CoStar (CSGP) 10-Q 23 Oct 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information Document - shares	9 Months Ended
	Sep. 30, 2019	Oct. 18, 2019
Document and Entity Information [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2019
Entity File Number	0-24531
Entity Registrant Name	COSTAR GROUP, INC.
Entity Central Index Key	0001057352
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	52-2091509
Entity Address, Address Line One	1331 L Street, NW
Entity Address, City or Town	Washington,
Entity Address, State or Province	DC
Entity Address, Postal Zip Code	20005
City Area Code	(202)
Local Phone Number	346-6500
Title of 12(b) Security	Common Stock ($0.01 par value)
Trading Symbol	CSGP
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		36,634,976

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Revenues	$ 352,808	$ 305,525	$ 1,024,993	$ 876,261
Cost of revenues	71,172	72,072	214,243	201,685
Gross profit	281,636	233,453	810,750	674,576
Operating expenses:
Selling and marketing (excluding customer base amortization)	101,582	89,251	308,751	290,706
Software development	32,639	26,173	89,022	75,357
General and administrative	45,530	39,012	127,943	117,658
Customer base amortization	7,616	8,329	22,473	22,948
Total operating expenses	187,367	162,765	548,189	506,669
Income from operations	94,269	70,688	262,561	167,907
Interest and other income	5,358	3,035	16,216	8,674
Interest and other expense	(704)	(717)	(2,133)	(2,135)
Income before income taxes	98,923	73,006	276,644	174,446
Income tax expense	20,304	14,247	49,608	19,621
Net income	$ 78,619	$ 58,759	$ 227,036	$ 154,825
Net income per share-basic (in dollars per share)	$ 2.16	$ 1.63	$ 6.26	$ 4.30
Net income per share-diluted (in dollars per share)	$ 2.15	$ 1.61	$ 6.20	$ 4.25
Weighted average outstanding shares-basic (in shares)	36,333	36,129	36,293	36,032
Weighted average outstanding shares-diluted (in shares)	36,652	36,518	36,615	36,439

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 78,619	$ 58,759	$ 227,036	$ 154,825
Other comprehensive loss, net of tax
Foreign currency translation adjustment	(2,161)	(361)	(2,548)	(1,469)
Total other comprehensive loss	(2,161)	(361)	(2,548)	(1,469)
Total comprehensive income	$ 76,458	$ 58,398	$ 224,488	$ 153,356

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 1,389,619	$ 1,100,416
Accounts receivable, less allowance of $4,858 and $5,709 as of September 30, 2019 and December 31, 2018, respectively	91,325	89,192
Prepaid expenses and other current assets	23,778	23,690
Total current assets	1,504,722	1,213,298
Long-term investments	10,070	10,070
Deferred income taxes, net	4,426	7,469
Property and equipment, net	107,989	83,303
Lease right-of-use assets	105,516	0
Goodwill	1,616,538	1,611,535
Intangible assets, net	259,444	288,911
Deferred commission costs, net	85,623	76,031
Deposits and other assets	6,155	7,432
Income tax receivable	14,908	14,908
Total assets	3,715,391	3,312,957
Current liabilities:
Accounts payable	8,082	6,327
Accrued wages and commissions	51,557	45,588
Accrued expenses	41,816	29,821
Deferred gain on the sale of building	0	2,523
Income taxes payable	9,116	14,288
Deferred rent	0	4,153
Lease liabilities	27,137	0
Deferred revenue	60,328	51,459
Total current liabilities	198,036	154,159
Deferred gain on the sale of building	0	13,669
Deferred rent	0	31,944
Deferred income taxes, net	83,120	69,857
Income taxes payable	17,172	17,386
Lease and other long-term liabilities	117,897	4,000
Total liabilities	416,225	291,015
Total stockholders’ equity	3,299,166	3,021,942
Total liabilities and stockholders’ equity	$ 3,715,391	$ 3,312,957

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Current assets:
Allowance for doubtful accounts	$ 4,858	$ 5,709

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS’ EQUITY - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]
Balance at January 1, 2019	$ 2,705,714	$ 361	$ 2,339,253	$ (9,020)	$ 375,120
Balance (in shares) at Dec. 31, 2017		36,107,000
Beginning balance at Dec. 31, 2017	2,651,250	$ 361	2,339,253	(9,020)	320,656
Net income	52,231				52,231
Other comprehensive income (loss)	951			951
Exercise of stock options (in shares)		111,000
Exercise of stock options	9,328	$ 1	9,327
Restricted stock grants (in shares)		114,000
Restricted stock grants	0	$ 1	(1)
Restricted stock grants surrendered (in shares)		(47,000)
Restricted stock grants surrendered	(15,392)		(15,392)
Stock-based compensation expense	10,335		10,335
Employee stock purchase plan (in shares)		4,000
Employee stock purchase plan	1,431		1,431
Stock issued for acquisitions (in shares)		103,000
Stock issued for acquisitions	36,366	$ 1	36,365
Balance (in shares) at Mar. 31, 2018		36,392,000
Ending balance at Mar. 31, 2018	2,800,964	$ 364	2,381,318	(8,069)	427,351
Balance (in shares) at Dec. 31, 2017		36,107,000
Beginning balance at Dec. 31, 2017	2,651,250	$ 361	2,339,253	(9,020)	320,656
Net income	154,825
Other comprehensive income (loss)	(1,469)
Balance (in shares) at Sep. 30, 2018		36,426,000
Ending balance at Sep. 30, 2018	2,928,314	$ 364	2,408,494	(10,489)	529,945
Balance (in shares) at Mar. 31, 2018		36,392,000
Beginning balance at Mar. 31, 2018	2,800,964	$ 364	2,381,318	(8,069)	427,351
Net income	43,835				43,835
Other comprehensive income (loss)	(2,059)			(2,059)
Exercise of stock options (in shares)		12,000
Exercise of stock options	2,261		2,261
Restricted stock grants (in shares)		12,000
Restricted stock grants	0		0
Restricted stock grants surrendered (in shares)		(29,000)
Restricted stock grants surrendered	(7,002)		(7,002)
Stock-based compensation expense	11,135		11,135
Employee stock purchase plan (in shares)		4,000
Employee stock purchase plan	1,484		1,484
Balance (in shares) at Jun. 30, 2018		36,391,000
Ending balance at Jun. 30, 2018	2,850,618	$ 364	2,389,196	(10,128)	471,186
Net income	58,759				58,759
Other comprehensive income (loss)	(361)			(361)
Exercise of stock options (in shares)		53,000
Exercise of stock options	10,366	$ 1	10,366
Restricted stock grants (in shares)		12,000
Restricted stock grants	0		0
Restricted stock grants surrendered (in shares)		(33,000)
Restricted stock grants surrendered	(1,271)	$ (1)	(1,271)
Stock-based compensation expense	8,883		8,883
Employee stock purchase plan (in shares)		3,000
Employee stock purchase plan	1,320	$ 0	1,320
Balance (in shares) at Sep. 30, 2018		36,426,000
Ending balance at Sep. 30, 2018	2,928,314	$ 364	2,408,494	(10,489)	529,945
Balance at January 1, 2019	3,033,999	$ 364	2,419,812	(11,688)	625,511
Balance (in shares) at Dec. 31, 2018		36,446,000
Beginning balance at Dec. 31, 2018	3,021,942	$ 364	2,419,812	(11,688)	613,454
Net income	85,169				85,169
Other comprehensive income (loss)	380			380
Exercise of stock options (in shares)		79,000
Exercise of stock options	10,638	$ 1	10,637
Restricted stock grants (in shares)		132,000
Restricted stock grants	0	$ 1	(1)
Restricted stock grants surrendered (in shares)		(43,000)
Restricted stock grants surrendered	(18,679)		(18,679)
Stock-based compensation expense	12,034		12,034
Management stock purchase plan	3,491
Employee stock purchase plan (in shares)		4,000
Employee stock purchase plan	1,582	$ 0	1,582
Balance (in shares) at Mar. 31, 2019		36,618,000
Ending balance at Mar. 31, 2019	3,128,614	$ 366	2,428,876	(11,308)	710,680
Balance (in shares) at Dec. 31, 2018		36,446,000
Beginning balance at Dec. 31, 2018	3,021,942	$ 364	2,419,812	(11,688)	613,454
Net income	227,036
Other comprehensive income (loss)	(2,548)
Balance (in shares) at Sep. 30, 2019		36,650,000
Ending balance at Sep. 30, 2019	3,299,166	$ 366	2,460,489	(14,236)	852,547
Balance (in shares) at Mar. 31, 2019		36,618,000
Beginning balance at Mar. 31, 2019	3,128,614	$ 366	2,428,876	(11,308)	710,680
Net income	63,248				63,248
Other comprehensive income (loss)	(767)			(767)
Exercise of stock options (in shares)		15,000
Exercise of stock options	3,001		3,001
Restricted stock grants (in shares)		4,000
Restricted stock grants	0		0
Restricted stock grants surrendered (in shares)		(17,000)
Restricted stock grants surrendered	(5,546)		(5,546)
Stock-based compensation expense	13,706		13,706
Employee stock purchase plan (in shares)		4,000
Employee stock purchase plan	1,815	$ 0	1,815
Balance (in shares) at Jun. 30, 2019		36,624,000
Ending balance at Jun. 30, 2019	3,204,071	$ 366	2,441,852	(12,075)	773,928
Net income	78,619				78,619
Other comprehensive income (loss)	(2,161)			(2,161)
Exercise of stock options (in shares)		20,000
Exercise of stock options	4,697		4,697
Restricted stock grants (in shares)		8,000
Restricted stock grants	0		0
Restricted stock grants surrendered (in shares)		(5,000)
Restricted stock grants surrendered	(815)		(815)
Stock-based compensation expense	13,002		13,002
Employee stock purchase plan (in shares)		3,000
Employee stock purchase plan	1,753	$ 0	1,753
Balance (in shares) at Sep. 30, 2019		36,650,000
Ending balance at Sep. 30, 2019	$ 3,299,166	$ 366	$ 2,460,489	$ (14,236)	$ 852,547

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Operating activities:
Net income	$ 227,036	$ 154,825
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	57,235	58,000
Amortization of deferred commissions costs	39,189	36,242
Amortization of debt issuance costs	657	657
Stock-based compensation expense	38,984	30,593
Deferred income taxes, net	13,288	7,644
Bad debt expense	7,458	4,519
Changes in operating assets and liabilities, net of acquisitions:
Accounts receivable	(8,797)	(19,038)
Prepaid expenses and other current assets	(754)	(2,952)
Deferred commissions	(48,791)	(41,421)
Lease right-of-use and other assets	16,326	(1,396)
Accounts payable and other liabilities	(4,339)	(13,443)
Deferred revenue	12,525	6,454
Net cash provided by operating activities	350,017	220,684
Investing activities:
Purchases of property and equipment and other assets	(44,162)	(21,801)
Cash paid for acquisitions, net of cash acquired	(13,721)	(340,074)
Net cash used in investing activities	(57,883)	(361,875)
Financing activities:
Repurchase of restricted stock to satisfy tax withholding obligations	(25,040)	(23,666)
Proceeds from exercise of stock options and employee stock purchase plan	22,970	25,768
Other financing activities	(123)	0
Net cash (used in) provided by financing activities	(2,193)	2,102
Effect of foreign currency exchange rates on cash and cash equivalents	(738)	(588)
Net increase (decrease) in cash and cash equivalents	289,203	(139,677)
Cash and cash equivalents at the beginning of period	1,100,416	1,211,463
Cash and cash equivalents at the end of period	1,389,619	1,071,786
Supplemental cash flow disclosures:
Interest paid	1,500	999
Income taxes paid	41,699	25,544
Supplemental non-cash investing and financing activities:
Stock issued in connection with acquisition - ForRent	0	36,366
Consideration owed for acquisitions	$ 1,650	$ 0

ORGANIZATION

ORGANIZATION	9 Months Ended
	Sep. 30, 2019
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 at least one year

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation The condensed 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. Interim Financial Statements The accompanying unaudited condensed consolidated financial statements of the Company have been prepared in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information. In the opinion of the Company’s management, the financial statements reflect all adjustments, consisting only of a normal recurring nature, necessary to present fairly the Company’s financial position at September 30, 2019 and December 31, 2018 , the results of its operations for the three and nine months ended September 30, 2019 and 2018, its comprehensive income for the three and nine months ended September 30, 2019 and 2018, its changes in stockholders' equity for the three and nine months ended September 30, 2019 and 2018, and its cash flows for the nine months ended September 30, 2019 and 2018. Certain notes and other information have been condensed or omitted from the interim financial statements presented in this Quarterly Report on Form 10-Q. Therefore, these financial statements should be read in conjunction with the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 . Use of Estimates The preparation of financial statements in conformity with 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, the useful lives and recoverability of long-lived and intangible assets, and goodwill; income taxes, the fair value of auction rate securities, accounting for business combinations, stock-based compensation, estimating the Company's incremental borrowing rate for its leases, 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 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. The Company’s subscription-based license agreements renew automatically, and a majority have a term of at least one year . 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. In limited circumstances, the Company's contracts with customers include promises to transfer multiple services, such as contracts for our subscription-based services and professional services. For these contracts, the Company accounts for individual performance obligations separately if they are distinct, which involves the determination of the standalone selling price for each distinct performance obligation. Deferred revenue results from amounts billed in advance to customers or cash received from customers in advance of the Company's fulfillment of its performance obligation(s) 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 the timing of revenue recognition. 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 of the Company's revenue recognition. 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. 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 $44 million and $38 million for the three months ended September 30, 2019 and 2018 , respectively, and $130 million and $108 million for the nine months ended September 30, 2019 and 2018 , respectively. Foreign Currency Translation The Company’s reporting currency is the U.S. dollar. The functional currency of the Company's foreign subsidiaries is the local currency. Assets and liabilities denominated in a foreign currency are translated into U.S. dollars using the exchange rates in effect as of the balance sheet date. 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 transactions denominated in a currency other than the functional currency of the entity are included in interest and other income (expense) in the condensed consolidated statements of operations using the average exchange rates in effect during the period. There were no material gains or losses from these transactions for the three and nine months ended September 30, 2019 and 2018 . Accumulated Other Comprehensive Loss The components of accumulated other comprehensive loss were as follows (in thousands): September 30, December 31, Foreign currency translation adjustment $ (13,506 ) $ (10,958 ) Net unrealized loss on investments, net of tax (730 ) (730 ) Total accumulated other comprehensive loss $ (14,236 ) $ (11,688 ) There were no amounts reclassified out of accumulated other comprehensive loss to the condensed consolidated statements of operations for the three and nine months ended September 30, 2019 and 2018 . See Note 5 for additional information regarding unrealized gains and losses recognized on investments. 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 condensed 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 following table sets forth the calculation of basic and diluted net income per share (in thousands, except per share data): Three Months Ended Nine Months Ended Numerator: 2019 2018 2019 2018 Net income $ 78,619 $ 58,759 $ 227,036 $ 154,825 Denominator: Denominator for basic net income per share — weighted-average outstanding shares 36,333 36,129 36,293 36,032 Effect of dilutive securities: Stock options and restricted stock awards 319 389 322 407 Denominator for diluted net income per share — weighted-average outstanding shares 36,652 36,518 36,615 36,439 Net income per share — basic $ 2.16 $ 1.63 $ 6.26 $ 4.30 Net income per share — diluted $ 2.15 $ 1.61 $ 6.20 $ 4.25 The Company’s potentially dilutive securities include outstanding stock options and unvested performance-based restricted stock, restricted stock and restricted stock units. Shares underlying unvested restricted stock that vest based on performance 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. Diluted net income per share considers the impact of potentially dilutive securities except when the inclusion of the potentially dilutive securities would have an anti-dilutive effect. The following table summarizes the shares underlying the unvested performance-based restricted stock and anti-dilutive stock options excluded from the basic and diluted earnings per share calculations (in thousands): Three Months Ended Nine Months Ended 2019 2018 2019 2018 Performance-based restricted stock 60 76 90 76 Anti-dilutive stock options — 39 48 76 Stock-Based Compensation Equity instruments issued in exchange for services performed by officers, directors, and employees of the Company are accounted for using a fair value based method where the fair value of such equity instruments is recognized as expense in the condensed consolidated statements of operations as they are earned. For stock-based awards that vest over a specific service period, compensation expense is measured based on the fair value of the awards at the grant date, 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 a performance condition, stock-based compensation expense is recognized based on the expected achievement of the related performance conditions at the end of each reporting period. If the Company's initial estimates of the achievement of the performance conditions change, the related stock-based compensation expense and timing may fluctuate from period to period based on those estimates. 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. For awards with both a performance 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 for stock options and restricted stock issued under equity incentive plans and stock purchases under the Employee Stock Purchase Plan included in the Company’s results of operations were as follows (in thousands): Three Months Ended Nine Months Ended 2019 2018 2019 2018 Cost of revenues $ 2,281 $ 1,382 $ 6,899 $ 4,410 Selling and marketing (excluding customer base amortization) 1,646 1,632 5,049 5,305 Software development 2,243 1,824 6,638 5,490 General and administrative 6,969 4,115 20,398 15,388 Total stock-based compensation expense $ 13,139 $ 8,953 $ 38,984 $ 30,593 Leases On January 1, 2019, the Company adopted Accounting Standards Update (“ASU”) 2016-02, Leases , later codified as Accounting Standards Codification ("ASC") 842 ( "ASC 842" ), using the modified retrospective method. For periods presented prior to the adoption date, the Company continues to follow its previous policy under ASC 840, Leases . Refer to the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, filed with the Securities and Exchange Commission for further details of the Company’s policy prior to adoption of ASC 842 . The determination of whether an arrangement contains a lease and the classification of a lease, if applicable, is made at lease commencement, at which time the Company also measures and recognizes a right-of-use ("ROU") asset, representing the Company’s right to use the underlying asset, and a lease liability, representing the Company’s obligation to make lease payments under the terms of the arrangement. For the purposes of recognizing ROU assets and lease liabilities associated with the Company’s leases, the Company has elected the practical expedient to not recognize a ROU asset or lease liability for short-term leases, which are leases with a term of twelve months or less. The lease term is defined as the noncancelable portion of the lease term plus any periods covered by an option to extend the lease if it is reasonably certain that that the option will be exercised. In determining the amount of lease payments used in measuring ROU assets and lease liabilities, the Company has elected the practical expedient not to separate non-lease components from lease components for all classes of underlying assets. Consideration considered part of the lease payments used to measure ROU assets and lease liabilities generally includes fixed payments and variable payments based on either an index or a rate. The ROU asset also includes any lease prepayments, offset by lease incentives. ROU assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. The rates implicit within the Company's leases are generally not determinable, therefore, the Company's incremental borrowing rate is used to determine the present value of lease payments. The determination of the Company’s incremental borrowing rate requires judgment. Because the Company currently has no outstanding debt, the incremental borrowing rate for each lease is primarily based on publicly-available information for companies within the same industry and with similar credit profiles as the Company. The rate is then adjusted for the impact of collateralization, the lease term and other specific terms included in the Company’s lease arrangements. The incremental borrowing rate is determined at lease commencement, or as of January 1, 2019 for operating leases in existence upon adoption of ASC 842 . The incremental borrowing rate is subsequently reassessed upon a modification to the lease arrangement. ROU assets are subsequently assessed for impairment in accordance with the Company’s accounting policy for long-lived assets. Lease costs related to the Company's operating leases are generally recognized as a single ratable lease cost over the lease term. See Note 7 for further discussion of the Company’s accounting for leases. Long-Lived Assets, Intangible Assets and Goodwill 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 the 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 undiscounted 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. Goodwill is tested annually for impairment by each reporting unit on October 1 of each year or more frequently if an event or other circumstance indicates that we may not recover the carrying value of the asset. 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 then determines the fair value of each reporting unit. 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. 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, otherwise, they are reflected as current and long-term assets. 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 regarding the Company's revolving credit facility. Business Combinations The Company allocates the purchase consideration related to business combinations to the identifiable tangible and intangible assets acquired, and liabilities assumed based on their estimated fair values. The purchase consideration 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 and other intangible assets, 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, determine their estimated fair value. 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 condensed 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 On January 1, 2019, the Company adopted ASU 2016-02, Leases , using the modified retrospective method which allows for the application of the transition provisions at the beginning of the period of adoption, rather than at the beginning of the earliest comparative period presented in these condensed consolidated financial statements. As permitted by the guidance, the Company elected to retain the original lease classification and historical accounting for initial direct costs for leases existing prior to the adoption date and did not reassess contracts entered into prior to the adoption date for the existence of a lease. The Company also did not recognize ROU assets and lease liabilities for short-term leases, which are leases in existence as of the adoption date with an original term of twelve months or less. As a result of the adoption of the standard, the Company recognized ROU assets of $116 million , including prepaid rent and deferred rent that was reclassified and recognized as of the adoption date as a component of the ROU assets, as well as lease liabilities of $150 million , on its condensed consolidated balance sheet. The assets and liabilities recognized upon application of the transition provisions were primarily associated with existing office leases. The Company also recognized a cumulative-effect adjustment to beginning retained earnings of $12 million , net of tax, as of January 1, 2019, to recognize the remaining deferred gain on the sale-leaseback of the Company's corporate headquarters building, pursuant to the guidance in ASC 842 . Recent Accounting Pronouncements Not Yet Adopted 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, including 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 ASC 350-40 to determine which implementation costs to defer and recognize as an asset. This guidance is effective for fiscal years beginning after December 15, 2019, including 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 June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which is designed to provide financial statement users with more information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. When determining such expected credit losses, the guidance requires companies to apply a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. This guidance is effective on a modified retrospective basis for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. The Company is currently evaluating the impact this guidance will have on its financial statements and related disclosures.

REVENUE FROM CONTRACTS WITH CUS

REVENUE FROM CONTRACTS WITH CUSTOMERS	9 Months Ended
	Sep. 30, 2019
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): Three Months Ended September 30, 2019 2018 North America International Total North America International Total Information and analytics CoStar Suite $ 149,187 $ 6,826 $ 156,013 $ 132,700 $ 6,384 $ 139,084 Information services 17,382 2,089 19,471 15,310 2,225 17,535 Online marketplaces Multifamily 125,707 — 125,707 104,778 — 104,778 Commercial property and land 51,508 109 51,617 44,128 — 44,128 Total revenues $ 343,784 $ 9,024 $ 352,808 $ 296,916 $ 8,609 $ 305,525 Nine Months Ended September 30, 2019 2018 North America International Total North America International Total Information and analytics CoStar Suite $ 436,070 $ 20,469 $ 456,539 $ 383,876 $ 19,381 $ 403,257 Information services 52,632 6,466 59,098 41,544 6,881 48,425 Online marketplaces Multifamily 360,463 — 360,463 297,254 — 297,254 Commercial property and land 148,418 475 148,893 127,325 — 127,325 Total revenues $ 997,583 $ 27,410 $ 1,024,993 $ 849,999 $ 26,262 $ 876,261 Deferred Revenue Changes in deferred revenue for the period were as follows (in thousands): Balance at December 31, 2018 $ 51,459 Revenue recognized in the current period from the amounts in the beginning balance (46,699 ) New deferrals, net of amounts recognized in the current period 59,224 Effects of foreign currency (237 ) Balance at September 30, 2019 (1) $ 63,747 __________________________ (1) Deferred revenue is comprised of $60 million of current liabilities and $4 million of noncurrent liabilities classified within lease and other long-term liabilities on the Company’s condensed consolidated balance sheet as of September 30, 2019 . Contract Assets The Company had contract assets of $3 million and $2 million as of September 30, 2019 and December 31, 2018 , respectively, 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 Commissions expense is included in selling and marketing expense in the Company's condensed consolidated statements of operations. Commissions expense activity for the three and nine months ended September 30, 2019 and 2018 was as follows (in thousands). The Company determined that no deferred commissions were impaired as of September 30, 2019 : Three Months Ended Nine Months Ended 2019 2018 2019 2018 Commissions incurred $ 20,371 $ 15,311 $ 62,766 $ 56,700 Commissions capitalized in the current period (15,394 ) (11,876 ) (48,791 ) (41,421 ) Amortization of deferred commissions costs 13,639 11,967 39,189 36,242 Total commissions expense $ 18,616 $ 15,402 $ 53,164 $ 51,521 Unsatisfied Performance Obligations Remaining contract consideration for which revenue has not been recognized due to unsatisfied performance obligations was approximately $213 million at September 30, 2019 , which the Company expects to recognize over the next five years . This amount does not include contract consideration for contracts with a duration of one year or less.

ACQUISITIONS

ACQUISITIONS	9 Months Ended
	Sep. 30, 2019
Business Combinations [Abstract]
ACQUISITIONS	ACQUISITIONS STR, Inc. and STR Global Ltd. On September 30, 2019, the Company entered into a securities purchase agreement to acquire all of the issued and outstanding equity interests of STR, Inc. and STR Global, Ltd. (together with STR, Inc., referred to as "STR"), for $450 million in cash, subject to certain adjustments per the definitive agreements. STR is a global leader in benchmarking and analytics for the hospitality industry. The combination of STR's and CoStar's offerings is expected to allow for the creation of valuable new and improved tools for industry participants. The transaction closed on October 22, 2019. The Company is in the process of accounting for the acquisition, including identifying the assets and liabilities assumed as part of the transaction. 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 has yielded 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): Final: February 21, 2018 Cash and cash equivalents $ 59 Accounts receivable 8,769 Indemnification asset 5,443 Goodwill 266,595 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 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 related to the determination of working capital as of the Acquisition Date and recognized in 2018 were not material. 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 had 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 goodwill that was 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, Contingencies , using a state-by-state assessment. The uncertain income tax position was determined in accordance with the provisions of ASC 740, Income Tax, 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 obligation for sales taxes in the state of Texas is limited to approximately $2 million . The total indemnification asset established as of the Acquisition Date was $5 million . $0.6 million and $0.9 million of the uncertain income tax position liability and related indemnification asset recognized as of the Acquisition Date was reversed during the three months ended September 30, 2019 and 2018 , respectively, 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 during the nine months ended September 30, 2018 . 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 funds remained in the escrow account after the employees were compensated and the defined six-month period ended, those funds were to be remitted to the Seller. The Company expensed all of the retention compensation as the services were performed in the post-combination period in 2018 . Other Acquisitions On June 12, 2019, the Company acquired Off Campus Partners, LLC ("OCP"), a provider of student housing marketplace content and technology to U.S. universities for $16 million . The purchase agreement required an initial payment of $14 million , net of cash acquired, at the time of closing, with the remainder of the purchase price payable one year following the acquisition date, subject to offset for indemnification claims or adjustments to the purchase price after final determination of closing net working capital. As part of the acquisition, the Company recorded goodwill and intangibles assets of $8 million and $9 million , respectively. The net assets of OCP were recorded at their estimated fair value. The estimated fair values are preliminary, subject to the final determination of working capital and completion of the Company's assessment of certain tax matters. On November 8, 2018, the Company acquired Cozy Services, Ltd. ("Cozy"), a 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 $52 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 Company's assessment of certain tax matters. Measurement period adjustments recognized in 2019 were not material. 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, with the remainder of the purchase price payable one year following the acquisition date, subject to offset for claims under the purchase agreement. 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 Company's assessment of certain tax matters. Measurement period adjustments recognized in 2019 were not material. 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 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 nine months ended September 30, 2018 combine the historical results of the Company and ForRent 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): Nine Months Ended September 30, 2018 Revenue $ 890,013 Net income $ 166,553 Net income per share - basic $ 4.62 Net income per share - diluted $ 4.57 Revenue attributable to ForRent from February 21, 2018 through September 30, 2018 was $50 million . Subsequent to the acquisition, the Company implemented an integration plan which included transitioning ForRent customers to arrangements in which the customer is able to advertise across the Company’s network of multifamily listing sites, including the legacy ForRent sites, for a single price. These arrangements do not specify revenues by site; however, all increases and decreases in combined revenue were allocated to ForRent for purposes of this pro forma financial information. This amount represents an estimate of ForRent revenue and is not necessarily indicative of how ForRent would have performed on a stand-alone basis. The Company began integrating the sales force and operations of ForRent after the closing of the acquisition in an effort 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 September 30, 2018.

INVESTMENTS

INVESTMENTS	9 Months Ended
	Sep. 30, 2019
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"), classified as available-for-sale and carried at fair value. Scheduled maturities of investments classified as available-for-sale as of September 30, 2019 are as follows (in thousands): Maturity Fair Value Due: October 1, 2019 — September 30, 2020 $ — October 1, 2020 — September 30, 2024 — October 1, 2024 — September 30, 2029 — After September 30, 2029 10,070 Available-for-sale investments $ 10,070 The Company had no realized gains or losses on its investments for each of the three and nine months ended September 30, 2019 and 2018 , respectively. Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis. Changes in unrealized holding gains and losses, net of the related tax effect, on available-for-sale securities are excluded from earnings and are reported as a separate component of accumulated other comprehensive loss in stockholders’ equity until realized. A decline in market value of any available-for-sale security below cost that is deemed to be other-than-temporary results in a reduction in carrying amount to fair value. The impairment is charged to earnings and a new cost basis for the security is established. Dividend and interest income are recognized when earned. As of September 30, 2019 , 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, 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 The unrealized losses on the Company’s investments as of September 30, 2019 and December 31, 2018 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 did not consider these investments to be other-than-temporarily impaired as of September 30, 2019 and December 31, 2018 . 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): September 30, December 31, 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 September 30, 2019 and December 31, 2018 .

FAIR VALUE

FAIR VALUE	9 Months Ended
	Sep. 30, 2019
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 September 30, 2019 (in thousands): Level 1 Level 2 Level 3 Total Assets: Money market funds $ 640,810 $ — $ — $ 640,810 Auction rate securities — — 10,070 10,070 Total assets measured at fair value $ 640,810 $ — $ 10,070 $ 650,880 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 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, 2017 to September 30, 2019 (in thousands): Auction Rate Securities Balance at December 31, 2017 $ 10,070 Decrease in unrealized loss included in accumulated other comprehensive loss — Balance at December 31, 2018 10,070 Decrease in unrealized loss included in accumulated other comprehensive loss — Balance at September 30, 2019 $ 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 September 30, 2019 , 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 on the Company’s condensed consolidated balance sheet as of September 30, 2019 . 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 September 30, 2019 . 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 September 30, 2019 and December 31, 2018 was approximately 5% and 6% , respectively. Selecting another discount rate within the range used in the discounted cash flow model would not result in a significant change to the fair value of the ARS. Based on this assessment of fair value, as of September 30, 2019 , the Company determined there was no decline in the fair value of its ARS investments. If the issuers are unable to successfully close future auctions and/or their credit ratings deteriorate, the Company may be required to record unrealized losses in accumulated other comprehensive loss or an other-than-temporary impairment charge to earnings on these investments.

LEASES

LEASES	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
LEASES	LEASES The Company has operating leases for its office facilities, data centers and certain vehicles, as well as finance leases for office equipment. The Company's leases have remaining terms of less than one year to ten years . The leases contain various renewal and termination options. The period which is subject to an option to extend the lease is included in the lease term if it is reasonably certain that the option will be exercised. The period which is subject to an option to terminate the lease is included if it is reasonably certain that the option will not be exercised. Lease costs related to the Company's operating leases included in the condensed consolidated statements of operations were as follows (in thousands): Three Months Ended Nine Months Ended Operating lease costs: 2019 2018 2019 2018 Cost of revenues $ 2,690 $ 3,088 $ 9,080 $ 8,983 Software development 1,091 857 3,070 2,482 Selling and marketing (excluding customer base amortization) 2,198 2,089 6,568 7,027 General and administrative 995 791 2,237 2,826 Total operating lease costs $ 6,974 $ 6,825 $ 20,955 $ 21,318 The impact of lease costs related to finance leases and short-term leases was not material for the three and nine months ended September 30, 2019 . Supplemental balance sheet information related to operating leases was as follows (in thousands): Balance Balance Sheet Location September 30, 2019 Long-term lease liabilities Lease and other long-term liabilities $ 110,477 Weighted-average remaining lease term in years 5.2 Weighted-average discount rate 4.1 % Balance sheet information related to finance leases was not material as of September 30, 2019 . Supplemental cash flow information related to leases was as follows (in thousands): Nine Months Ended Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows used in operating leases $ 24,114 ROU assets obtained in exchange for lease obligations: Operating leases $ 7,048 Maturities of operating lease liabilities at September 30, 2019 were as follows (in thousands): October 1, 2019 - September 30, 2020 $ 32,345 October 1, 2020 - September 30, 2021 30,368 October 1, 2021 - September 30, 2022 27,540 October 1, 2022 - September 30, 2023 26,388 October 1, 2023 - September 30, 2024 22,968 Thereafter 13,926 Total lease payments 153,535 Less imputed interest (15,921 ) Present value of lease liabilities $ 137,614 Future minimum lease payments as of December 31, 2018 were 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

GOODWILL

GOODWILL	9 Months Ended
	Sep. 30, 2019
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, 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 Acquisitions, including measurement period adjustments 6,428 — 6,428 Effect of foreign currency translation — (1,425 ) (1,425 ) Goodwill, September 30, 2019 $ 1,579,516 $ 37,022 $ 1,616,538 The Company recorded goodwill of approximately $8 million in connection with the June 2019 acquisition of OCP, a provider of student housing marketplace content and technology to universities in the United States. The Company recorded goodwill of approximately $53 million in connection with the November 8, 2018 acquisition of Cozy, a 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 Company recorded a measurement period adjustment during the nine months ended September 30, 2019 which resulted in a $1 million reduction to the initial amount of Cozy goodwill recognized. 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 $267 million

INTANGIBLE ASSETS

INTANGIBLE ASSETS	9 Months Ended
	Sep. 30, 2019
Intangible Assets, Net (Excluding Goodwill) [Abstract]
INTANGIBLE ASSETS	INTANGIBLE ASSETS Intangible assets consist of the following (in thousands, except amortization period data): September 30, December 31, Weighted- Average Amortization Period (in years) Acquired technology $ 101,892 $ 103,128 5 Accumulated amortization (88,841 ) (85,344 ) Acquired technology, net 13,051 17,784 Acquired customer base 346,924 339,574 10 Accumulated amortization (221,033 ) (199,405 ) Acquired customer base, net 125,891 140,169 Acquired trade names and other intangible assets 199,818 199,752 12 Accumulated amortization (79,316 ) (68,794 ) Acquired trade names and other intangible assets, net 120,502 130,958 Intangible assets, net $ 259,444 $ 288,911

LONG-TERM DEBT

LONG-TERM DEBT	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
LONG-TERM DEBT	LONG-TERM DEBT On October 19, 2017, the Company entered into an amended and restated 2017 Credit Agreement (the "2017 Credit Agreement"), which amended and restated in its entirety, the existing 2014 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. 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 September 30, 2019 and December 31, 2018 , 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 September 30, 2019 . The Company had no outstanding long-term debt at September 30, 2019 and December 31, 2018 . For the three and nine months ended September 30, 2019 , the Company recognized interest expense of $0.7 million and $2.1 million , including amortization of debt issuance costs of $0.2 million and $0.7 million and commitment fees of $0.5 million and $1.4 million, on its revolving credit facility, respectively. For the three and nine months ended September 30, 2018 , the Company recognized interest expense of $0.7 million and $2.1 million , including amortization of debt issuance costs of $0.2 million and $0.7 million and commitment fees of $0.5 million and $1.4 million, on its revolving credit facility, respectively. The Company had $3 million and $3 million of deferred debt issuance costs included in deposits and other assets on the Company's condensed consolidated balance sheets as of September 30, 2019 and December 31, 2018

INCOME TAXES

INCOME TAXES	9 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The income tax provision reflects an effective tax rate of approximately 21% and 20% for the three months ended September 30, 2019 and 2018 , respectively, and 18% and 11% for the nine months ended September 30, 2019 and 2018 , respectively. The increase in the effective tax rate for the three and nine months ended September 30, 2019 , was primarily due to higher income before income taxes during those periods, as well as, discrete items for state research and development tax credits recognized for the nine months ended September 30, 2018

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	9 Months Ended
	Sep. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES The Company leases office facilities under various non-cancelable operating leases. The leases contain various renewal options. See Note 7 for further discussion of the Company's operating lease commitments. In addition, the Company has other commitments related to purchase obligations for goods and services. 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	9 Months Ended
	Sep. 30, 2019
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): Three Months Ended Nine Months Ended 2019 2018 2019 2018 EBITDA North America $ 115,973 $ 94,088 $ 326,648 $ 229,905 International (2,882 ) (2,967 ) (6,852 ) (3,998 ) Total EBITDA $ 113,091 $ 91,121 $ 319,796 $ 225,907 The reconciliation of net income to EBITDA consists of the following (in thousands): Three Months Ended Nine Months Ended 2019 2018 2019 2018 Net income $ 78,619 $ 58,759 $ 227,036 $ 154,825 Amortization of acquired intangible assets in cost of revenues 4,957 5,310 15,503 15,242 Amortization of acquired intangible assets in operating expenses 7,586 8,329 22,443 22,948 Depreciation and other amortization 6,279 6,794 19,289 19,810 Interest and other income (5,358 ) (3,035 ) (16,216 ) (8,674 ) Interest and other expense 704 717 2,133 2,135 Income tax expense 20,304 14,247 49,608 19,621 EBITDA $ 113,091 $ 91,121 $ 319,796 $ 225,907 Summarized information by operating segment consists of the following (in thousands): September 30, December 31, Property and equipment, net: North America $ 104,239 $ 79,493 International 3,750 3,810 Total property and equipment, net $ 107,989 $ 83,303 Goodwill: North America $ 1,579,516 $ 1,573,088 International 37,022 38,447 Total goodwill $ 1,616,538 $ 1,611,535 Assets: North America $ 3,638,588 $ 3,253,035 International 76,803 59,922 Total assets $ 3,715,391 $ 3,312,957 Liabilities: North America $ 385,254 $ 272,776 International 30,971 18,239 Total liabilities $ 416,225 $ 291,015

SUMMARY OF SIGNIFICANT ACCOUN_2

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with 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, the useful lives and recoverability of long-lived and intangible assets, and goodwill; income taxes, the fair value of auction rate securities, accounting for business combinations, stock-based compensation, estimating the Company's incremental borrowing rate for its leases, 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 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. The Company’s subscription-based license agreements renew automatically, and a majority have a term of at least one year . 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. In limited circumstances, the Company's contracts with customers include promises to transfer multiple services, such as contracts for our subscription-based services and professional services. For these contracts, the Company accounts for individual performance obligations separately if they are distinct, which involves the determination of the standalone selling price for each distinct performance obligation. Deferred revenue results from amounts billed in advance to customers or cash received from customers in advance of the Company's fulfillment of its performance obligation(s) 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 the timing of revenue recognition. 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.
Cost of Revenues	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.
Advertising Costs	Advertising Costs
Foreign Currency Translation	Foreign Currency Translation The Company’s reporting currency is the U.S. dollar. The functional currency of the Company's foreign subsidiaries is the local currency. Assets and liabilities denominated in a foreign currency are translated into U.S. dollars using the exchange rates in effect as of the balance sheet date. 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
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 condensed 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.
Stock-Based Compensation	Stock-Based Compensation Equity instruments issued in exchange for services performed by officers, directors, and employees of the Company are accounted for using a fair value based method where the fair value of such equity instruments is recognized as expense in the condensed consolidated statements of operations as they are earned. For stock-based awards that vest over a specific service period, compensation expense is measured based on the fair value of the awards at the grant date, 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 a performance condition, stock-based compensation expense is recognized based on the expected achievement of the related performance conditions at the end of each reporting period. If the Company's initial estimates of the achievement of the performance conditions change, the related stock-based compensation expense and timing may fluctuate from period to period based on those estimates. 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. For awards with both a performance 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.
Leases	Leases On January 1, 2019, the Company adopted Accounting Standards Update (“ASU”) 2016-02, Leases , later codified as Accounting Standards Codification ("ASC") 842 ( "ASC 842" ), using the modified retrospective method. For periods presented prior to the adoption date, the Company continues to follow its previous policy under ASC 840, Leases . Refer to the Company’s Annual Report on Form 10-K for the year ended December 31, 2018, filed with the Securities and Exchange Commission for further details of the Company’s policy prior to adoption of ASC 842 . The determination of whether an arrangement contains a lease and the classification of a lease, if applicable, is made at lease commencement, at which time the Company also measures and recognizes a right-of-use ("ROU") asset, representing the Company’s right to use the underlying asset, and a lease liability, representing the Company’s obligation to make lease payments under the terms of the arrangement. For the purposes of recognizing ROU assets and lease liabilities associated with the Company’s leases, the Company has elected the practical expedient to not recognize a ROU asset or lease liability for short-term leases, which are leases with a term of twelve months or less. The lease term is defined as the noncancelable portion of the lease term plus any periods covered by an option to extend the lease if it is reasonably certain that that the option will be exercised. In determining the amount of lease payments used in measuring ROU assets and lease liabilities, the Company has elected the practical expedient not to separate non-lease components from lease components for all classes of underlying assets. Consideration considered part of the lease payments used to measure ROU assets and lease liabilities generally includes fixed payments and variable payments based on either an index or a rate. The ROU asset also includes any lease prepayments, offset by lease incentives. ROU assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. The rates implicit within the Company's leases are generally not determinable, therefore, the Company's incremental borrowing rate is used to determine the present value of lease payments. The determination of the Company’s incremental borrowing rate requires judgment. Because the Company currently has no outstanding debt, the incremental borrowing rate for each lease is primarily based on publicly-available information for companies within the same industry and with similar credit profiles as the Company. The rate is then adjusted for the impact of collateralization, the lease term and other specific terms included in the Company’s lease arrangements. The incremental borrowing rate is determined at lease commencement, or as of January 1, 2019 for operating leases in existence upon adoption of ASC 842 . The incremental borrowing rate is subsequently reassessed upon a modification to the lease arrangement. ROU assets are subsequently assessed for impairment in accordance with the Company’s accounting policy for long-lived assets.
Long-Lived Assets, Intangible 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 the 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 undiscounted 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.
Goodwill	Goodwill is tested annually for impairment by each reporting unit on October 1 of each year or more frequently if an event or other circumstance indicates that we may not recover the carrying value of the asset. 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 then determines the fair value of each reporting unit. 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.
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, otherwise, they are reflected as current and long-term assets. 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 related to business combinations to the identifiable tangible and intangible assets acquired, and liabilities assumed based on their estimated fair values. The purchase consideration 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 and other intangible assets, 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, determine their estimated fair value. 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 condensed 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 On January 1, 2019, the Company adopted ASU 2016-02, Leases , using the modified retrospective method which allows for the application of the transition provisions at the beginning of the period of adoption, rather than at the beginning of the earliest comparative period presented in these condensed consolidated financial statements. As permitted by the guidance, the Company elected to retain the original lease classification and historical accounting for initial direct costs for leases existing prior to the adoption date and did not reassess contracts entered into prior to the adoption date for the existence of a lease. The Company also did not recognize ROU assets and lease liabilities for short-term leases, which are leases in existence as of the adoption date with an original term of twelve months or less. As a result of the adoption of the standard, the Company recognized ROU assets of $116 million , including prepaid rent and deferred rent that was reclassified and recognized as of the adoption date as a component of the ROU assets, as well as lease liabilities of $150 million , on its condensed consolidated balance sheet. The assets and liabilities recognized upon application of the transition provisions were primarily associated with existing office leases. The Company also recognized a cumulative-effect adjustment to beginning retained earnings of $12 million , net of tax, as of January 1, 2019, to recognize the remaining deferred gain on the sale-leaseback of the Company's corporate headquarters building, pursuant to the guidance in ASC 842 . Recent Accounting Pronouncements Not Yet Adopted 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, including 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 ASC 350-40 to determine which implementation costs to defer and recognize as an asset. This guidance is effective for fiscal years beginning after December 15, 2019, including 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 June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which is designed to provide financial statement users with more information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. When determining such expected credit losses, the guidance requires companies to apply a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. This guidance is effective on a modified retrospective basis for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. The Company is currently evaluating the impact this guidance will have on its financial statements and related disclosures.
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"), classified as available-for-sale and carried at fair value.

SUMMARY OF SIGNIFICANT ACCOUN_3

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Schedule of accumulated other comprehensive loss	The components of accumulated other comprehensive loss were as follows (in thousands): September 30, December 31, Foreign currency translation adjustment $ (13,506 ) $ (10,958 ) Net unrealized loss on investments, net of tax (730 ) (730 ) Total accumulated other comprehensive loss $ (14,236 ) $ (11,688 )
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): Three Months Ended Nine Months Ended Numerator: 2019 2018 2019 2018 Net income $ 78,619 $ 58,759 $ 227,036 $ 154,825 Denominator: Denominator for basic net income per share — weighted-average outstanding shares 36,333 36,129 36,293 36,032 Effect of dilutive securities: Stock options and restricted stock awards 319 389 322 407 Denominator for diluted net income per share — weighted-average outstanding shares 36,652 36,518 36,615 36,439 Net income per share — basic $ 2.16 $ 1.63 $ 6.26 $ 4.30 Net income per share — diluted $ 2.15 $ 1.61 $ 6.20 $ 4.25
Schedule of anti-dilutive securities excluded from computation of earnings per share	The following table summarizes the shares underlying the unvested performance-based restricted stock and anti-dilutive stock options excluded from the basic and diluted earnings per share calculations (in thousands): Three Months Ended Nine Months Ended 2019 2018 2019 2018 Performance-based restricted stock 60 76 90 76 Anti-dilutive stock options — 39 48 76
Stock-based compensation expense for stock options and restricted stock	Stock-based compensation expense for stock options and restricted stock issued under equity incentive plans and stock purchases under the Employee Stock Purchase Plan included in the Company’s results of operations were as follows (in thousands): Three Months Ended Nine Months Ended 2019 2018 2019 2018 Cost of revenues $ 2,281 $ 1,382 $ 6,899 $ 4,410 Selling and marketing (excluding customer base amortization) 1,646 1,632 5,049 5,305 Software development 2,243 1,824 6,638 5,490 General and administrative 6,969 4,115 20,398 15,388 Total stock-based compensation expense $ 13,139 $ 8,953 $ 38,984 $ 30,593

REVENUE FROM CONTRACTS WITH C_2

REVENUE FROM CONTRACTS WITH CUSTOMERS (Tables)	9 Months Ended
	Sep. 30, 2019
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): Three Months Ended September 30, 2019 2018 North America International Total North America International Total Information and analytics CoStar Suite $ 149,187 $ 6,826 $ 156,013 $ 132,700 $ 6,384 $ 139,084 Information services 17,382 2,089 19,471 15,310 2,225 17,535 Online marketplaces Multifamily 125,707 — 125,707 104,778 — 104,778 Commercial property and land 51,508 109 51,617 44,128 — 44,128 Total revenues $ 343,784 $ 9,024 $ 352,808 $ 296,916 $ 8,609 $ 305,525 Nine Months Ended September 30, 2019 2018 North America International Total North America International Total Information and analytics CoStar Suite $ 436,070 $ 20,469 $ 456,539 $ 383,876 $ 19,381 $ 403,257 Information services 52,632 6,466 59,098 41,544 6,881 48,425 Online marketplaces Multifamily 360,463 — 360,463 297,254 — 297,254 Commercial property and land 148,418 475 148,893 127,325 — 127,325 Total revenues $ 997,583 $ 27,410 $ 1,024,993 $ 849,999 $ 26,262 $ 876,261
Contract with Customer, Asset and Liability	Changes in deferred revenue for the period were as follows (in thousands): Balance at December 31, 2018 $ 51,459 Revenue recognized in the current period from the amounts in the beginning balance (46,699 ) New deferrals, net of amounts recognized in the current period 59,224 Effects of foreign currency (237 ) Balance at September 30, 2019 (1) $ 63,747 __________________________ (1) Deferred revenue is comprised of $60 million of current liabilities and $4 million of noncurrent liabilities classified within lease and other long-term liabilities on the Company’s condensed consolidated balance sheet as of September 30, 2019 .
Schedule of Commissions Expense	Commissions expense activity for the three and nine months ended September 30, 2019 and 2018 was as follows (in thousands). The Company determined that no deferred commissions were impaired as of September 30, 2019 : Three Months Ended Nine Months Ended 2019 2018 2019 2018 Commissions incurred $ 20,371 $ 15,311 $ 62,766 $ 56,700 Commissions capitalized in the current period (15,394 ) (11,876 ) (48,791 ) (41,421 ) Amortization of deferred commissions costs 13,639 11,967 39,189 36,242 Total commissions expense $ 18,616 $ 15,402 $ 53,164 $ 51,521

ACQUISITIONS (Tables)

ACQUISITIONS (Tables)	9 Months Ended
	Sep. 30, 2019
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): Final: February 21, 2018 Cash and cash equivalents $ 59 Accounts receivable 8,769 Indemnification asset 5,443 Goodwill 266,595 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
Business Acquisition, Pro Forma Information	The unaudited pro forma financial information was as follows (in thousands, except per share data): Nine Months Ended September 30, 2018 Revenue $ 890,013 Net income $ 166,553 Net income per share - basic $ 4.62 Net income per share - diluted $ 4.57

INVESTMENTS (Tables)

INVESTMENTS (Tables)	9 Months Ended
	Sep. 30, 2019
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 September 30, 2019 are as follows (in thousands): Maturity Fair Value Due: October 1, 2019 — September 30, 2020 $ — October 1, 2020 — September 30, 2024 — October 1, 2024 — September 30, 2029 — After September 30, 2029 10,070 Available-for-sale investments $ 10,070
Schedule of available for sale securities reconciliation	As of September 30, 2019 , 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, 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
Schedule of unrealized loss on investments for twelve months or longer	The components of the Company’s investments in an unrealized loss position for twelve months or longer were as follows (in thousands): September 30, December 31, 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)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Summary of fair value hierarchy for its 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 September 30, 2019 (in thousands): Level 1 Level 2 Level 3 Total Assets: Money market funds $ 640,810 $ — $ — $ 640,810 Auction rate securities — — 10,070 10,070 Total assets measured at fair value $ 640,810 $ — $ 10,070 $ 650,880 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
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, 2017 to September 30, 2019 (in thousands): Auction Rate Securities Balance at December 31, 2017 $ 10,070 Decrease in unrealized loss included in accumulated other comprehensive loss — Balance at December 31, 2018 10,070 Decrease in unrealized loss included in accumulated other comprehensive loss — Balance at September 30, 2019 $ 10,070

LEASES (Tables)

LEASES (Tables)	9 Months Ended
	Sep. 30, 2019
Leases [Abstract]
Components of lease expense	Lease costs related to the Company's operating leases included in the condensed consolidated statements of operations were as follows (in thousands): Three Months Ended Nine Months Ended Operating lease costs: 2019 2018 2019 2018 Cost of revenues $ 2,690 $ 3,088 $ 9,080 $ 8,983 Software development 1,091 857 3,070 2,482 Selling and marketing (excluding customer base amortization) 2,198 2,089 6,568 7,027 General and administrative 995 791 2,237 2,826 Total operating lease costs $ 6,974 $ 6,825 $ 20,955 $ 21,318
Supplemental Balance Sheet Information Related to Leases	Supplemental balance sheet information related to operating leases was as follows (in thousands): Balance Balance Sheet Location September 30, 2019 Long-term lease liabilities Lease and other long-term liabilities $ 110,477 Weighted-average remaining lease term in years 5.2 Weighted-average discount rate 4.1 %
Schedule of Supplemental Cash Flow Information for Leases	Supplemental cash flow information related to leases was as follows (in thousands): Nine Months Ended Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows used in operating leases $ 24,114 ROU assets obtained in exchange for lease obligations: Operating leases $ 7,048
Lessee, Operating Lease, Liability, Maturity	Future minimum lease payments as of December 31, 2018 were 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 Maturities of operating lease liabilities at September 30, 2019 were as follows (in thousands): October 1, 2019 - September 30, 2020 $ 32,345 October 1, 2020 - September 30, 2021 30,368 October 1, 2021 - September 30, 2022 27,540 October 1, 2022 - September 30, 2023 26,388 October 1, 2023 - September 30, 2024 22,968 Thereafter 13,926 Total lease payments 153,535 Less imputed interest (15,921 ) Present value of lease liabilities $ 137,614
Lessee, Operating Lease Prior to ASC 842 Adoption	Future minimum lease payments as of December 31, 2018 were 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

GOODWILL (Tables)

GOODWILL (Tables)	9 Months Ended
	Sep. 30, 2019
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, 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 Acquisitions, including measurement period adjustments 6,428 — 6,428 Effect of foreign currency translation — (1,425 ) (1,425 ) Goodwill, September 30, 2019 $ 1,579,516 $ 37,022 $ 1,616,538

INTANGIBLE ASSETS (Tables)

INTANGIBLE ASSETS (Tables)	9 Months Ended
	Sep. 30, 2019
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): September 30, December 31, Weighted- Average Amortization Period (in years) Acquired technology $ 101,892 $ 103,128 5 Accumulated amortization (88,841 ) (85,344 ) Acquired technology, net 13,051 17,784 Acquired customer base 346,924 339,574 10 Accumulated amortization (221,033 ) (199,405 ) Acquired customer base, net 125,891 140,169 Acquired trade names and other intangible assets 199,818 199,752 12 Accumulated amortization (79,316 ) (68,794 ) Acquired trade names and other intangible assets, net 120,502 130,958 Intangible assets, net $ 259,444 $ 288,911

SEGMENT REPORTING (Tables)

SEGMENT REPORTING (Tables)	9 Months Ended
	Sep. 30, 2019
Segment Reporting [Abstract]
Summarized information by operating segment	Summarized information by operating segment consists of the following (in thousands): Three Months Ended Nine Months Ended 2019 2018 2019 2018 EBITDA North America $ 115,973 $ 94,088 $ 326,648 $ 229,905 International (2,882 ) (2,967 ) (6,852 ) (3,998 ) Total EBITDA $ 113,091 $ 91,121 $ 319,796 $ 225,907
Reconciliation of net income to EBITDA	The reconciliation of net income to EBITDA consists of the following (in thousands): Three Months Ended Nine Months Ended 2019 2018 2019 2018 Net income $ 78,619 $ 58,759 $ 227,036 $ 154,825 Amortization of acquired intangible assets in cost of revenues 4,957 5,310 15,503 15,242 Amortization of acquired intangible assets in operating expenses 7,586 8,329 22,443 22,948 Depreciation and other amortization 6,279 6,794 19,289 19,810 Interest and other income (5,358 ) (3,035 ) (16,216 ) (8,674 ) Interest and other expense 704 717 2,133 2,135 Income tax expense 20,304 14,247 49,608 19,621 EBITDA $ 113,091 $ 91,121 $ 319,796 $ 225,907
Summarized information by operating segment, assets and liabilities	Summarized information by operating segment consists of the following (in thousands): September 30, December 31, Property and equipment, net: North America $ 104,239 $ 79,493 International 3,750 3,810 Total property and equipment, net $ 107,989 $ 83,303 Goodwill: North America $ 1,579,516 $ 1,573,088 International 37,022 38,447 Total goodwill $ 1,616,538 $ 1,611,535 Assets: North America $ 3,638,588 $ 3,253,035 International 76,803 59,922 Total assets $ 3,715,391 $ 3,312,957 Liabilities: North America $ 385,254 $ 272,776 International 30,971 18,239 Total liabilities $ 416,225 $ 291,015

ORGANIZATION (Details)

ORGANIZATION (Details)	9 Months Ended
	Sep. 30, 2019operating_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	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Term of subscription-based license agreements (in years)			1 year
Deferred sales commission, amortization period			3 years
Advertising expense	$ 44	$ 38	$ 130	$ 108

SUMMARY OF SIGNIFICANT ACCOUN_5

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES , FOREIGN CURRENCY AND ACCUMULATED OTHER COMPREHENSIVE LOSS, NET OF TAX (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Accounting Policies [Abstract]
Material gains or losses from foreign currency transactions	$ 0	$ 0	$ 0	$ 0
Accumulated Other Comprehensive Loss Net of Tax [Abstract]
Foreign currency translation adjustment	(13,506,000)		(13,506,000)		$ (10,958,000)
Net unrealized loss on investments, net of tax	(730,000)		(730,000)		(730,000)
Total accumulated other comprehensive loss	(14,236,000)		(14,236,000)		$ (11,688,000)
Reclassification out of accumulated other comprehensive loss	$ 0	$ 0	$ 0	$ 0

SUMMARY OF SIGNIFICANT ACCOUN_6

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES , NET INCOME PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income	$ 78,619	$ 63,248	$ 85,169	$ 58,759	$ 43,835	$ 52,231	$ 227,036	$ 154,825
Denominator:
Denominator for basic net income per share - weighted-average outstanding shares (in shares)	36,333			36,129			36,293	36,032
Effect of dilutive securities:
Stock options and restricted stock awards	319			389			322	407
Denominator for diluted net income per share — weighted average outstanding shares (in shares)	36,652			36,518			36,615	36,439
Net income per share - basic (in dollars per share)	$ 2.16			$ 1.63			$ 6.26	$ 4.30
Net income per share - diluted (in dollars per share)	$ 2.15			$ 1.61			$ 6.20	$ 4.25
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	0			39			48	76
Performance Based Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	60			76			90	76

SUMMARY OF SIGNIFICANT ACCOUN_7

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES , STOCK BASED COMPENSATION EXPENSE (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	$ 13,139	$ 8,953	$ 38,984	$ 30,593
Cost of Revenues [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	2,281	1,382	6,899	4,410
Selling and Marketing (excluding customer base amortization) [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	1,646	1,632	5,049	5,305
Software Development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	2,243	1,824	6,638	5,490
General and Administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Compensation expense	$ 6,969	$ 4,115	$ 20,398	$ 15,388

SUMMARY OF SIGNIFICANT ACCOUN_8

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES , RECENT ACCOUNTING PRONOUCEMENTS (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease right-of-use assets	$ 105,516		$ 0
Lease liability	$ 137,614
Cumulative effect of adoption of new accounting standard, net of tax		$ 12,057		$ 54,464
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease right-of-use assets		116,000
Lease liability		150,000
Cumulative effect of adoption of new accounting standard, net of tax		$ 12,000

REVENUE FROM CONTRACTS WITH C_3

REVENUE FROM CONTRACTS WITH CUSTOMERS , DISAGGREGATED REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 352,808	$ 305,525	$ 1,024,993	$ 876,261
Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Revenues	156,013	139,084	456,539	403,257
Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Revenues	19,471	17,535	59,098	48,425
Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Revenues	125,707	104,778	360,463	297,254
Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Revenues	51,617	44,128	148,893	127,325
North America [Member]
Disaggregation of Revenue [Line Items]
Revenues	343,784	296,916	997,583	849,999
North America [Member] | Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Revenues	149,187	132,700	436,070	383,876
North America [Member] | Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Revenues	17,382	15,310	52,632	41,544
North America [Member] | Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Revenues	125,707	104,778	360,463	297,254
North America [Member] | Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Revenues	51,508	44,128	148,418	127,325
International [Member]
Disaggregation of Revenue [Line Items]
Revenues	9,024	8,609	27,410	26,262
International [Member] | Information And Analytics [Member] | CoStar Suite [Member]
Disaggregation of Revenue [Line Items]
Revenues	6,826	6,384	20,469	19,381
International [Member] | Information And Analytics [Member] | Information services [Member]
Disaggregation of Revenue [Line Items]
Revenues	2,089	2,225	6,466	6,881
International [Member] | Online Marketplaces [Member] | Multifamily Online Marketplace [Member]
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
International [Member] | Online Marketplaces [Member] | Commercial property and land [Member]
Disaggregation of Revenue [Line Items]
Revenues	$ 109	$ 0	$ 475	$ 0

REVENUE FROM CONTRACTS WITH C_4

REVENUE FROM CONTRACTS WITH CUSTOMERS , CONTRACT ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Change in Contract with Customer, Liability [Roll Forward]
Beginning balance	$ 51,459
Revenue recognized in the current period from the amounts in the beginning balance	(46,699)
New deferrals, net of amounts recognized in the current period	59,224
Effects of foreign currency	(237)
Ending balance	63,747
Current liability	60,328	$ 51,459
Noncurrent liability	$ 4,000

REVENUE FROM CONTRACTS WITH C_5

REVENUE FROM CONTRACTS WITH CUSTOMERS , CONTRACT ASSETS (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Revenue from Contract with Customer [Abstract]
Contract with customer, asset, gross	$ 3	$ 2

REVENUE FROM CONTRACTS WITH C_6

REVENUE FROM CONTRACTS WITH CUSTOMERS , COMMISSIONS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
Commissions incurred	$ 20,371	$ 15,311	$ 62,766	$ 56,700
Commissions capitalized in the current period	(15,394)	(11,876)	(48,791)	(41,421)
Amortization of deferred commissions costs	13,639	11,967	39,189	36,242
Total commissions expense	$ 18,616	$ 15,402	$ 53,164	$ 51,521

REVENUE FROM CONTRACTS WITH C_7

REVENUE FROM CONTRACTS WITH CUSTOMERS , UNSATISFIED PERFORMANCE OBLIGATIONS (Details) $ in Millions	Sep. 30, 2019USD ($)
Revenue from Contract with Customer [Abstract]
Revenue, remaining performance obligation	$ 213
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period	5 years

ACQUISITIONS (Details)

ACQUISITIONS (Details) £ in Millions	Oct. 22, 2019USD ($)	Jun. 12, 2019USD ($)	Nov. 08, 2018USD ($)	Oct. 12, 2018GBP (£)	Oct. 12, 2018USD ($)	Feb. 21, 2018USD ($)shares	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Jun. 30, 2019USD ($)	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
Goodwill							$ 1,616,538,000		$ 1,611,535,000		$ 1,283,457,000
Cash paid for acquisitions, net of cash acquired							13,721,000	$ 340,074,000
Acquisitions							6,428,000		$ 329,938,000
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) | shares						103,280
Purchase price, shares issued						$ 36,000,000
Goodwill						266,595,000
Goodwill, expected tax deductible amount						8,000,000
Indemnification asset						6,260,000
State uncertain income tax position liability						2,047,000
Indemnification asset established as of the Acquisition Date						$ 5,443,000
Indemnification asset							600,000	900,000
Transaction costs								3,000,000
Off Campus Partners [Member]
Business Acquisition [Line Items]
Goodwill							$ 8,000,000			$ 8,000,000
Cash paid for acquisitions, net of cash acquired		$ 16,000,000
Initial payments to acquire businesses, net of cash acquired		14,000,000
Finite-lived intangible assets acquired		$ 9,000,000
Cozy Services Ltd [Member]
Business Acquisition [Line Items]
Cash paid for acquisitions, net of cash acquired			$ 65,000,000
Acquisitions			52,000,000
Finite-lived intangible assets acquired			$ 11,000,000
Realla Ltd [Member]
Business Acquisition [Line Items]
Cash paid for acquisitions, net of cash acquired				£ 12	$ 15,000,000
Initial payments to acquire businesses, net of cash acquired				10	13,000,000
Acquisitions				8	10,000,000
Finite-lived intangible assets acquired				£ 4	$ 5,000,000
Customer Relationships [Member] | ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Weighted-average amortization period						10 years
Database Rights [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
Acquired 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								$ 12,000,000
Subsequent Event [Member] | STR Inc and STR Global Ltd [Member]
Business Acquisition [Line Items]
Cash payment	$ 450,000,000

ACQUISITIONS , SCHEDULE OF IDEN

ACQUISITIONS , SCHEDULE OF IDENTIFIED ASSETS AND LIABILITIES ASSUMED (Details) - USD ($) $ in Thousands	Jun. 12, 2019	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Feb. 21, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Cash paid for acquisitions, net of cash acquired		$ 13,721	$ 340,074
Goodwill		1,616,538			$ 1,611,535		$ 1,283,457
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
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
Off Campus Partners [Member]
Business Acquisition [Line Items]
Cash paid for acquisitions, net of cash acquired	$ 16,000
Initial payments to acquire businesses, net of cash acquired	14,000
Goodwill		$ 8,000		$ 8,000
Finite-lived intangible assets acquired	$ 9,000

ACQUISITIONS , SCHEDULE OF PRO

ACQUISITIONS , SCHEDULE OF PRO FORMA INFORMATION (Details) - USD ($) $ / shares in Units, $ in Thousands	7 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Business Combinations [Abstract]
Revenue		$ 890,013
Net income		$ 166,553
Net income per share - basic (usd per share)		$ 4.62
Net income per share - diluted (usd per share)		$ 4.57
Revenue since acquisition	$ 50,000

INVESTMENTS, SCHEDULED MATURITI

INVESTMENTS, SCHEDULED MATURITIES AND REALIZED GAINS AND LOSSES (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt Maturities Fair Value [Abstract]
October 1, 2019 — September 30, 2020	$ 0		$ 0
October 1, 2020 — September 30, 2024	0		0
October 1, 2024 — September 30, 2029	0		0
After September 30, 2029	10,070,000		10,070,000
Available-for-sale investments	10,070,000		10,070,000
Available-for-sale securities, gross realized gains	0	$ 0	0	$ 0
Available-for-sale securities, gross realized losses	$ 0	$ 0	$ 0	$ 0

INVESTMENTS, AVAILABLE-FOR-SALE

INVESTMENTS, AVAILABLE-FOR-SALE SECURITIES (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Available-for-sale Securities Reconciliation [Abstract]
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
Available-For-Sale Securities, Unrealized Loss Positions [Abstract]
Continuous unrealized loss position, 12 months or more, aggregated fair value	10,070,000	10,070,000
Continuous unrealized loss position, 12 months or more, gross unrealized losses	(730,000)	(730,000)
Continuous unrealized loss position, 12 months or less, gross unrealized losses	0	0
Auction Rate Securities [Member]
Available-for-sale Securities Reconciliation [Abstract]
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
Available-For-Sale Securities, Unrealized Loss Positions [Abstract]
Continuous unrealized loss position, 12 months or more, aggregated fair value	10,070,000	10,070,000
Continuous unrealized loss position, 12 months or more, gross unrealized losses	$ (730,000)	$ (730,000)

FAIR VALUE (Details)

FAIR VALUE (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2019USD ($)	Dec. 31, 2018USD ($)
Assets:
Total assets measured at fair value	$ 650,880,000	$ 600,637,000
Unobservable inputs assets (level 3) [Roll forward]
Temporary impairment of the auction rates security investments, net of unrealized gain	(730,000)	(730,000)
Money Market Funds [Member]
Assets:
Total assets measured at fair value	640,810,000	590,567,000
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	640,810,000	590,567,000
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	640,810,000	590,567,000
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	10,070,000
Decrease in unrealized loss included in accumulated other comprehensive loss	0	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 underlying securities involved in auction rate securities	20 years
Par value of company held auction rate securities	$ 11,000,000
Temporary impairment of the auction rates security investments, net of unrealized gain	$ 0
Weighted Average [Member] | Measurement Input, Discount Rate [Member]
Unobservable inputs assets (level 3) [Roll forward]
Discount rate	0.05	0.06

LEASES (Narrative) (Details)

LEASES (Narrative) (Details)	Sep. 30, 2019
Minimum [Member]
Lessee, Lease, Description [Line Items]
Term of contract	1 year
Maximum [Member]
Lessee, Lease, Description [Line Items]
Term of contract	10 years

LEASES (Lease Cost) (Details)

LEASES (Lease Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Lessee, Lease, Description [Line Items]
Total operating lease costs	$ 6,974	$ 6,825	$ 20,955	$ 21,318
Cost of Revenues [Member]
Lessee, Lease, Description [Line Items]
Total operating lease costs	2,690	3,088	9,080	8,983
Software Development [Member]
Lessee, Lease, Description [Line Items]
Total operating lease costs	1,091	857	3,070	2,482
Selling and Marketing [Member]
Lessee, Lease, Description [Line Items]
Total operating lease costs	2,198	2,089	6,568	7,027
General and Administrative [Member]
Lessee, Lease, Description [Line Items]
Total operating lease costs	$ 995	$ 791	$ 2,237	$ 2,826

LEASES (Supplemental Balance Sh

LEASES (Supplemental Balance Sheet Information Related to Leases) (Details) $ in Thousands	Sep. 30, 2019USD ($)
Operating leases
Long-term lease liabilities	$ 110,477
Weighted-average remaining lease term in years	5 years 2 months 12 days
Weighted-average discount rate	4.10%

LEASES (Supplemental Cash Flow

LEASES (Supplemental Cash Flow Information Related to Leases) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019USD ($)
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows used in operating leases	$ 24,114
ROU assets obtained in exchange for lease obligations:
Operating leases	$ 7,048

LEASES (Maturities of Operating

LEASES (Maturities of Operating Lease Liabilities) (Details) $ in Thousands	Sep. 30, 2019USD ($)
Leases [Abstract]
October 1, 2019 - September 30, 2020	$ 32,345
October 1, 2020 - September 30, 2021	30,368
October 1, 2021 - September 30, 2022	27,540
October 1, 2022 - September 30, 2023	26,388
October 1, 2023 - September 30, 2024	22,968
Thereafter	13,926
Total lease payments	153,535
Less imputed interest	(15,921)
Present value of lease liabilities	$ 137,614

LEASES (Lessee, Operating Lease

LEASES (Lessee, Operating Lease Prior to ASC 842 Adoption) (Details) $ in Thousands	Dec. 31, 2018USD ($)
Leases [Abstract]
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

GOODWILL (Details)

GOODWILL (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, beginning balance	$ 1,611,535	$ 1,283,457
Acquisitions	6,428	329,938
Effect of foreign currency translation	(1,425)	(1,860)
Goodwill, ending balance	1,616,538	1,611,535
North America [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	1,573,088	1,253,494
Acquisitions	6,428	319,594
Effect of foreign currency translation	0	0
Goodwill, ending balance	1,579,516	1,573,088
International [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance	38,447	29,963
Acquisitions	0	10,344
Effect of foreign currency translation	(1,425)	(1,860)
Goodwill, ending balance	$ 37,022	$ 38,447

GOODWILL (Narrative) (Details)

GOODWILL (Narrative) (Details) $ in Thousands, £ in Millions	Nov. 08, 2018USD ($)	Oct. 12, 2018GBP (£)	Oct. 12, 2018USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2018USD ($)	Jun. 30, 2019USD ($)	Feb. 21, 2018USD ($)	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
Goodwill				$ 1,616,538	$ 1,611,535			$ 1,283,457
Goodwill acquired				6,428	$ 329,938
ForRent, Division Of DE Holdings, Inc. [Member]
Business Acquisition [Line Items]
Goodwill							$ 266,595
Realla Ltd [Member]
Business Acquisition [Line Items]
Goodwill acquired		£ 8	$ 10,000
Cozy Services Ltd [Member]
Business Acquisition [Line Items]
Goodwill, gross	$ 53,000
Goodwill increase (decrease)				(1,000)
Goodwill acquired	$ 52,000
Off Campus Partners [Member]
Business Acquisition [Line Items]
Goodwill				$ 8,000		$ 8,000

INTANGIBLE ASSETS (Details)

INTANGIBLE ASSETS (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	$ 259,444	$ 288,911
Acquired database technology [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	101,892	103,128
Finite-lived intangible assets, accumulated amortization	(88,841)	(85,344)
Finite-lived intangible assets, net	$ 13,051	17,784
Weighted-average amortization period	5 years
Acquired Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 346,924	339,574
Finite-lived intangible assets, accumulated amortization	(221,033)	(199,405)
Finite-lived intangible assets, net	$ 125,891	140,169
Weighted-average amortization period	10 years
Acquired trade names and other intangible assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 199,818	199,752
Finite-lived intangible assets, accumulated amortization	(79,316)	(68,794)
Finite-lived intangible assets, net	$ 120,502	$ 130,958
Weighted-average amortization period	12 years

LONG-TERM DEBT (Details)

LONG-TERM DEBT (Details) - USD ($)	Oct. 19, 2017	Apr. 02, 2014	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Debt Instrument [Line Items]
Debt outstanding							$ 0
Interest expense, debt			$ 700,000	$ 700,000	$ 2,100,000	$ 2,100,000
Amortization of debt issuance costs			200,000	200,000	700,000	700,000
Commitment fee			500,000	$ 500,000	1,400,000	$ 1,400,000
Capitalized debt issuance costs			3,000,000		3,000,000		$ 3,000,000
Letter of Credit [Member]
Debt Instrument [Line Items]
Debt outstanding			$ 200,000		$ 200,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Term of loan		5 years
Revolving Credit Facility [Member] | 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 750,000,000
Term of loan	5 years
Covenant compliance, secured leverage ratio	3.50
Covenant compliance, total leverage ratio	4.50
Letter of Credit [Member] | 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 20,000,000
Interest rate, increase (decrease)	2.00%
Letter of Credit [Member] | 2017 Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on federal funds rate (in percent)	1.00%
Letter of Credit [Member] | 2017 Credit Agreement [Member] | Federal Funds Rate [Member]
Debt Instrument [Line Items]
Basis spread on federal funds rate (in percent)	0.50%
Initial Basis Spread [Member] | Letter of Credit [Member] | 2017 Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on federal funds rate (in percent)	1.25%
Initial Basis Spread One Month LIBOR [Member] | Letter of Credit [Member] | 2017 Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on federal funds rate (in percent)	0.25%

INCOME TAXES (Details)

INCOME TAXES (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Effective income tax rate (in percent)	21.00%	20.00%	18.00%	11.00%

SEGMENT REPORTING (Details)

SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Mar. 31, 2019USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Sep. 30, 2019USD ($)operating_segments	Sep. 30, 2018USD ($)
Segment Reporting Information [Line Items]
Number of business segments | operating_segments							2
Reconciliation of EBITDA to net income (loss) [Abstract]
Net income	$ 78,619	$ 63,248	$ 85,169	$ 58,759	$ 43,835	$ 52,231	$ 227,036	$ 154,825
Amortization of acquired intangible assets in cost of revenues	4,957			5,310			15,503	15,242
Amortization of acquired intangible assets in operating expenses	7,586			8,329			22,443	22,948
Depreciation and other amortization	6,279			6,794			19,289	19,810
Interest and other income	(5,358)			(3,035)			(16,216)	(8,674)
Interest and other expense	704			717			2,133	2,135
Income tax expense	20,304			14,247			49,608	19,621
EBITDA	113,091			91,121			319,796	225,907
North America [Member]
Reconciliation of EBITDA to net income (loss) [Abstract]
EBITDA	115,973			94,088			326,648	229,905
International [Member]
Reconciliation of EBITDA to net income (loss) [Abstract]
EBITDA	$ (2,882)			$ (2,967)			$ (6,852)	$ (3,998)

SEGMENT REPORTING, ASSETS AND L

SEGMENT REPORTING, ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Property and equipment, net	$ 107,989	$ 83,303
Goodwill	1,616,538	1,611,535	$ 1,283,457
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	3,715,391	3,312,957
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	416,225	291,015
North America [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	104,239	79,493
Goodwill	1,579,516	1,573,088	1,253,494
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	3,638,588	3,253,035
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	385,254	272,776
International [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	3,750	3,810
Goodwill	37,022	38,447	$ 29,963
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	76,803	59,922
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	$ 30,971	$ 18,239

Uncategorized Items - csgp-0930

Label	Element	Value
Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 12,057,000
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 54,464,000