (in years)	7 years 2 months 8 days
Weighted- average exercise price (in dollars per share)	$ 182.75
Options Exercisable [Abstract]
Number of shares (in shares) | shares	33,033
Weighted-average exercise price (in dollars per share)	$ 182.75
$188.23 - $197.37
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of exercise price, minimum, (in dollars per share)	188.23
Range of exercise price, maximum, (in dollars per share)	$ 197.37
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares	34,264
Weighted-average remaining contractual life (in years)	6 years 2 months 1 day
Weighted- average exercise price (in dollars per share)	$ 193.69
Options Exercisable [Abstract]
Number of shares (in shares) | shares	34,264
Weighted-average exercise price (in dollars per share)	$ 193.69
$197.38 - $202.98
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of exercise price, minimum, (in dollars per share)	197.38
Range of exercise price, maximum, (in dollars per share)	$ 202.98
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares	34,600
Weighted-average remaining contractual life (in years)	5 years 1 month 28 days
Weighted- average exercise price (in dollars per share)	$ 201.04
Options Exercisable [Abstract]
Number of shares (in shares) | shares	34,600
Weighted-average exercise price (in dollars per share)	$ 201.04
$202.99 - $273.52
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of exercise price, minimum, (in dollars per share)	202.99
Range of exercise price, maximum, (in dollars per share)	$ 273.52
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares	82,500
Weighted-average remaining contractual life (in years)	8 years 1 month 28 days
Weighted- average exercise price (in dollars per share)	$ 204.91
Options Exercisable [Abstract]
Number of shares (in shares) | shares	24,364
Weighted-average exercise price (in dollars per share)	$ 204.91
$273.53 - $342.13
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of exercise price, minimum, (in dollars per share)	273.53
Range of exercise price, maximum, (in dollars per share)	$ 342.13
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares	76,000
Weighted-average remaining contractual life (in years)	9 years 1 month 28 days
Weighted- average exercise price (in dollars per share)	$ 342.13
Options Exercisable [Abstract]
Number of shares (in shares) | shares	0
Weighted-average exercise price (in dollars per share)	$ 0
$54.51 - $342.13
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Range of exercise price, minimum, (in dollars per share)	54.51
Range of exercise price, maximum, (in dollars per share)	$ 342.13
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares) | shares	344,276
Weighted-average remaining contractual life (in years)	7 years 11 days
Weighted- average exercise price (in dollars per share)	$ 212.28
Options Exercisable [Abstract]
Number of shares (in shares) | shares	185,405
Weighted-average exercise price (in dollars per share)	$ 165.31

EMPLOYEE BENEFIT PLANS (Restric

EMPLOYEE BENEFIT PLANS (Restrictive Stock Award Activity) (Details)	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Performance Based RSAs Without Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares) | shares	363,883
Granted (in shares) | shares	134,209
Vested (in shares) | shares	(151,995)
Canceled (in shares) | shares	(41,936)
Unvested restricted stock at end of period (in shares) | shares	304,161
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) | $ / shares	$ 206.59
Granted (in dollars per share) | $ / shares	358.51
Vested (in dollars per share) | $ / shares	193.70
Canceled (in dollars per share) | $ / shares	258.19
Unvested restricted stock at end of period (in dollars per share) | $ / shares	$ 272.95
Performance-based RSAs - with Market Condition [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares) | shares	85,200
Granted (in shares) | shares	26,160
Vested (in shares) | shares	(27,360)
Canceled (in shares) | shares	(7,680)
Unvested restricted stock at end of period (in shares) | shares	76,320
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) | $ / shares	$ 205.08
Granted (in dollars per share) | $ / shares	380.24
Vested (in dollars per share) | $ / shares	361.20
Canceled (in dollars per share) | $ / shares	361.20
Unvested restricted stock at end of period (in dollars per share) | $ / shares	$ 193.44

EMPLOYEE BENEFIT PLANS (Restr_2

EMPLOYEE BENEFIT PLANS (Restrictive Stock Units) (Details) - Restricted Stock Units (RSUs) [Member]	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares) | shares	962
Granted (in shares) | shares	214
Vested (in shares) | shares	(324)
Canceled (in shares) | shares	0
Unvested restricted stock at end of period (in shares) | shares	852
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) | $ / shares	$ 190.27
Granted (in dollars per share) | $ / shares	342.13
Vested (in dollars per share) | $ / shares	189.08
Canceled (in dollars per share) | $ / shares	0
Unvested restricted stock at end of period (in dollars per share) | $ / shares	$ 228.86

EMPLOYEE BENEFIT PLANS (Employe

EMPLOYEE BENEFIT PLANS (Employee 401(k) Plan, Employee Pension Plan, Registered Retirement Savings Plan and Employee Stock Purchase Plan) (Details) - USD ($)	Sep. 15, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Maximum percentage of employee total compensation matched by employer (in hundredths)		100.00%
Maximum annual employee contribution		4.00%	4.00%	4.00%
Company match to employee contributions		$ 12,000,000	$ 10,000,000	$ 9,000,000
Administrative expense		$ 0	$ 0	$ 0
Registered Retirement Savings Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Maximum percentage of employee total compensation matched by employer (in hundredths)		100.00%	100.00%	100.00%
Maximum annual employee contribution		4.00%
Company match to employee contributions		$ 58,000	$ 43,000	$ 10,000
Employee Stock Purchase Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of purchase price of company's common stock to the market price		90.00%
Increase in shares of common stock issued pursuant to stock plan (in shares)	100,000
Shares available, employee stock purchase plan (in shares)		65,174	80,022
Shares of Company's common stock purchased during the period (in shares)		14,848	13,790
Foreign Plan [Member] | Pension Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Maximum percentage of employee total compensation matched by employer (in hundredths)		6.00%	6.00%	6.00%
Company match to employee contributions		$ 500,000	$ 400,000	$ 400,000

QUARTERLY RESULTS OF OPERATIO_3

QUARTERLY RESULTS OF OPERATIONS (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
Revenues	$ 315,571	$ 305,525	$ 297,018	$ 273,718	$ 253,991	$ 247,533	$ 237,153	$ 226,553	$ 1,191,832	$ 965,230	$ 837,630
Cost of revenues	68,248	72,072	67,136	62,477	58,301	55,483	55,273	51,346
Gross profit	247,323	233,453	229,882	211,241	195,690	192,050	181,880	175,207	921,899	744,827	663,816
Operating expenses	141,666	162,765	186,108	157,796	144,932	134,537	153,997	137,545	648,335	571,011	518,911
Income from operations	105,657	70,688	43,774	53,445	50,758	57,513	27,883	37,662	273,564	173,816	144,905
Interest and other income	4,607	3,035	2,652	2,987	2,455	555	605	429	13,281	4,044	1,773
Interest and other expense	(695)	(717)	(728)	(690)	(734)	(2,901)	(2,693)	(2,686)	(2,830)	(9,014)	(10,016)
Loss on debt extinguishment					(3,788)	0	0	0	0	(3,788)	0
Income before income taxes	109,569	73,006	45,698	55,742	48,691	55,167	25,795	35,405	284,015	165,058	136,662
Income tax expense	26,060	14,247	1,863	3,511	4,487	20,990	3,611	13,275	45,681	42,363	51,591
Net income	$ 83,509	$ 58,759	$ 43,835	$ 52,231	$ 44,204	$ 34,177	$ 22,184	$ 22,130	$ 238,334	$ 122,695	$ 85,071
Net income (loss) per share — basic (in dollars per share)	$ 2.31	$ 1.63	$ 1.22	$ 1.46	$ 1.24	$ 1.05	$ 0.68	$ 0.69	$ 6.61	$ 3.70	$ 2.64
Net income (loss) per share — diluted (in dollars per share)	$ 2.29	$ 1.61	$ 1.20	$ 1.44	$ 1.22	$ 1.04	$ 0.68	$ 0.68	$ 6.54	$ 3.66	$ 2.62

Schedule II Valuation and Qua_2

Schedule II Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-09, Valuation and Qualifying Accounts [Abstract]
Balance at beginning of year	$ 6,469	$ 6,344	$ 7,478
Charged to expense	6,542	5,690	7,358
Write-offs, net of recoveries	7,302	5,565	8,492
Balance at end of year	$ 5,709	$ 6,469	$ 6,344