Gartner (IT) 10-Q 2 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 31, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	GARTNER INC
Entity Central Index Key	749,251
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding (in shares)		90,651,439

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 630,016	$ 474,233
Fees receivable, net of allowances of $10,000 and $7,400, respectively	874,283	643,013
Deferred commissions	143,063	141,410
Prepaid expenses and other current assets	164,921	84,540
Total current assets	1,812,283	1,343,196
Property, equipment and leasehold improvements, net	216,021	121,606
Goodwill	3,145,046	738,453
Intangible assets, net	1,564,465	76,801
Other assets	273,677	87,279
Total Assets	7,011,492	2,367,335
Current liabilities:
Accounts payable and accrued liabilities	550,207	440,771
Deferred revenues	1,512,227	989,478
Current portion of long-term debt	419,601	30,000
Total current liabilities	2,482,035	1,460,249
Long-term debt, net of deferred financing fees	2,922,229	664,391
Other liabilities	743,818	181,817
Total Liabilities	6,148,082	2,306,457
Stockholders’ Equity
Preferred stock, $.01 par value, 5,000,000 shares authorized; none issued or outstanding	0	0
Common stock, $.0005 par value, 250,000,000 shares authorized; 163,602,067 shares issued for September 30, 2017 and 156,234,415 shares issued for December 31, 2016	82	78
Additional paid-in capital	1,748,610	863,127
Accumulated other comprehensive loss, net	(1,216)	(49,683)
Accumulated earnings	1,539,978	1,644,005
Treasury stock, at cost, 72,956,127 and 73,583,172 common shares, respectively	(2,424,044)	(2,396,649)
Total Stockholders’ Equity	863,410	60,878
Total Liabilities and Stockholders’ Equity	$ 7,011,492	$ 2,367,335

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 10,000	$ 7,400
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	5,000,000	5,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in dollars per share)	$ 0.0005	$ 0.0005
Common stock, shares authorized (in shares)	250,000,000	250,000,000
Common stock, shares issued (in shares)	163,602,067	156,234,415
Treasury stock, common shares (in shares)	72,956,127	73,583,172

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues:
Research	$ 653,443	$ 466,877	$ 1,778,481	$ 1,371,157
Consulting	72,117	73,707	242,404	237,876
Events	44,953	33,475	171,427	132,290
Talent Assessment & Other	57,572	0	104,673	0
Total revenues	828,085	574,059	2,296,985	1,741,323
Costs and expenses:
Cost of services and product development	332,207	223,122	921,820	666,585
Selling, general and administrative	421,163	269,902	1,133,633	799,322
Depreciation	17,340	9,531	45,637	27,390
Amortization of intangibles	51,224	6,221	123,014	18,614
Acquisition and integration charges	30,500	16,557	142,104	32,958
Total costs and expenses	852,434	525,333	2,366,208	1,544,869
Operating (loss) income	(24,349)	48,726	(69,223)	196,454
Interest expense, net	(38,762)	(5,932)	(88,624)	(19,294)
Other income, net	1,171	1,954	1,653	5,086
(Loss) income before income taxes	(61,940)	44,748	(156,194)	182,246
(Benefit) provision for income taxes	(13,760)	14,264	(52,166)	55,149
Net (loss) income	$ (48,180)	$ 30,484	$ (104,028)	$ 127,097
Net (loss) income per share:
Basic (in dollars per share)	$ (0.53)	$ 0.37	$ (1.19)	$ 1.54
Diluted (in dollars per share)	$ (0.53)	$ 0.36	$ (1.19)	$ 1.52
Weighted average shares outstanding:
Basic (in shares)	90,624	82,638	87,585	82,549
Diluted (in shares)	90,624	83,803	87,585	83,761

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive (Loss) Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (48,180)	$ 30,484	$ (104,028)	$ 127,097
Other comprehensive (loss) income, net of tax:
Foreign currency translation adjustments	46,319	(3,032)	51,714	2,371
Interest rate swaps – net change in deferred loss	1,302	3,214	(3,396)	(5,865)
Pension – net change in deferred actuarial loss	52	37	149	112
Other comprehensive (loss) income, net of tax	47,673	219	48,467	(3,382)
Comprehensive (loss) income	$ (507)	$ 30,703	$ (55,561)	$ 123,715

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities:
Net (loss) income	$ (104,028)	$ 127,097
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization	168,651	46,004
Stock-based compensation expense	67,930	36,128
Deferred taxes	(99,450)	(13,415)
Amortization and write-off of deferred financing fees	13,236	2,611
Changes in assets and liabilities, net of acquisitions:
Fees receivable, net	(15,090)	26,242
Deferred commissions	3,231	12,376
Prepaid expenses and other current assets	(32,159)	(43,402)
Other assets	(76,548)	21,597
Deferred revenues	198,353	114,197
Accounts payable, accrued, and other liabilities	108,141	(47,172)
Cash provided by operating activities	232,267	282,263
Investing activities:
Additions to property, equipment and leasehold improvements	(75,619)	(36,877)
Acquisitions - cash paid (net of cash acquired)	(2,634,809)	(29,363)
Cash used in investing activities	(2,710,428)	(66,240)
Financing activities:
Proceeds from employee stock purchase plan	8,550	6,931
Proceeds from borrowings	3,025,000	747,500
Payments for deferred financing fees	(51,171)	(4,975)
Payments on borrowings	(339,624)	(827,500)
Purchases of treasury stock	(37,188)	(52,889)
Cash provided by (used in) financing activities	2,605,567	(130,933)
Net increase in cash and cash equivalents	127,406	85,090
Effects of exchange rates on cash and cash equivalents	28,377	7,668
Cash and cash equivalents, beginning of period	474,233	372,976
Cash and cash equivalents, end of period	$ 630,016	$ 465,734

Business and Basis of Presentat

Business and Basis of Presentation	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Business and Basis of Presentation	Business and Basis of Presentation Business. Gartner, Inc. (NYSE: IT) is the world’s leading research and advisory company. The company helps business leaders across all major functions in every industry and enterprise size with the objective insights they need to make the right decisions. Gartner's comprehensive suite of services delivers strategic advice and proven best practices to help clients succeed in their mission-critical priorities. Gartner is headquartered in Stamford, Connecticut, U.S.A., and has more than 14,000 associates serving clients in over 11,000 enterprises in approximately 100 countries. Gartner delivers its products and services globally through four business segments: Research, Consulting, Events, and Talent Assessment & Other. When used in these notes, the terms “Gartner,” “Company,” “we,” “us,” or “our” refer to Gartner, Inc. and its consolidated subsidiaries. The Company acquired two businesses during 2017, L2, Inc. ("L2") and CEB Inc. ("CEB"). Note 2 — Acquisitions provides additional information regarding these acquisitions. As a result of these acquisitions, the Company has made certain changes to its reportable segments effective June 30, 2017, which are described in Note 5 — Segment Information. Basis of presentation . The accompanying interim condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”), as defined in the Financial Accounting Standards Board ("FASB") Accounting Standards Codification (“ASC”) Topic 270 for interim financial information and with the applicable instructions of the U.S. Securities & Exchange Commission (“SEC”) Rule 10-01 of Regulation S-X on Form 10-Q and should be read in conjunction with the consolidated financial statements and related notes of the Company filed in its Annual Report on Form 10-K for the year ended December 31, 2016 . The fiscal year of Gartner is the twelve-month calendar period from January 1 through December 31. In the opinion of management, all normal recurring accruals and adjustments considered necessary for a fair presentation of financial position, results of operations and cash flows at the dates and for the periods presented herein have been included. The results of operations for the three and nine months ended September 30, 2017 may not be indicative of the results of operations for the remainder of 2017 or beyond. Principles of consolidation . The accompanying interim condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated. Use of estimates . The preparation of the accompanying interim condensed consolidated financial statements requires management to make estimates and assumptions about future events. These estimates and the underlying assumptions affect the amounts of assets and liabilities reported, disclosures about contingent assets and liabilities, and reported amounts of revenues and expenses. Such estimates include the valuation of fees receivable, goodwill, intangible assets, and other long-lived assets, as well as tax accruals and other liabilities. In addition, estimates are used in revenue recognition, income tax expense, performance-based compensation charges, depreciation and amortization. Management believes its use of estimates in these interim condensed consolidated financial statements to be reasonable. Management continually evaluates and revises its estimates using historical experience and other factors, including the general economic environment and actions it may take in the future. Management adjusts these estimates when facts and circumstances dictate. However, these estimates may involve significant uncertainties and judgments and cannot be determined with precision. In addition, these estimates are based on management’s best judgment at a point in time. As a result, differences between our estimates and actual results could be material and would be reflected in the Company’s consolidated financial statements in future periods. Adoption of new accounting standards. The Company did not adopt any significant new accounting standards during the nine months ended September 30, 2017. Accounting standards issued but not yet adopted. The FASB has issued accounting standards that have not yet become effective and that may impact the Company’s consolidated financial statements or related disclosures in future periods. These standards and their potential impact are discussed below: Targeted Improvements to Accounting for Hedging Activities - In August 2017, the FASB issued Accounting Standards Update ("ASU") No. 2017-12, " Derivatives and Hedging (Topic 815) " ("ASU No. 2017-12"). ASU No. 2017-12 is intended to improve the financial reporting of hedging relationships to better portray economic results of an entity’s risk management activities in its financial statements. In addition to that main objective, the amendments would make certain targeted improvements to simplify the application of the hedge accounting guidance in current U.S. GAAP. ASU No. 2017-12 is effective for Gartner on January 1, 2019. We are currently evaluating the potential impact of ASU No. 2017-12 on the Company's consolidated financial statements. Distinguishing Liabilities from Equity — In July 2017, the FASB issued Accounting Standards Update ("ASU") No. 2017-11, " Earnings Per Share, Distinguishing Liabilities from Equity, and Derivatives and Hedging" ("ASU No. 2017-11"). ASU No. 2017-11 is intended to simplify the accounting for financial instruments with characteristics of liabilities and equity. Among the issues addressed are: (i) determining whether an instrument (or embedded feature) is indexed to an entity’s own stock; (ii) distinguishing liabilities from equity for mandatorily redeemable financial instruments of certain nonpublic entities; and (iii) identifying mandatorily redeemable non-controlling interests. ASU No. 2017-11 is effective for Gartner on January 1, 2019. We are currently evaluating the potential impact of ASU No. 2017-11 on the Company's consolidated financial statements. Stock Compensation Award Modifications — In May 2017, the FASB issued ASU No. 2017-09, " Compensation—Stock Compensation - Scope of Modification Accounting " ("ASU No. 2017-09"). ASU No. 2017-09 provides guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting. ASU No. 2017-09 is effective for Gartner on January 1, 2018. We have concluded that the adoption of ASU No. 2017-09 will not have a material impact on the Company's consolidated financial statements. Retirement Benefits Cost Presentation — In March 2017, the FASB issued ASU No. 2017-07, "Compensation—Retirement Benefits" ("ASU No. 2017-07"). ASU No. 2017-07 improves the reporting of net benefit cost in the financial statements, and provides additional guidance on the presentation of net benefit cost in the income statement and clarifies the components eligible for capitalization. ASU No. 2017-07 is effective for Gartner on January 1, 2018. We have concluded that the adoption of ASU No. 2017-07 will not have a material impact on the Company's consolidated financial statements. Partial Sales of Non-financial Asset s — In February 2017, the FASB issued ASU No. 2017-05, "Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Non-financial Assets" ("ASU No. 2017-05"). ASU No. 2017-05 clarifies the scope of the FASB’s recently established guidance on non-financial asset de-recognition as well as the accounting for partial sales of non-financial assets. It conforms the de-recognition guidance on non-financial assets with the model for revenue transactions. ASU No. 2017-05 is effective for Gartner on January 1, 2018. We have concluded that the adoption of ASU No. 2017-05 will not have a material impact on the Company's consolidated financial statements. Goodwill Impairment — In January 2017, the FASB issued ASU No. 2017-04, " Intangibles—Goodwill and Other - Simplifying the Test for Goodwill Impairment " ("ASU No. 2017-04"). ASU No. 2017-04 simplifies the determination of the amount of goodwill to be potentially charged off by eliminating Step 2 of the goodwill impairment test. ASU No. 2017-04 is effective for Gartner on January 1, 2020. We have concluded that the adoption of ASU No. 2017-04 will not have a material impact on the Company's consolidated financial statements. Definition of a Business — In January 2017, the FASB issued ASU No. 2017-01, "Clarifying the Definition of a Business" ("ASU No. 2017-01"), which is effective for Gartner on January 1, 2018. ASU No. 2017-01 changes the U.S. GAAP definition of a business which can impact the accounting for asset purchases, acquisitions, goodwill impairment, and other assessments. We have concluded that the adoption of ASU No. 2017-01 will not have a material impact on the Company's consolidated financial statements. Presentation of Restricted Cash — In November 2016, the FASB issued ASU No. 2016-18, "Restricted Cash" ("ASU No. 2016-18"). ASU No. 2016-18 requires that amounts generally described as restricted cash and restricted cash equivalents be presented with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. If different, a reconciliation of the cash balances reported in the cash flow statement and the balance sheet would need to be provided along with explanatory information. ASU No. 2016-18 is effective for Gartner on January 1, 2018. The adoption of ASU No. 2016-18 will require the Company to disclose restricted cash and, as a result, will change the presentation of the consolidated statements of cash flows. Income Taxes — In October 2016, the FASB issued ASU No. 2016-16, "Intra-Entity Transfers of Assets Other Than Inventory" ("ASU No. 2016-16"). ASU No. 2016-16 accelerates the recognition of taxes on certain intra-entity transactions and is effective for Gartner on January 1, 2018. Current U.S. GAAP requires deferral of the income tax implications of an intercompany sale of assets until the assets are sold to a third party or recovered through use. Under the new rule, the seller’s tax effects and the buyer’s deferred taxes on post-adoption asset transfers will be immediately recognized upon the sale. On the date of adoption of ASU No. 2016-16 any taxes attributable to pre-2018 intra-entity transfers that were previously deferred will be accelerated and recorded to retained earnings as permitted by the transition rules. ASU 2016-16 could have a material impact on our consolidated financial statements in the future depending on the nature, size, and tax consequences of future intra-entity transfers, if any. Statement of Cash Flows — In August 2016, the FASB issued ASU No. 2016-15, "Classification of Certain Cash Receipts and Cash Payments" ("ASU No. 2016-15"). ASU No. 2016-15 sets forth classification requirements for certain cash flow transactions. ASU No. 2016-15 is effective for Gartner on January 1, 2018. We have concluded that the adoption of ASU No. 2016-15 will not have a material impact on the Company's consolidated financial statements. Financial Instrument Credit Losses — In June 2016, the FASB issued ASU No. 2016-13, " Financial Instruments—Credit Losses" ("ASU No. 2016-13"). ASU No. 2016-13 amends the current financial instrument impairment model by requiring entities to use a forward-looking approach based on expected losses to estimate credit losses on certain types of financial instruments, including trade receivables. ASU No. 2016-13 is effective for Gartner on January 1, 2020, with early adoption permitted. We are currently evaluating the potential impact of ASU No. 2016-13 on our consolidated financial statements. Leases — In February 2016, the FASB issued ASU No. 2016-02, " Leases " ("ASU No. 2016-02") which will require significant changes in the accounting and disclosure for lease arrangements. Currently under U.S. GAAP, lease arrangements that meet certain criteria are considered operating leases and are not recorded on the balance sheet. All of the Company's existing lease arrangements are accounted for as operating leases and are thus not recorded on the Company's balance sheet. ASU No. 2016-02 will significantly change the accounting for leases since a right-of-use ("ROU") model must be used in which the lessee must record a ROU asset and a lease liability on the balance sheet for leases with terms longer than 12 months. Leases will be classified as either finance or operating arrangements, with classification affecting the pattern of expense recognition in the income statement. ASU No. 2016-02 also requires expanded disclosures about leasing arrangements. ASU No. 2016-02 will be effective for Gartner on January 1, 2019. We are currently evaluating the impact of ASU No. 2016-02 on our consolidated financial statements. Financial Instruments Recognition and Measurement — In January 2016, the FASB issued ASU No. 2016-01, " Financial Instruments Overall - Recognition and Measurement of Financial Assets and Liabilities " ("ASU No. 2016-01") to address certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. Among the significant changes required by ASU No. 2016-01 is that equity investments will be measured at fair value with changes in fair value recognized in net income. ASU No. 2016-01 will be effective for Gartner on January 1, 2018. We have concluded that the adoption of ASU No. 2016-01 will not have a material impact on the Company's consolidated financial statements. Revenue Recognition — In May 2014, the FASB issued ASU No. 2014-09, " Revenue from Contracts with Customers " ("ASU No. 2014-09"). ASU No. 2014-09 and related amendments require changes in revenue recognition policies as well as enhanced disclosures. ASU No. 2014-09 is intended to clarify the principles for recognizing revenue by removing inconsistencies and weaknesses in existing revenue recognition rules; provide a more robust framework for addressing revenue recognition issues; improve comparability of revenue recognition practices across entities, industries, jurisdictions and capital markets; and provide more useful information to users of financial statements through improved disclosures. The Company has completed an initial assessment of the impact of ASU No. 2014-09 on its existing revenue recognition policies and plans to adopt the rule on January 1, 2018 using the cumulative effect method of adoption. ASU No. 2014-09 also requires significantly expanded disclosures around the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers, which the Company is currently compiling. While the Company has not fully completed its assessment of the impact of ASU No. 2014-09, primarily the completion of our review of the recently-acquired CEB's revenue recognition policies, based on the analysis completed to date, the Company does not currently anticipate that the new rule will have a material impact on its consolidated financial statements. The FASB also continues to work on a number of other significant accounting standards which if issued could materially impact the Company's accounting policies and disclosures in future periods. However, since these standards have not yet been issued, the effective dates and potential impact are unknown.

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2017
Business Combinations [Abstract]
Acquisitions	Acquisitions The Company accounts for business acquisitions in accordance with the acquisition method of accounting as prescribed by FASB ASC Topic 805, Business Combinations. The acquisition method of accounting requires the Company to record the net assets and liabilities acquired based on their estimated fair values as of the acquisition date, with any excess of the consideration transferred over the estimated fair value of the net assets acquired, including identifiable intangible assets, to be recorded to goodwill. Under the acquisition method, the operating results of acquired companies are included in the Company's consolidated financial statements beginning on the date of acquisition. The Company completed the following business acquisitions during 2017: CEB On April 5, 2017, the Company acquired 100% of the outstanding capital stock of CEB for an aggregate purchase price of $3.5 billion . The consideration transferred by Gartner included approximately $2.7 billion in cash and $818.7 million in fair value of Gartner common shares. CEB was a publicly-held company headquartered in Arlington, Virginia with approximately 4,900 employees. CEB's primary business is to serve as a leading provider of subscription-based, best practice research and analysis focusing on human resources, sales, finance, IT, and legal. CEB serves executives and professionals at corporate and middle market institutions in over 70 countries. L2 On March 9, 2017 , the Company acquired 100% of the outstanding capital stock of L2, a privately-held firm based in New York City with 150 employees, for an aggregate purchase price of $134.2 million . L2 is a subscription-based research business that benchmarks the digital performance of brands. Total Consideration Transferred The following table summarizes the aggregate consideration paid or payable for these acquisitions (in thousands): Aggregate consideration (1): CEB L2 Total Cash paid at close (2), (3) $ 2,687,704 $ 134,199 $ 2,821,903 Additional cash paid (2) 12,465 — 12,465 Fair value of Gartner equity awards (4) 818,660 — 818,660 Total (5) $ 3,518,829 $ 134,199 $ 3,653,028 (1) Includes the total consideration transferred for 100% of the outstanding capital stock of the acquired businesses. (2) The cash paid at close represents the gross contractual amount paid. The Company paid the additional $12.5 million in cash in third quarter 2017. Net of cash acquired from these businesses and for cash flow reporting purposes, the Company paid $2.63 billion in cash. (3) The Company borrowed a total of approximately $2.78 billion in conjunction with the CEB acquisition (see Note 7 — Debt for additional information). (4) Consists of the fair value of (i) Gartner common stock issued (see Note 8 — Equity for additional information) and (ii) stock-based compensation replacement awards. (5) The Company may also be required to pay up to an additional $20.8 million in cash for L2 which is contingent on the achievement of certain employment conditions by several key employees. This amount is being recognized as compensation expense over approximately three years. The allocation of the purchase price for the L2 and CEB acquisitions are preliminary with respect to various matters, to include leases and leasehold improvements; tax contingencies; and other items. The Company expects to complete the allocation of the purchase price by the end of the accounting measurement period for the respective acquisition. Preliminary Allocation of Purchase Price The following table summarizes the preliminary allocation of the purchase price to the fair value of the assets acquired and liabilities assumed for the acquisitions of L2 and CEB (in thousands): CEB (3) L2 (4) Total Assets: Cash $ 194,706 $ 4,852 $ 199,558 Fees receivable 175,440 8,277 183,717 Prepaid expenses and other current assets 52,032 1,167 53,199 Property, equipment and leasehold improvements 51,751 663 52,414 Goodwill (1) 2,267,087 109,779 2,376,866 Finite-lived intangible assets (2) 1,574,100 15,890 1,589,990 Other assets 182,720 12,321 195,041 Total assets $ 4,497,836 $ 152,949 $ 4,650,785 Liabilities: Accounts payable and accrued liabilities $ 130,544 $ 3,050 $ 133,594 Deferred revenues (current) 246,472 13,200 259,672 Other liabilities 601,991 2,500 604,491 Total liabilities $ 979,007 $ 18,750 $ 997,757 Net assets acquired $ 3,518,829 $ 134,199 $ 3,653,028 (1) The Company believes the goodwill resulting from the acquisitions is supportable based on anticipated synergies. For CEB, among the factors contributing to the anticipated synergies are a broader market presence, expanded product offerings and market opportunities, and an acceleration of CEB's growth by leveraging Gartner's global infrastructure and best practices in sales productivity and other areas. None of the recorded goodwill is expected to be deductible for tax purposes. See Note 6 — Goodwill and Intangible Assets for additional information. (2) All of the acquired intangible assets are finite-lived. The determination of the fair value of the finite-lived intangible assets required management judgment and the consideration of a number of factors. In determining the fair values, management primarily relied on income valuation methodologies, in particular discounted cash flow models. The use of discounted cash flow models required the use of estimates, significant among them projected cash flows related to the particular asset; the useful lives of the particular assets; the selection of royalty and discount rates used in the models; and certain published industry benchmark data. In establishing the estimated useful lives of the finite-lived intangible assets, the Company relied on both internally-generated data for similar assets as well as certain published industry benchmark data. We believe the values we have assigned to the finite-lived intangible assets are both reasonable and supportable. See Note 6 — Goodwill and Intangible Assets for additional information regarding the finite-lived intangible assets. (3) The Company's financial statements include the operating results of CEB beginning on April 5, 2017, the date of acquisition. CEB's operating results and the related goodwill are being reported as part of the Company's Research, Events, and Talent Assessment & Other segments. The Company recorded certain measurement period adjustments for the CEB preliminary purchase price allocation during the third quarter of 2017, primarily related to certain tenant improvement incentives, which increased Prepaid expenses and Other assets, with a reduction to goodwill. Had the Company acquired CEB in prior periods, the impact to the Company's operating results would have been material, and as a result the following pro forma consolidated financial information (unaudited) is presented as if CEB had been acquired by the Company on January 1, 2016 (in thousands, except per share amounts): Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Pro forma total revenue $ 891,003 $ 756,083 $ 2,664,450 $ 2,233,942 Pro forma net (loss) (7,283 ) (40,825 ) (71,122 ) (133,379 ) Pro forma basic and diluted (loss) per share (0.08 ) (0.45 ) (0.79 ) (1.48 ) The pro forma results have been prepared in accordance with U.S. GAAP and include the following pro forma adjustments: (a) An increase in interest expense and amortization of debt issuance costs related to the financing of the CEB acquisition. Note 7 — Debt provides further information regarding the Company's borrowings related to the CEB acquisition; (b) A decrease in revenue as a result of the required fair value adjustment to deferred revenue; and (c) An adjustment for additional depreciation and amortization expense as a result of the preliminary purchase price allocation for finite-lived intangible assets and property, equipment, and leasehold improvements. (4) The Company's financial statements include the operating results of L2 beginning on March 9, 2017, the acquisition date. L2's operating results were not material to the Company's consolidated operating results and segment results for either the three or nine months ended September 30, 2017. Had the Company acquired L2 in prior periods, the impact to the Company's operating results would not have been material, and as a result pro forma financial information for L2 for prior periods has not been presented. L2's operating results and the related goodwill are being reported as part of the Company's Research segment. The Company recognized $30.5 million and $142.1 million of acquisition and integration charges in the three and nine months ended September 30, 2017, respectively, compared to $16.6 million and $33.0 million in the three and nine months ended September 30, 2016. The additional charges during 2017 primarily consisted of higher professional fees, severance, stock-based compensation charges, and accruals for exit costs for certain office space that the Company does not intend to occupy at a new building in Arlington, Virginia that was related to the CEB acquisition. The following table presents a summary of the amounts related to this space for the period ended September 30, 2017 (in thousands): For the period ended September 30, 2017 Liability balance at December 31, 2016 $ — Charges during the six months ended June 30, 2017 20,149 Liability balance at June 30, 2017 20,149 Charges and adjustments during the three months ended September 30, 2017 974 Liability balance at September 30, 2017 $ 21,123

Net (Loss) Income per Share

Net (Loss) Income per Share	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Net (Loss) Income per Share	Net (Loss) Income per Share The following table sets forth the calculation of basic and diluted (loss) income per share (in thousands, except per share data): Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Numerator: Net (loss) income used for calculating basic and diluted (loss) income per common share $ (48,180 ) $ 30,484 $ (104,028 ) $ 127,097 Denominator: Weighted average number of common shares used in the calculation of basic (loss) income per share 90,624 82,638 87,585 82,549 Common stock equivalents associated with stock-based compensation plans (1), (2) — 1,165 — 1,212 Shares used in the calculation of diluted (loss) income per share 90,624 83,803 87,585 83,761 Basic (loss) income per share $ (0.53 ) $ 0.37 $ (1.19 ) $ 1.54 Diluted (loss) income per share $ (0.53 ) $ 0.36 $ (1.19 ) $ 1.52 (1) For the three and nine months ended September 30, 2016, certain common stock equivalents were not included in the computation of diluted income per share because the effect would have been anti-dilutive. These common share equivalents totaled less than 0.2 million for both 2016 periods. (2) Approximately 1.4 million common stock equivalents for both the three and nine months ended September 30, 2017 were completely excluded from the calculation of diluted (loss) per share because they were anti-dilutive.

Stock-Based Compensation

Stock-Based Compensation	9 Months Ended
	Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	Stock-Based Compensation The Company grants stock-based compensation awards as an incentive for employees and directors to contribute to the Company’s long-term success. The Company currently awards stock-settled stock appreciation rights, service-based and performance-based restricted stock units, and common stock equivalents. As of September 30, 2017 , the Company had 5.5 million shares of its common stock, par value $.0005 per share (the “Common Stock”), available for stock-based compensation awards under its 2014 Long-Term Incentive Plan. The Company accounts for stock-based compensation awards in accordance with FASB ASC Topics 505 and 718 and SEC Staff Accounting Bulletins No. 107 and No. 110. Stock-based compensation expense is based on the fair value of the award on the date of grant. The Company recognizes stock-based compensation expense over the period that the related service is performed, which is generally the same as the vesting period of the underlying award. Currently, the Company issues treasury shares upon the exercise, release or settlement of stock-based compensation awards. Determining the appropriate fair value model and calculating the fair value of stock-based compensation awards requires the use of certain complex and subjective assumptions, including the expected life of a stock-based compensation award and Common Stock price volatility. In addition, determining the appropriate periodic stock-based compensation expense requires management to estimate the likelihood of the achievement of certain performance targets. The assumptions used in calculating the fair values of stock-based compensation awards and the related periodic expense represent management’s best estimates, which involve inherent uncertainties and the application of judgment. As a result, if circumstances change and the Company deems it necessary in the future to modify the assumptions it made or to use different assumptions, or if the quantity and nature of the Company’s stock-based compensation awards changes, then the amount of expense may need to be adjusted and future stock-based compensation expense could be materially different from what has been recorded in the current period. Stock-Based Compensation Expense The Company recognized the following stock-based compensation expense by award type and expense category line item in the periods indicated (in millions): Three Months Ended Nine Months Ended September 30, September 30, Award type 2017 2016 2017 2016 Stock appreciation rights $ 1.1 $ 1.3 $ 5.2 $ 4.3 Restricted stock units 13.2 8.0 62.3 31.3 Common stock equivalents 0.1 0.2 0.5 0.5 Total (1) $ 14.4 $ 9.5 $ 68.0 $ 36.1 Three Months Ended Nine Months Ended September 30, September 30, Expense category line item 2017 2016 2017 2016 Cost of services and product development $ 6.1 $ 4.6 $ 21.5 $ 17.0 Selling, general and administrative 7.1 4.9 30.8 19.1 Acquisition and integration charges (2) 1.2 — 15.7 — Total (1) $ 14.4 $ 9.5 $ 68.0 $ 36.1 (1) Includes charges of $2.1 million and $2.5 million during the three months ended September 30, 2017 and 2016 , respectively, and $22.6 million and $16.5 million during the nine months ended September 30, 2017 and 2016, respectively, for awards to retirement-eligible employees. Those awards vest on an accelerated basis. (2) These charges are primarily the result of the acceleration of the vesting of certain restricted stock units related to the CEB acquisition. As of September 30, 2017 , the Company had $ 97.4 million of total unrecognized stock-based compensation cost, which is expected to be expensed over the remaining weighted-average service period of approximately 2.5 years. Stock-Based Compensation Awards The disclosures presented below provide information regarding the Company’s stock-based compensation awards, all of which are classified as equity awards in accordance with FASB ASC Topic 505. Stock Appreciation Rights Stock-settled stock appreciation rights ("SARs") permit the holder to participate in the appreciation of the value of the Common Stock. After the applicable vesting criteria have been satisfied, SARs are settled in shares of Common Stock upon exercise by the employee. SARs vest ratably over a four -year service period and expire seven years from the date of grant. The fair value of a SARs award is recognized as compensation expense on a straight-line basis over four years . SARs have only been awarded to the Company’s executive officers. When SARs are exercised, the number of shares of Common Stock issued is calculated as follows: (1) the total proceeds from the exercise of the SARs award (calculated as the closing price of the Common Stock on the date of exercise less the exercise price of the SARs award, multiplied by the number of SARs exercised) is divided by (2) the closing price of the Common Stock as reported on the New York Stock Exchange on the date of exercise. The Company withholds a portion of the shares of the Common Stock issued upon exercise to satisfy statutory tax withholding requirements. SARs recipients do not have any stockholder rights until the shares of Common Stock are issued in respect of the award, which is subject to the prior satisfaction of the vesting and other criteria relating to such grants. The following table summarizes changes in SARs outstanding during the nine months ended September 30, 2017 : Stock Appreciation Rights ("SARs") (in millions) Per Share Weighted Average Exercise Price Per Share Weighted Average Grant Date Fair Value Weighted Average Remaining Contractual Term (Years) Outstanding at December 31, 2016 1.3 $ 66.22 $ 15.77 4.40 Granted 0.3 99.07 22.02 6.35 Exercised (0.3 ) 51.25 14.74 n/a Outstanding at September 30, 2017 (1) (2) 1.3 76.64 17.36 4.57 Vested and exercisable at September 30, 2017 (2) 0.6 $ 64.71 $ 15.60 3.49 n/a=not applicable. (1) As of September 30, 2017 , 0.7 million of the total SARs outstanding were unvested. The Company expects that substantially all of those unvested awards will vest in future periods. (2) As of September 30, 2017 , the total SARs outstanding had an intrinsic value of $62.8 million . On such date, SARs vested and exercisable had an intrinsic value of $33.4 million . The fair value of a SARs award is determined on the date of grant using the Black-Scholes-Merton valuation model with the following weighted average assumptions: Nine Months Ended September 30, 2017 2016 Expected dividend yield (1) — % — % Expected stock price volatility (2) 22 % 22 % Risk-free interest rate (3) 1.8 % 1.1 % Expected life in years (4) 4.5 4.4 (1) The expected dividend yield assumption was based on both the Company's historical and anticipated dividend payouts. Historically, the Company has not paid cash dividends on its Common Stock. (2) The determination of expected stock price volatility was based on both historical Common Stock prices and implied volatility from publicly traded options in the Common Stock. (3) The risk-free interest rate was based on the yield of a U.S. Treasury security with a maturity similar to the expected life of the award. (4) The expected life represents the Company’s estimate of the weighted average period of time the SARs are expected to be outstanding (that is, the period between the service inception date and the expected exercise date). Restricted Stock Units Restricted stock units ("RSUs") give the awardee the right to receive shares of Common Stock when the vesting conditions are met and certain restrictions lapse. Each RSU that vests entitles the awardee to one share of Common Stock. RSU awardees do not have any of the rights of a Gartner stockholder, including voting rights and the right to receive dividends and distributions, until the shares are released. The fair value of an RSU award is determined on the date of grant based on the closing price of the Common Stock as reported on the New York Stock Exchange on that date. Service-based RSUs vest ratably over four years and are expensed on a straight-line basis over the vesting period. Performance-based RSUs are subject to the satisfaction of both performance and service conditions, vest ratably over four years and are expensed on an accelerated basis over the vesting period. The following table summarizes the changes in RSUs outstanding during the nine months ended September 30, 2017 : Restricted Stock Units ("RSUs") (in millions) Per Share Weighted Average Grant Date Fair Value Outstanding at December 31, 2016 1.3 $ 73.19 Granted (1) (2) 1.1 105.45 Vested and released (0.7 ) 78.44 Forfeited (0.1 ) 93.18 Outstanding at September 30, 2017 (3) (4) 1.6 $ 91.37 (1) The 1.1 million of RSUs granted during the nine months ended September 30, 2017 consisted of 0.2 million of performance-based RSUs awarded to executives and 0.9 million of service-based RSUs awarded to non-executive employees and non-management board members. The 0.2 million of performance-based RSUs represents the target amount of the grant for the year, which is tied to an increase in the Company's total contract value for 2017. Total contract value for this determination represents the value attributable to all of the Company's subscription-related contracts but not including CEB contract value. The final number of performance-based RSUs that will ultimately be awarded for 2017 ranges from 0% to 200% of the target amount, with the final number dependent on the actual increase in total contract value for 2017 as measured on December 31, 2017. If the specified minimum level of achievement is not met, the performance-based RSUs will be forfeited in their entirety and any previously recorded compensation expense will be reversed. (2) Includes 0.6 million of RSUs awarded to employees that joined Gartner as a result of the CEB acquisition. (3) The Company expects that substantially all of the RSUs outstanding will vest in future periods. (4) As of September 30, 2017, the weighted average remaining contractual term of the RSUs outstanding was approximately 1.4 years. Common Stock Equivalents Common stock equivalents ("CSEs") are convertible into Common Stock. Each CSE entitles the holder to one share of Common Stock. Members of our Board of Directors receive their directors’ fees in CSEs unless they opt to receive up to 50% of those fees in cash. Generally, CSEs have no defined term and are converted into shares of Common Stock when service as a director terminates unless the director has elected an accelerated release. The fair value of a CSE award is determined on the date of grant based on the closing price of the Common Stock as reported on the New York Stock Exchange on that date. CSEs vest immediately and as a result they are recorded as expense on the date of grant. The following table summarizes the changes in CSEs outstanding during the nine months ended September 30, 2017 : Common Stock Equivalents ("CSEs") Per Share Weighted Average Grant Date Fair Value Outstanding at December 31, 2016 107,338 $ 20.74 Granted 4,382 118.75 Converted to shares of Common Stock upon grant (3,177 ) 118.71 Outstanding at September 30, 2017 108,543 $ 21.82 Employee Stock Purchase Plan The Company has an employee stock purchase plan (the “ESP Plan”) under which eligible employees are permitted to purchase shares of Common Stock through payroll deductions, which may not exceed 10% of an employee’s compensation (or $23,750 in any calendar year), at a price equal to 95% of the closing price of the Common Stock as reported on the New York Stock Exchange at the end of each offering period. As of September 30, 2017 , the Company had 0.8 million shares available for purchase under the ESP Plan. The ESP Plan is considered non-compensatory under FASB ASC Topic 718 and, as a result, the Company does not record stock-based compensation expense for employee share purchases. The Company received $8.6 million and $6.9 million in cash from employee share purchases under the ESP Plan during the nine months ended September 30, 2017 and 2016 , respectively.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Segment Information	Segment Information On April 5, 2017, Gartner completed its acquisition of CEB. With the CEB acquisition, Gartner is reporting four business segments reflecting the Company’s enlarged scale and breadth of advisory services aligned to the mission-critical priorities of virtually all functional business leaders across every industry and size of enterprise worldwide. The Company’s reportable segments are as follows: • Research - includes our previous Gartner Research segment as well as the results of CEB’s core subscription-based best practice and decision support research activities. In addition, Research now includes our Strategic Advisory Services ("SAS") business, which was previously included in the Consulting segment. Research consists primarily of subscription-based research products, access to research inquiry, peer networking services, and membership programs. • Consulting - includes our previous Gartner Consulting segment except, as noted above, the results of our SAS business are now included in the Research segment. Consulting consists primarily of consulting and measurement engagements. • Events - includes our previous Gartner Events segment and the results of CEB’s former Evanta business and destination event activities. Events consists of various symposia, conferences, exhibitions, and destination activities. • Talent Assessment & Other - this is a new segment for Gartner and it includes CEB's previously disclosed Talent Assessment business as well as certain CEB non-subscription based talent products and services. On October 4, 2017, the Company announced that it has initiated a process to explore and evaluate strategic alternatives for the Talent Assessment business, which is a substantial part of the Talent Assessment & Other segment. See Note 14 — Subsequent Event for additional information. The Company evaluates segment performance and allocates resources based on gross contribution margin. Gross contribution, as presented in the table below, is defined as operating income excluding certain Cost of services and product development expenses, Selling, general and administrative expenses, Depreciation, Amortization of intangibles, and Acquisition and integration charges. Certain bonus and fringe benefit costs included in consolidated Cost of services and product development are not allocated to segment expense. The accounting policies used by the reportable segments are the same as those used by the Company. There are no intersegment revenues. The Company does not identify or allocate assets, including capital expenditures, by reportable segment. Accordingly, assets are not reported by segment because the information is not available by segment and is not reviewed in the evaluation of segment performance or in making decisions in the allocation of resources. The following tables present operating information about the Company’s reportable segments for the periods indicated (in thousands). The segment results for the three and nine months ended September 30, 2017 include the results of CEB beginning on the acquisition date: Three Months Ended September 30, 2017 Research Consulting Events Talent Assessment & Other Consolidated Revenues $ 653,443 $ 72,117 $ 44,953 $ 57,572 $ 828,085 Gross contribution 436,222 16,188 16,011 31,215 499,636 Corporate and other expenses (523,985 ) Operating (loss) $ (24,349 ) Three Months Ended September 30, 2016 Research Consulting Events Talent Assessment & Other Consolidated Revenues (1) $ 466,877 $ 73,707 $ 33,475 $ — $ 574,059 Gross contribution (1) 322,646 18,215 14,529 — 355,390 Corporate and other expenses (306,664 ) Operating income $ 48,726 Nine Months Ended September 30, 2017 Research Consulting Events Talent Assessment & Other Consolidated Revenues $ 1,778,481 $ 242,404 $ 171,427 $ 104,673 $ 2,296,985 Gross contribution 1,187,906 71,558 79,313 48,512 1,387,289 Corporate and other expenses (1,456,512 ) Operating (loss) $ (69,223 ) Nine Months Ended September 30, 2016 Research Consulting Events Talent Assessment & Other Consolidated Revenues (1) $ 1,371,157 $ 237,876 $ 132,290 $ — $ 1,741,323 Gross contribution (1) 954,276 71,110 63,574 — 1,088,960 Corporate and other expenses (892,506 ) Operating income $ 196,454 (1) Effective June 30, 2017, the Company began reporting the results of its SAS business in Research whereas previously the SAS business was reported with Consulting. As a result, revenues of $5.4 million and $20.2 million pertaining to the three and nine months ended September 30, 2016, respectively, were reclassified from Consulting to Research to be comparable with the current year presentation. Gross contribution of $3.4 million and $13.2 million for the three and nine months ended September 30, 2016, respectively, was also reclassified from Consulting to Research. The following table provides a reconciliation of total segment gross contribution to net (loss) income for the periods indicated (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Total segment gross contribution $ 499,636 $ 355,390 $ 1,387,289 $ 1,088,960 Costs and expenses: Cost of services and product development - unallocated (1) 3,758 4,453 12,124 14,222 Selling, general and administrative 421,163 269,902 1,133,633 799,322 Depreciation and amortization 68,564 15,752 168,651 46,004 Acquisition and integration charges 30,500 16,557 142,104 32,958 Operating (loss) income (24,349 ) 48,726 (69,223 ) 196,454 Interest expense and other, net 37,591 3,978 86,971 14,208 (Benefit) provision for income taxes (13,760 ) 14,264 (52,166 ) 55,149 Net (loss) income $ (48,180 ) $ 30,484 $ (104,028 ) $ 127,097 (1) The unallocated amounts consist of certain bonus and related fringe costs recorded in consolidated Cost of services and product development expense that are not allocated to segment expense. The Company's policy is to only allocate bonus and related fringe charges to segments for up to 100% of the segment employee's target bonus. Amounts above 100% are absorbed by corporate.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	9 Months Ended
	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill Goodwill represents the excess of the purchase price of acquired businesses over the estimated fair values of the tangible and identifiable intangible net assets acquired. Evaluations of the recoverability of goodwill are performed in accordance with FASB ASC Topic 350, which requires an annual assessment of potential goodwill impairment at the reporting unit level and whenever events or changes in circumstances indicate that the carrying value of goodwill may not be recoverable. The annual assessment of the recoverability of recorded goodwill can be based on either a qualitative or quantitative assessment or a combination of the two approaches. Both methods utilize estimates which, in turn, require judgments and assumptions regarding future trends and events. As a result, both the precision and reliability of the resulting estimates are subject to uncertainty. If our goodwill impairment evaluation determines that the fair value of a reporting unit is less than its related carrying amount, we may recognize an impairment charge. In connection with our most recent annual impairment test of goodwill during the quarter ended September 30, 2017, which indicated no impairment of recorded goodwill, the Company utilized the qualitative approach in assessing the fair values of its reporting units relative to their respective carrying values. The following table presents changes to the carrying amount of goodwill by reportable segment during the nine months ended September 30, 2017 (in thousands): Research Consulting Events Talent Assessment & Other Total Balance at December 31, 2016 (1) $ 595,450 $ 96,480 $ 46,523 $ — $ 738,453 Additions due to acquisitions (2) 1,717,750 — 123,423 535,693 2,376,866 Foreign currency translation impact 23,183 1,375 1,462 3,707 29,727 Balance at September 30, 2017 $ 2,336,383 $ 97,855 $ 171,408 $ 539,400 $ 3,145,046 (1) The Company does not have any accumulated goodwill impairment losses. (2) The goodwill additions are due to the acquisitions of CEB and L2 during April 2017 and March 2017, respectively (see Note 2 for additional information regarding our recent acquisitions). Finite-Lived Intangible Assets The following tables present reconciliations of the carrying amounts of the Company's finite-lived intangible assets as of the dates indicated (in thousands): September 30, 2017 Customer Software Content Other Total Gross cost at December 31, 2016 $ 63,369 $ 16,025 $ 3,728 $ 33,645 $ 116,767 Additions due to acquisitions (1) 1,251,000 181,000 143,500 14,490 1,589,990 Write-off of fully amortized intangible assets — — (4,227 ) — (4,227 ) Foreign currency translation impact 12,073 1,194 6,080 496 19,843 Gross cost 1,326,442 198,219 149,081 48,631 1,722,373 Accumulated amortization (2) (75,173 ) (24,419 ) (36,884 ) (21,432 ) (157,908 ) Balance at September 30, 2017 $ 1,251,269 $ 173,800 $ 112,197 $ 27,199 $ 1,564,465 December 31, 2016 Customer Software Content Other Total Gross cost $ 63,369 $ 16,025 $ 3,728 $ 33,645 $ 116,767 Accumulated amortization (2) (16,744 ) (8,904 ) (2,033 ) (12,285 ) (39,966 ) Balance at December 31, 2016 $ 46,625 $ 7,121 $ 1,695 $ 21,360 $ 76,801 (1) The additions were primarily due to the acquisitions of CEB and L2 during April 2017 and March 2017, respectively (see Note 2 for additional information regarding our recent acquisitions). (2) Finite-lived intangible assets are amortized using the straight-line method over the following periods: Customer relationships— 4 to 13 years ; Software— 3 to 7 years ; Content— 1.5 to 5 years ; and Other (consisting of trade names and non-competes) — 2 to 5 years . Amortization expense related to finite-lived intangible assets was $51.2 million and $6.2 million during the three months ended September 30, 2017 and 2016 , respectively, and $123.0 million and $18.6 million during the nine months ended September 30, 2017 and 2016, respectively. The estimated future amortization expense by year for finite-lived intangible assets will be as follows (in thousands): 2017 (remaining three months) $ 59,743 2018 222,102 2019 165,855 2020 158,981 2021 138,639 Thereafter 819,145 $ 1,564,465

Debt

Debt	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Debt	Debt 2016 Credit Agreement The Company entered into a term loan and revolving credit facility on June 17, 2016 (the "2016 Credit Agreement"). As discussed below, the 2016 Credit Agreement was amended three times during 2017 in conjunction with the acquisition of CEB. As of September 30, 2017, the 2016 Credit Agreement provides for a $1.5 billion Term loan A facility, a $500.0 million Term loan B facility, and a $1.2 billion revolving credit facility. The 2016 Credit Agreement contains certain customary restrictive loan covenants, including, among others, financial covenants that apply a maximum leverage ratio and a minimum interest expense coverage ratio, and covenants limiting Gartner’s ability to incur indebtedness, grant liens, make acquisitions, merge, dispose of assets, pay dividends, repurchase stock, make investments and enter into certain transactions with affiliates. The Company was in full compliance with the covenants as of September 30, 2017. The Company borrowed a total of $2.78 billion for the CEB acquisition. The Company borrowed $1.675 billion under the 2016 Credit Agreement, which consisted of $900.0 million under an increased Term loan A facility, $500.0 million under a new Term loan B facility, and $275.0 million on its existing revolving credit facility. The $1.675 billion drawn under the 2016 Credit Agreement, along with the funds raised through the issuance of $800.0 million Senior Notes and $300.0 million 364 -day Bridge Credit Facility discussed below, were used to fund the CEB acquisition and related costs. As discussed below, the funds borrowed under the 364 -day Bridge Credit Facility were completely repaid by September 30, 2017. On January 20, 2017, the Company entered into a First Amendment to the 2016 Credit Agreement, which was entered into to permit the acquisition of CEB and the incurrence of additional debt to finance, in part, the acquisition and repay certain debt of CEB, and to modify certain covenants. On March 20, 2017, the Company entered into a Second Amendment to the 2016 Credit Agreement. The Second Amendment was also entered into in connection with the acquisition of CEB and was executed primarily to extend the maturity date of the Term loan A facility and revolving credit facility through March 20, 2022 and to revise the interest rate and amortization schedule. On April 5, 2017, in conjunction with the closing of the CEB acquisition, the Company entered into a Third Amendment to the 2016 Credit Agreement with its lenders which increased the aggregate principal amount of the existing Term loan A facility by $900.0 million and added a new incremental Term loan B facility in an aggregate principal amount of $500.0 million . The Term loan A facility will be repaid in 16 consecutive quarterly installments that commenced on June 30, 2017, plus a final payment to be made on March 20, 2022. The additional amount drawn under the Term loan A facility has the same maturity date and is subject to the same interest, repayment terms, amortization schedules, representations and warranties, affirmative and negative covenants and events of default as the amounts outstanding under such facility prior to entry by the Company into the Incremental Amendment. The revolving credit facility may be borrowed, repaid, and re-borrowed through March 20, 2022, at which time all amounts must be repaid. Amounts borrowed under the Term loan A facility and the revolving credit facility bear interest at a rate equal to, at the Company's option, either: (i) the greatest of: (x) the Administrative Agent’s prime rate; (y) the average rate on Federal Reserve Board of New York rate plus 1/2 of 1% ; and (z) the eurodollar rate (adjusted for statutory reserves) plus 1% , in each case plus a margin equal to between 0.125% and 1.50% depending on Gartner’s consolidated leverage ratio as of the end of the four consecutive fiscal quarters most recently ended; or (ii) the eurodollar rate (adjusted for statutory reserves) plus a margin equal to between 1.125% and 2.50% , depending on Gartner’s leverage ratio as of the end of the four consecutive fiscal quarters most recently ended. The Term loan B facility contains representations and warranties, affirmative and negative covenants and events of default that are the same as the Term loan A facility and revolving credit facility, except that a breach of financial maintenance covenants will not result in an event of default under the Term loan B facility unless the lenders under the revolving credit facility and Term loan A facility have accelerated the revolving loans and Term loan A loans and terminated their commitments thereunder. Additionally, the Term loan B facility includes mandatory prepayment requirements related to asset sales (subject to reinvestment), debt incurrence (other than permitted debt) and excess cash flow, subject to certain limitations described therein. The Term loan B facility will mature on April 5, 2024 and amounts outstanding thereunder will bear interest at a rate per annum equal to, at the option of Gartner, (i) adjusted LIBOR plus 2.00% or (ii) an alternate base rate plus 1.00% . 364 -day Bridge Credit Facility On April 5, 2017, in conjunction with the acquisition of CEB, the Company entered into a senior unsecured 364 -day Bridge Credit Facility in an aggregate principal amount of $300.0 million , which was immediately drawn down to fund a portion of the purchase price associated with the CEB acquisition. The Company repaid $100.0 million of the 364 -day Bridge Credit Facility during the second quarter of 2017 and the remaining $200.0 million was repaid during the third quarter of 2017. Senior Notes On March 30, 2017 , in conjunction with the acquisition of CEB, the Company issued $800.0 million aggregate principal amount of 5.125% Senior Notes due 2025 (the “Senior Notes”). The proceeds of the Senior Notes were also used to fund a portion of the purchase price associated with the CEB acquisition. The Senior Notes were issued at an issue price of 100.00% and bear interest at a fixed rate of 5.125% per annum. Starting on October 1, 2017, interest on the Senior Notes will be payable on April 1 and October 1 of each year. The Senior Notes will mature on April 1, 2025. The Company may redeem some or all of the Senior Notes at any time on or after April 1, 2020 for cash at the redemption prices set forth in the Note Indenture, plus accrued and unpaid interest to, but not including, the redemption date. Prior to April 1, 2020, the Company may redeem up to 40% of the aggregate principal amount of the Senior Notes with the proceeds of certain equity offerings at a redemption price of 105.125% plus accrued and unpaid interest to, but not including, the redemption date. In addition, the Company may redeem some or all of the Senior Notes prior to April 1, 2020, at a redemption price of 100% of the principal amount of the Senior Notes plus accrued and unpaid interest to, but not including, the redemption date, plus a “make-whole” premium. If the Company experiences specific kinds of change of control, it will be required to offer to purchase the Senior Notes at a price equal to 101% of the principal amount thereof plus accrued and unpaid interest. The Senior Notes are the Company’s general unsecured senior obligations, and are effectively subordinated to all of the Company’s existing and future secured indebtedness to the extent of the value of the collateral securing such indebtedness, structurally subordinated to all existing and future indebtedness and other liabilities of the Company’s non-guarantor subsidiaries, equal in right of payment to all of the Company’s and Company’s guarantor subsidiaries’ existing and future senior indebtedness and senior in right of payment to all of the Company’s future subordinated indebtedness, if any. Outstanding Borrowings - September 30, 2017 The following table summarizes the Company’s total outstanding borrowings (in thousands): Balance Balance September 30, December 31, Description: 2017 2016 2016 Credit Agreement - Term loan A facility (1) $ 1,447,875 $ 585,000 2016 Credit Agreement - Term loan B facility (1) 497,500 — 2016 Credit Agreement - Revolving credit facility (1), (2) 640,000 115,000 Senior notes (3) 800,000 — Other (4) 2,500 2,500 Principal amount outstanding (5) $ 3,387,875 $ 702,500 Less: deferred financing fees (6) (46,045 ) (8,109 ) Net balance sheet carrying amount $ 3,341,830 $ 694,391 (1) The contractual annualized interest rate as of September 30, 2017 on the Term loan A and B facilities was 3.23% , which consisted of a floating eurodollar base rate of 1.23% plus a margin of 2.00% . The contractual annualized interest rate on the revolving credit facility was 3.73% , which consisted of a floating eurodollar base rate of 1.23% plus a margin of 2.50% . However, the Company has interest rate swap contracts which effectively convert the floating eurodollar base rates on a portion of the amounts outstanding to a fixed base rate. (2) The Company had $534.0 million of available borrowing capacity on the revolver (not including the expansion feature) as of September 30, 2017 . (3) Consists of $800.0 million principal amount of Senior Notes outstanding, which the Company issued on March 30, 2017 to finance in part the CEB acquisition. The Senior Notes pay a fixed rate of 5.125% and have an eight year maturity. (4) Consists of a $2.5 million State of Connecticut economic development loan with a 3.00% fixed rate of interest. The loan was originated in 2012 and has a 10 year maturity. Principal payments are deferred for the first five years and the loan may be repaid at any point by the Company without penalty. (5) The average annual effective rates on the Company's total debt outstanding for the three and nine months ended September 30, 2017 , including the effect of its interest rate swaps discussed below, were 4.01% and 3.73% , respectively. (6) The deferred financing fees are being amortized to Interest expense, net over the term of the respective debt obligation. During the nine months ended September 30, 2017, the Company paid $51.2 million in additional deferred financing fees and recorded a charge of approximately $6.1 million for the write-off of deferred financing fees related to the prior financing arrangement. Interest Rate Swaps The Company has fixed-for-floating interest rate swap contracts which it designates as accounting hedges of the forecasted interest payments on the Company’s variable rate borrowings. The Company pays base fixed rates on the swaps and in return receives a floating eurodollar base rate on 30 day notional borrowings. The Company accounts for the interest rate swaps as cash flow hedges in accordance with FASB ASC Topic 815. Since the swaps hedge forecasted interest payments, changes in the fair value of the swaps are recorded in accumulated other comprehensive income (loss), a component of equity, as long as the swaps continue to be highly effective hedges of the designated interest rate risk. Any ineffective portion of change in the fair value of the hedges is recorded in earnings. All of the swaps were highly effective hedges of the forecasted interest payments as of September 30, 2017 . The interest rate swaps had a total negative fair value (liability) to the Company of $8.0 million at September 30, 2017 , which is deferred and recorded in Accumulated other comprehensive loss, net of tax effect.

Equity

Equity	9 Months Ended
	Sep. 30, 2017
Stockholders' Equity Note [Abstract]
Equity	Equity Share Repurchase Program The Company has a $1.2 billion board approved authorization to repurchase the Company's common stock, of which $1.1 billion remained available as of September 30, 2017. The Company may repurchase its common stock from time to time in amounts and at prices the Company deems appropriate, subject to the availability of stock, prevailing market conditions, the trading price of the stock, the Company’s financial performance and other conditions. Repurchases may be made through open market purchases, private transactions or other transactions and will be funded from cash on hand and borrowings under the Company’s credit arrangement. The Company’s recent share repurchase activity is presented in the following table: Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Number of shares repurchased (1) 26,725 9,882 348,236 542,792 Cash paid for repurchased shares (in thousands) (2) $ 3,275 $ 922 $ 37,188 $ 52,889 (1) The average purchase price for repurchased shares was $122.53 and $106.79 for the three and nine months ended September 30, 2017 , respectively, and $93.33 and $84.17 for the three and nine months ended September 30, 2016. (2) The cash paid for the nine months ended September 30, 2016 includes $7.2 million for share repurchases that were executed in late December 2015 and were settled in early January 2016. Share Issuance Related to the Acquisition of CEB On April 5, 2017, the Company completed the CEB acquisition and issued 7.4 million of its common shares at a fair value of $109.65 per common share. Note 2 — Acquisitions provides additional information regarding the CEB acquisition. The fair value of the Company's common stock was determined based on an average of the high and low prices of the common stock as reported by the New York Stock Exchange on April 5, 2017, the date of the acquisition. Accumulated Other Comprehensive (Loss) Income ("AOCL/I") The following tables disclose information about changes in AOCL/I by component and the related amounts reclassified out of AOCL/I to income during the periods indicated (net of tax, in thousands) (1): For the three months ended September 30, 2017 : Interest Rate Swaps Defined Benefit Pension Plans Foreign Currency Translation Adjustments Total Balance - June 30, 2017 $ (6,107 ) $ (5,700 ) $ (37,082 ) $ (48,889 ) Changes during the period: Change in AOCL/I before reclassifications to income 9 — 46,319 46,328 Reclassifications from AOCL/I to income during the period (2), (3) 1,293 52 — 1,345 Other comprehensive (loss) income for the period 1,302 52 46,319 47,673 Balance – September 30, 2017 $ (4,805 ) $ (5,648 ) $ 9,237 $ (1,216 ) For the three months ended September 30, 2016 : Interest Rate Defined Foreign Total Balance – June 30, 2016 $ (12,158 ) $ (4,757 ) $ (31,088 ) $ (48,003 ) Changes during the period: Change in AOCL/I before reclassifications to income 2,076 — (3,032 ) (956 ) Reclassifications from AOCL/I to income during the period (2), (3) 1,138 37 — 1,175 Other comprehensive (loss) income for the period 3,214 37 (3,032 ) 219 Balance – September 30, 2016 $ (8,944 ) $ (4,720 ) $ (34,120 ) $ (47,784 ) For the nine months ended September 30, 2017: Interest Rate Defined Foreign Total Balance – December 31, 2016 $ (1,409 ) $ (5,797 ) $ (42,477 ) $ (49,683 ) Changes during the period: Change in AOCL/I before reclassifications to income (6,912 ) — 51,714 44,802 Reclassifications from AOCL/I to income during the period (2), (3) 3,516 149 — 3,665 Other comprehensive (loss) income for the period (3,396 ) 149 51,714 48,467 Balance – September 30, 2017 $ (4,805 ) $ (5,648 ) $ 9,237 $ (1,216 ) For the nine months ended September 30, 2016: Interest Rate Defined Foreign Total Balance – December 31, 2015 $ (3,079 ) $ (4,832 ) $ (36,491 ) $ (44,402 ) Changes during the period: Change in AOCL/I before reclassifications to income (9,376 ) — 2,371 (7,005 ) Reclassifications from AOCL/I to income during the period (2), (3) 3,511 112 — 3,623 Other comprehensive (loss) income for the period (5,865 ) 112 2,371 (3,382 ) Balance – September 30, 2016 $ (8,944 ) $ (4,720 ) $ (34,120 ) $ (47,784 ) (1) Amounts in parentheses represent debits (deferred losses). (2) The reclassifications related to interest rate swaps (cash flow hedges) were recorded in Interest expense, net of tax effect. See Note 10 – Derivatives and Hedging for information regarding the hedges. (3) The reclassifications related to defined benefit pension plans were recorded in Selling, general and administrative expense, net of tax effect. See Note 12 – Employee Benefits for information regarding the Company’s defined benefit pension plans.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The provision for income taxes for the three months ended September 30, 2017 was a benefit of $(13.8) million on a pretax loss of $(61.9) million compared to an expense of $14.3 million on pretax income of $44.7 million in the three months ended September 30, 2016 . The effective income tax rate was 22.2% for the three months ended September 30, 2017 and 31.9% for the same period in 2016 . The quarter-over-quarter change in the effective income tax rate was primarily attributable to increases in non-deductible acquisition and integration charges. The provision for income taxes for the nine months ended September 30, 2017 was a benefit of $(52.2) million on a pretax loss of $(156.2) million compared to an expense of $55.1 million on pretax income of $182.2 million in the nine months ended September 30, 2016. The effective income tax rate was 33.4% for the nine months ended September 30, 2017 and 30.3% for the same period in 2016 . The change in the effective income tax rate was primarily attributable to a favorable estimated mix of earnings and an increase in stock-based compensation benefits. As of September 30, 2017 and December 31, 2016 , the Company had gross unrecognized tax benefits of $72.2 million and $37.1 million , respectively. The increase is largely attributable to unrecognized tax benefits of CEB included in the purchase price allocation. It is reasonably possible that gross unrecognized tax benefits will decrease by approximately $8.0 million within the next 12 months, due to the anticipated closure of audits and the expiration of certain statutes of limitation. In July 2015, the United States Tax Court (the “Court”) issued an opinion relating to the treatment of stock-based compensation expense in an inter-company cost-sharing arrangement. In its opinion, the Court held that affiliated companies may exclude stock-based compensation expense from their cost-sharing arrangement. The Internal Revenue Service is appealing the decision. Because of uncertainty related to the final resolution of this litigation and the recognition of potential benefits to the Company, the Company has not recorded any financial statement benefit associated with this decision. The Company will monitor developments related to this case and the potential impact of those developments on the Company’s consolidated financial statements.

Derivatives and Hedging

Derivatives and Hedging	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives and Hedging	Derivatives and Hedging The Company enters into a limited number of derivative contracts to mitigate the cash flow risk associated with changes in interest rates on variable rate debt and changes in foreign exchange rates on forecasted foreign currency transactions. The Company accounts for its outstanding derivative contracts in accordance with FASB ASC Topic 815, which requires all derivatives, including derivatives designated as accounting hedges, to be recorded on the balance sheet and recognized at fair value. The following tables provide information regarding the Company’s outstanding derivatives contracts as of the dates indicated (in thousands, except for number of outstanding contracts): September 30, 2017 Derivative Contract Type Number of Outstanding Contracts Notional Amounts Fair Value Asset (Liability), Net (3) Balance Sheet Line Item Unrealized Loss Recorded in OCI Interest rate swaps (1) 5 $ 1,400,000 $ (8,009 ) Other liabilities $ (4,805 ) Foreign currency forwards (2) 29 102,262 (88 ) Accrued liabilities — Total 34 $ 1,502,262 $ (8,097 ) $ (4,805 ) December 31, 2016 Derivative Contract Type Number of Outstanding Contracts Notional Amounts Fair Value Asset (Liability), Net (3) Balance Sheet Line Item Unrealized Loss Recorded in OCI Interest rate swaps (1) 3 $ 700,000 $ (2,349 ) Other liabilities $ (1,409 ) Foreign currency forwards (2) 84 86,946 (320 ) Accrued liabilities — Total 87 $ 786,946 $ (2,669 ) $ (1,409 ) (1) The swaps have been designated and are accounted for as cash flow hedges of the forecasted interest payments on borrowings. As a result, changes in fair value of the swaps are deferred and are recorded in AOCL/I, net of tax effect (see Note 7 — Debt for additional information). (2) The Company has foreign exchange transaction risk since it typically enters into transactions in the normal course of business that are denominated in foreign currencies that differ from the local functional currency. The Company enters into short-term foreign currency forward exchange contracts to mitigate the cash flow risk associated with changes in foreign currency rates on forecasted foreign currency transactions. These contracts are accounted for at fair value with realized and unrealized gains and losses recognized in Other expense, net since the Company does not designate these contracts as hedges for accounting purposes. All of the contracts outstanding at September 30, 2017 matured by the end of October 2017. (3) See Note 11 — Fair Value Disclosures for the determination of the fair value of these instruments. At September 30, 2017 , all of the Company’s derivative counterparties were investment grade financial institutions. The Company did not have any collateral arrangements with its derivative counterparties, and none of the derivative contracts contained credit-risk related contingent features. The following table provides information regarding amounts recognized in the Condensed Consolidated Statements of Operations for derivative contracts for the periods indicated (in thousands): Three Months Ended Nine Months Ended September 30, September 30, Amount recorded in: 2017 2016 2017 2016 Interest expense, net (1) $ 2,154 $ 1,895 $ 5,860 $ 5,851 Other (income), net (2) (1,764 ) (814 ) (960 ) (239 ) Total expense, net $ 390 $ 1,081 $ 4,900 $ 5,612 (1) Consists of interest expense from interest rate swap contracts. (2) Consists of net realized and unrealized gains and losses on foreign currency forward contracts.

Fair Value Disclosures

Fair Value Disclosures	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair Value Disclosures	Fair Value Disclosures The Company’s financial instruments include cash equivalents, fees receivable from customers, accounts payable, and accruals which are normally short-term in nature. The Company believes the carrying amounts of these financial instruments reasonably approximate their fair value due to their short-term nature. The Company’s financial instruments also include its outstanding borrowings. The Company believes the carrying amount of its variable-rate debt reasonably approximates its fair value because the rate of interest on those borrowings reflects current market rates of interest for similar instruments with comparable maturities. The Company enters into a limited number of derivatives transactions but does not enter into repurchase agreements, securities lending transactions, or master netting arrangements. Receivables or payables that result from derivatives transactions are recorded gross in the Condensed Consolidated Balance Sheets. FASB ASC Topic 820 provides a framework for the measurement of fair value and a valuation hierarchy based upon the transparency of inputs used in the valuation of assets and liabilities. Classification within the hierarchy is based upon the lowest level of input that is significant to the resulting fair value measurement. The valuation hierarchy contains three levels. Level 1 measurements consist of quoted prices in active markets for identical assets or liabilities. Level 2 measurements include significant other observable inputs such as quoted prices for similar assets or liabilities in active markets; identical assets or liabilities in inactive markets; observable inputs such as interest rates and yield curves; and other market-corroborated inputs. Level 3 measurements include significant unobservable inputs, such as internally-created valuation models. The Company does not currently utilize Level 3 valuation inputs to remeasure any of its assets or liabilities. However, level 3 inputs may be used by the Company in its required annual impairment review of recorded goodwill. Information regarding the periodic assessment of the Company’s goodwill is included in Note 6 — Goodwill and Intangible Assets. The Company does not typically transfer assets or liabilities between different levels of the fair value hierarchy. The following table presents the fair value of certain financial assets and liabilities (in thousands): Fair Value Fair Value Description: September 30, December 31, Assets: Values based on Level 1 inputs: Deferred compensation plan assets (1) $ 12,478 $ 10,247 Total Level 1 inputs 12,478 10,247 Values based on Level 2 inputs: Deferred compensation plan assets (1) 31,050 27,847 Foreign currency forward contracts (2) 64 165 Total Level 2 inputs 31,114 28,012 Total Assets $ 43,592 $ 38,259 Liabilities: Values based on Level 2 inputs: Deferred compensation plan liabilities (1) $ 48,793 $ 43,075 Foreign currency forward contracts (2) 152 485 Interest rate swap contracts (3) 8,009 2,349 Senior Notes due 2025 (4) 844,448 — Total Level 2 inputs 901,402 45,909 Total Liabilities $ 901,402 $ 45,909 (1) The Company has a deferred compensation plan for the benefit of certain highly compensated officers, managers and other key employees. The assets consist of investments in money market and mutual funds, and company-owned life insurance contracts, all of which are valued based on Level 1 or Level 2 valuation inputs. The related deferred compensation plan liabilities are recorded at fair value, or the estimated amount needed to settle the liability, which the Company considers to be a Level 2 input. (2) The Company enters into foreign currency forward exchange contracts to hedge the effects of adverse fluctuations in foreign currency exchange rates. Valuation of the foreign currency forward contracts is based on observable foreign currency exchange rates in active markets, which the Company considers a Level 2 input. (3) The Company has interest rate swap contracts which hedge the risk of variability from interest payments on its borrowings (see Note 7 — Debt). The fair value of the swaps is based on mark-to-market valuations prepared by a third-party broker. The valuations are based on observable interest rates from recently executed market transactions and other observable market data, which the Company considers Level 2 inputs. The Company independently corroborates the reasonableness of the valuations prepared by the third-party broker through the use of an electronic quotation service. (4) As discussed in Note 7 — Debt, the Company issued $800.0 million of principal amount fixed-rate Senior Notes due 2025 on March 30, 2017. The estimated fair value of the notes was derived from quoted market prices provided by an independent dealer which the Company considers to be a Level 2 input.

Employee Benefits

Employee Benefits	9 Months Ended
	Sep. 30, 2017
Retirement Benefits [Abstract]
Employee Benefits	Employee Benefits Defined-Benefit Pension Plans The Company has defined-benefit pension plans in several of its international locations. Benefits paid under these plans are based on years of service and level of employee compensation. The Company’s defined-benefit pension plans are accounted for in accordance with FASB ASC Topics 715 and 960. Net periodic pension expense was $0.9 million and $2.4 million for the three and nine months ended September 30, 2017 , respectively, and $0.9 million and $2.4 million for the three and nine months ended September 30, 2016, respectively.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Contingencies The Company is involved in legal proceedings and litigation arising in the ordinary course of business. We believe that the potential liability, if any, in excess of amounts already accrued from all proceedings, claims and litigation will not have a material effect on our financial position, cash flows, or results of operations when resolved in a future period. The Company has various agreements that may obligate us to indemnify the other party with respect to certain matters. Generally, these indemnification clauses are included in contracts arising in the normal course of business under which we customarily agree to hold the other party harmless against losses arising from a breach of representations related to such matters as title to assets sold and licensed or certain intellectual property rights. It is not possible to predict the maximum potential amount of future payments under these indemnification agreements due to the conditional nature of the Company’s obligations and the unique facts of each particular agreement. Historically, payments made by us under these agreements have not been material. As of September 30, 2017 , the Company did not have any material payment obligations under any such indemnification agreements.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Event On October 4, 2017, the Company announced that it initiated a process to explore and evaluate strategic alternatives for its Talent Assessment business, which is a substantial part of its Talent Assessment & Other segment. The Talent Assessment & Other segment is entirely composed of business operations that were acquired as part of the CEB acquisition. As part of this process, the Company intends to consider a range of strategic options, which may include, among other things, a sale of the business. There is no timetable for completion of the review process and the Company does not intend to announce any updates until the conclusion of the strategic review. There can be no assurance that this review will result in a transaction being announced or agreed upon.

Business and Basis of Present21

Business and Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of presentation	Basis of presentation . The accompanying interim condensed consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”), as defined in the Financial Accounting Standards Board ("FASB") Accounting Standards Codification (“ASC”) Topic 270 for interim financial information and with the applicable instructions of the U.S. Securities & Exchange Commission (“SEC”) Rule 10-01 of Regulation S-X on Form 10-Q and should be read in conjunction with the consolidated financial statements and related notes of the Company filed in its Annual Report on Form 10-K for the year ended December 31, 2016 . The fiscal year of Gartner is the twelve-month calendar period from January 1 through December 31. In the opinion of management, all normal recurring accruals and adjustments considered necessary for a fair presentation of financial position, results of operations and cash flows at the dates and for the periods presented herein have been included. The results of operations for the three and nine months ended September 30, 2017 may not be indicative of the results of operations for the remainder of 2017 or beyond.
Principles of consolidation	Principles of consolidation . The accompanying interim condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated.
Use of estimates	Use of estimates . The preparation of the accompanying interim condensed consolidated financial statements requires management to make estimates and assumptions about future events. These estimates and the underlying assumptions affect the amounts of assets and liabilities reported, disclosures about contingent assets and liabilities, and reported amounts of revenues and expenses. Such estimates include the valuation of fees receivable, goodwill, intangible assets, and other long-lived assets, as well as tax accruals and other liabilities. In addition, estimates are used in revenue recognition, income tax expense, performance-based compensation charges, depreciation and amortization. Management believes its use of estimates in these interim condensed consolidated financial statements to be reasonable. Management continually evaluates and revises its estimates using historical experience and other factors, including the general economic environment and actions it may take in the future. Management adjusts these estimates when facts and circumstances dictate. However, these estimates may involve significant uncertainties and judgments and cannot be determined with precision. In addition, these estimates are based on management’s best judgment at a point in time. As a result, differences between our estimates and actual results could be material and would be reflected in the Company’s consolidated financial statements in future periods.
Adoption of new accounting standards and Accounting standards issued but not yet adopted	Adoption of new accounting standards. The Company did not adopt any significant new accounting standards during the nine months ended September 30, 2017. Accounting standards issued but not yet adopted. The FASB has issued accounting standards that have not yet become effective and that may impact the Company’s consolidated financial statements or related disclosures in future periods. These standards and their potential impact are discussed below: Targeted Improvements to Accounting for Hedging Activities - In August 2017, the FASB issued Accounting Standards Update ("ASU") No. 2017-12, " Derivatives and Hedging (Topic 815) " ("ASU No. 2017-12"). ASU No. 2017-12 is intended to improve the financial reporting of hedging relationships to better portray economic results of an entity’s risk management activities in its financial statements. In addition to that main objective, the amendments would make certain targeted improvements to simplify the application of the hedge accounting guidance in current U.S. GAAP. ASU No. 2017-12 is effective for Gartner on January 1, 2019. We are currently evaluating the potential impact of ASU No. 2017-12 on the Company's consolidated financial statements. Distinguishing Liabilities from Equity — In July 2017, the FASB issued Accounting Standards Update ("ASU") No. 2017-11, " Earnings Per Share, Distinguishing Liabilities from Equity, and Derivatives and Hedging" ("ASU No. 2017-11"). ASU No. 2017-11 is intended to simplify the accounting for financial instruments with characteristics of liabilities and equity. Among the issues addressed are: (i) determining whether an instrument (or embedded feature) is indexed to an entity’s own stock; (ii) distinguishing liabilities from equity for mandatorily redeemable financial instruments of certain nonpublic entities; and (iii) identifying mandatorily redeemable non-controlling interests. ASU No. 2017-11 is effective for Gartner on January 1, 2019. We are currently evaluating the potential impact of ASU No. 2017-11 on the Company's consolidated financial statements. Stock Compensation Award Modifications — In May 2017, the FASB issued ASU No. 2017-09, " Compensation—Stock Compensation - Scope of Modification Accounting " ("ASU No. 2017-09"). ASU No. 2017-09 provides guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting. ASU No. 2017-09 is effective for Gartner on January 1, 2018. We have concluded that the adoption of ASU No. 2017-09 will not have a material impact on the Company's consolidated financial statements. Retirement Benefits Cost Presentation — In March 2017, the FASB issued ASU No. 2017-07, "Compensation—Retirement Benefits" ("ASU No. 2017-07"). ASU No. 2017-07 improves the reporting of net benefit cost in the financial statements, and provides additional guidance on the presentation of net benefit cost in the income statement and clarifies the components eligible for capitalization. ASU No. 2017-07 is effective for Gartner on January 1, 2018. We have concluded that the adoption of ASU No. 2017-07 will not have a material impact on the Company's consolidated financial statements. Partial Sales of Non-financial Asset s — In February 2017, the FASB issued ASU No. 2017-05, "Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Non-financial Assets" ("ASU No. 2017-05"). ASU No. 2017-05 clarifies the scope of the FASB’s recently established guidance on non-financial asset de-recognition as well as the accounting for partial sales of non-financial assets. It conforms the de-recognition guidance on non-financial assets with the model for revenue transactions. ASU No. 2017-05 is effective for Gartner on January 1, 2018. We have concluded that the adoption of ASU No. 2017-05 will not have a material impact on the Company's consolidated financial statements. Goodwill Impairment — In January 2017, the FASB issued ASU No. 2017-04, " Intangibles—Goodwill and Other - Simplifying the Test for Goodwill Impairment " ("ASU No. 2017-04"). ASU No. 2017-04 simplifies the determination of the amount of goodwill to be potentially charged off by eliminating Step 2 of the goodwill impairment test. ASU No. 2017-04 is effective for Gartner on January 1, 2020. We have concluded that the adoption of ASU No. 2017-04 will not have a material impact on the Company's consolidated financial statements. Definition of a Business — In January 2017, the FASB issued ASU No. 2017-01, "Clarifying the Definition of a Business" ("ASU No. 2017-01"), which is effective for Gartner on January 1, 2018. ASU No. 2017-01 changes the U.S. GAAP definition of a business which can impact the accounting for asset purchases, acquisitions, goodwill impairment, and other assessments. We have concluded that the adoption of ASU No. 2017-01 will not have a material impact on the Company's consolidated financial statements. Presentation of Restricted Cash — In November 2016, the FASB issued ASU No. 2016-18, "Restricted Cash" ("ASU No. 2016-18"). ASU No. 2016-18 requires that amounts generally described as restricted cash and restricted cash equivalents be presented with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. If different, a reconciliation of the cash balances reported in the cash flow statement and the balance sheet would need to be provided along with explanatory information. ASU No. 2016-18 is effective for Gartner on January 1, 2018. The adoption of ASU No. 2016-18 will require the Company to disclose restricted cash and, as a result, will change the presentation of the consolidated statements of cash flows. Income Taxes — In October 2016, the FASB issued ASU No. 2016-16, "Intra-Entity Transfers of Assets Other Than Inventory" ("ASU No. 2016-16"). ASU No. 2016-16 accelerates the recognition of taxes on certain intra-entity transactions and is effective for Gartner on January 1, 2018. Current U.S. GAAP requires deferral of the income tax implications of an intercompany sale of assets until the assets are sold to a third party or recovered through use. Under the new rule, the seller’s tax effects and the buyer’s deferred taxes on post-adoption asset transfers will be immediately recognized upon the sale. On the date of adoption of ASU No. 2016-16 any taxes attributable to pre-2018 intra-entity transfers that were previously deferred will be accelerated and recorded to retained earnings as permitted by the transition rules. ASU 2016-16 could have a material impact on our consolidated financial statements in the future depending on the nature, size, and tax consequences of future intra-entity transfers, if any. Statement of Cash Flows — In August 2016, the FASB issued ASU No. 2016-15, "Classification of Certain Cash Receipts and Cash Payments" ("ASU No. 2016-15"). ASU No. 2016-15 sets forth classification requirements for certain cash flow transactions. ASU No. 2016-15 is effective for Gartner on January 1, 2018. We have concluded that the adoption of ASU No. 2016-15 will not have a material impact on the Company's consolidated financial statements. Financial Instrument Credit Losses — In June 2016, the FASB issued ASU No. 2016-13, " Financial Instruments—Credit Losses" ("ASU No. 2016-13"). ASU No. 2016-13 amends the current financial instrument impairment model by requiring entities to use a forward-looking approach based on expected losses to estimate credit losses on certain types of financial instruments, including trade receivables. ASU No. 2016-13 is effective for Gartner on January 1, 2020, with early adoption permitted. We are currently evaluating the potential impact of ASU No. 2016-13 on our consolidated financial statements. Leases — In February 2016, the FASB issued ASU No. 2016-02, " Leases " ("ASU No. 2016-02") which will require significant changes in the accounting and disclosure for lease arrangements. Currently under U.S. GAAP, lease arrangements that meet certain criteria are considered operating leases and are not recorded on the balance sheet. All of the Company's existing lease arrangements are accounted for as operating leases and are thus not recorded on the Company's balance sheet. ASU No. 2016-02 will significantly change the accounting for leases since a right-of-use ("ROU") model must be used in which the lessee must record a ROU asset and a lease liability on the balance sheet for leases with terms longer than 12 months. Leases will be classified as either finance or operating arrangements, with classification affecting the pattern of expense recognition in the income statement. ASU No. 2016-02 also requires expanded disclosures about leasing arrangements. ASU No. 2016-02 will be effective for Gartner on January 1, 2019. We are currently evaluating the impact of ASU No. 2016-02 on our consolidated financial statements. Financial Instruments Recognition and Measurement — In January 2016, the FASB issued ASU No. 2016-01, " Financial Instruments Overall - Recognition and Measurement of Financial Assets and Liabilities " ("ASU No. 2016-01") to address certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. Among the significant changes required by ASU No. 2016-01 is that equity investments will be measured at fair value with changes in fair value recognized in net income. ASU No. 2016-01 will be effective for Gartner on January 1, 2018. We have concluded that the adoption of ASU No. 2016-01 will not have a material impact on the Company's consolidated financial statements. Revenue Recognition — In May 2014, the FASB issued ASU No. 2014-09, " Revenue from Contracts with Customers " ("ASU No. 2014-09"). ASU No. 2014-09 and related amendments require changes in revenue recognition policies as well as enhanced disclosures. ASU No. 2014-09 is intended to clarify the principles for recognizing revenue by removing inconsistencies and weaknesses in existing revenue recognition rules; provide a more robust framework for addressing revenue recognition issues; improve comparability of revenue recognition practices across entities, industries, jurisdictions and capital markets; and provide more useful information to users of financial statements through improved disclosures. The Company has completed an initial assessment of the impact of ASU No. 2014-09 on its existing revenue recognition policies and plans to adopt the rule on January 1, 2018 using the cumulative effect method of adoption. ASU No. 2014-09 also requires significantly expanded disclosures around the nature, amount, timing and uncertainty of revenue and cash flows arising from contracts with customers, which the Company is currently compiling. While the Company has not fully completed its assessment of the impact of ASU No. 2014-09, primarily the completion of our review of the recently-acquired CEB's revenue recognition policies, based on the analysis completed to date, the Company does not currently anticipate that the new rule will have a material impact on its consolidated financial statements. The FASB also continues to work on a number of other significant accounting standards which if issued could materially impact the Company's accounting policies and disclosures in future periods. However, since these standards have not yet been issued, the effective dates and potential impact are unknown.
Acquisitions	The Company accounts for business acquisitions in accordance with the acquisition method of accounting as prescribed by FASB ASC Topic 805, Business Combinations. The acquisition method of accounting requires the Company to record the net assets and liabilities acquired based on their estimated fair values as of the acquisition date, with any excess of the consideration transferred over the estimated fair value of the net assets acquired, including identifiable intangible assets, to be recorded to goodwill. Under the acquisition method, the operating results of acquired companies are included in the Company's consolidated financial statements beginning on the date of acquisition.

Acquisitions (Tables)

Acquisitions (Tables)	9 Months Ended
	Sep. 30, 2017
Business Combinations [Abstract]
Schedule of aggregate purchase price for acquisitions	The following table summarizes the aggregate consideration paid or payable for these acquisitions (in thousands): Aggregate consideration (1): CEB L2 Total Cash paid at close (2), (3) $ 2,687,704 $ 134,199 $ 2,821,903 Additional cash paid (2) 12,465 — 12,465 Fair value of Gartner equity awards (4) 818,660 — 818,660 Total (5) $ 3,518,829 $ 134,199 $ 3,653,028 (1) Includes the total consideration transferred for 100% of the outstanding capital stock of the acquired businesses. (2) The cash paid at close represents the gross contractual amount paid. The Company paid the additional $12.5 million in cash in third quarter 2017. Net of cash acquired from these businesses and for cash flow reporting purposes, the Company paid $2.63 billion in cash. (3) The Company borrowed a total of approximately $2.78 billion in conjunction with the CEB acquisition (see Note 7 — Debt for additional information). (4) Consists of the fair value of (i) Gartner common stock issued (see Note 8 — Equity for additional information) and (ii) stock-based compensation replacement awards. (5) The Company may also be required to pay up to an additional $20.8 million in cash for L2 which is contingent on the achievement of certain employment conditions by several key employees. This amount is being recognized as compensation expense over approximately three years.
Summary of the allocation of the purchase price to the fair value of the assets and liabilities assumed	The following table summarizes the preliminary allocation of the purchase price to the fair value of the assets acquired and liabilities assumed for the acquisitions of L2 and CEB (in thousands): CEB (3) L2 (4) Total Assets: Cash $ 194,706 $ 4,852 $ 199,558 Fees receivable 175,440 8,277 183,717 Prepaid expenses and other current assets 52,032 1,167 53,199 Property, equipment and leasehold improvements 51,751 663 52,414 Goodwill (1) 2,267,087 109,779 2,376,866 Finite-lived intangible assets (2) 1,574,100 15,890 1,589,990 Other assets 182,720 12,321 195,041 Total assets $ 4,497,836 $ 152,949 $ 4,650,785 Liabilities: Accounts payable and accrued liabilities $ 130,544 $ 3,050 $ 133,594 Deferred revenues (current) 246,472 13,200 259,672 Other liabilities 601,991 2,500 604,491 Total liabilities $ 979,007 $ 18,750 $ 997,757 Net assets acquired $ 3,518,829 $ 134,199 $ 3,653,028 (1) The Company believes the goodwill resulting from the acquisitions is supportable based on anticipated synergies. For CEB, among the factors contributing to the anticipated synergies are a broader market presence, expanded product offerings and market opportunities, and an acceleration of CEB's growth by leveraging Gartner's global infrastructure and best practices in sales productivity and other areas. None of the recorded goodwill is expected to be deductible for tax purposes. See Note 6 — Goodwill and Intangible Assets for additional information. (2) All of the acquired intangible assets are finite-lived. The determination of the fair value of the finite-lived intangible assets required management judgment and the consideration of a number of factors. In determining the fair values, management primarily relied on income valuation methodologies, in particular discounted cash flow models. The use of discounted cash flow models required the use of estimates, significant among them projected cash flows related to the particular asset; the useful lives of the particular assets; the selection of royalty and discount rates used in the models; and certain published industry benchmark data. In establishing the estimated useful lives of the finite-lived intangible assets, the Company relied on both internally-generated data for similar assets as well as certain published industry benchmark data. We believe the values we have assigned to the finite-lived intangible assets are both reasonable and supportable. See Note 6 — Goodwill and Intangible Assets for additional information regarding the finite-lived intangible assets. (3) The Company's financial statements include the operating results of CEB beginning on April 5, 2017, the date of acquisition. CEB's operating results and the related goodwill are being reported as part of the Company's Research, Events, and Talent Assessment & Other segments. The Company recorded certain measurement period adjustments for the CEB preliminary purchase price allocation during the third quarter of 2017, primarily related to certain tenant improvement incentives, which increased Prepaid expenses and Other assets, with a reduction to goodwill. Had the Company acquired CEB in prior periods, the impact to the Company's operating results would have been material, and as a result the following pro forma consolidated financial information (unaudited) is presented as if CEB had been acquired by the Company on January 1, 2016 (in thousands, except per share amounts): Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Pro forma total revenue $ 891,003 $ 756,083 $ 2,664,450 $ 2,233,942 Pro forma net (loss) (7,283 ) (40,825 ) (71,122 ) (133,379 ) Pro forma basic and diluted (loss) per share (0.08 ) (0.45 ) (0.79 ) (1.48 ) The pro forma results have been prepared in accordance with U.S. GAAP and include the following pro forma adjustments: (a) An increase in interest expense and amortization of debt issuance costs related to the financing of the CEB acquisition. Note 7 — Debt provides further information regarding the Company's borrowings related to the CEB acquisition; (b) A decrease in revenue as a result of the required fair value adjustment to deferred revenue; and (c) An adjustment for additional depreciation and amortization expense as a result of the preliminary purchase price allocation for finite-lived intangible assets and property, equipment, and leasehold improvements. (4) The Company's financial statements include the operating results of L2 beginning on March 9, 2017, the acquisition date. L2's operating results were not material to the Company's consolidated operating results and segment results for either the three or nine months ended September 30, 2017. Had the Company acquired L2 in prior periods, the impact to the Company's operating results would not have been material, and as a result pro forma financial information for L2 for prior periods has not been presented. L2's operating results and the related goodwill are being reported as part of the Company's Research segment.
Schedule of pro forma financial information	Had the Company acquired CEB in prior periods, the impact to the Company's operating results would have been material, and as a result the following pro forma consolidated financial information (unaudited) is presented as if CEB had been acquired by the Company on January 1, 2016 (in thousands, except per share amounts): Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Pro forma total revenue $ 891,003 $ 756,083 $ 2,664,450 $ 2,233,942 Pro forma net (loss) (7,283 ) (40,825 ) (71,122 ) (133,379 ) Pro forma basic and diluted (loss) per share (0.08 ) (0.45 ) (0.79 ) (1.48 )
Summary of Amounts Related to Exited Space	The following table presents a summary of the amounts related to this space for the period ended September 30, 2017 (in thousands): For the period ended September 30, 2017 Liability balance at December 31, 2016 $ — Charges during the six months ended June 30, 2017 20,149 Liability balance at June 30, 2017 20,149 Charges and adjustments during the three months ended September 30, 2017 974 Liability balance at September 30, 2017 $ 21,123

Net (Loss) Income per Share (Ta

Net (Loss) Income per Share (Tables)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Schedule of basic and diluted (loss) earnings per share calculations	The following table sets forth the calculation of basic and diluted (loss) income per share (in thousands, except per share data): Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Numerator: Net (loss) income used for calculating basic and diluted (loss) income per common share $ (48,180 ) $ 30,484 $ (104,028 ) $ 127,097 Denominator: Weighted average number of common shares used in the calculation of basic (loss) income per share 90,624 82,638 87,585 82,549 Common stock equivalents associated with stock-based compensation plans (1), (2) — 1,165 — 1,212 Shares used in the calculation of diluted (loss) income per share 90,624 83,803 87,585 83,761 Basic (loss) income per share $ (0.53 ) $ 0.37 $ (1.19 ) $ 1.54 Diluted (loss) income per share $ (0.53 ) $ 0.36 $ (1.19 ) $ 1.52 (1) For the three and nine months ended September 30, 2016, certain common stock equivalents were not included in the computation of diluted income per share because the effect would have been anti-dilutive. These common share equivalents totaled less than 0.2 million for both 2016 periods. (2) Approximately 1.4 million common stock equivalents for both the three and nine months ended September 30, 2017 were completely excluded from the calculation of diluted (loss) per share because they were anti-dilutive.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	9 Months Ended
	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of stock-based compensation expense by award type	The Company recognized the following stock-based compensation expense by award type and expense category line item in the periods indicated (in millions): Three Months Ended Nine Months Ended September 30, September 30, Award type 2017 2016 2017 2016 Stock appreciation rights $ 1.1 $ 1.3 $ 5.2 $ 4.3 Restricted stock units 13.2 8.0 62.3 31.3 Common stock equivalents 0.1 0.2 0.5 0.5 Total (1) $ 14.4 $ 9.5 $ 68.0 $ 36.1 (1) Includes charges of $2.1 million and $2.5 million during the three months ended September 30, 2017 and 2016 , respectively, and $22.6 million and $16.5 million during the nine months ended September 30, 2017 and 2016, respectively, for awards to retirement-eligible employees. Those awards vest on an accelerated basis.
Schedule of stock-based compensation expense by expense category	Three Months Ended Nine Months Ended September 30, September 30, Expense category line item 2017 2016 2017 2016 Cost of services and product development $ 6.1 $ 4.6 $ 21.5 $ 17.0 Selling, general and administrative 7.1 4.9 30.8 19.1 Acquisition and integration charges (2) 1.2 — 15.7 — Total (1) $ 14.4 $ 9.5 $ 68.0 $ 36.1 (1) Includes charges of $2.1 million and $2.5 million during the three months ended September 30, 2017 and 2016 , respectively, and $22.6 million and $16.5 million during the nine months ended September 30, 2017 and 2016, respectively, for awards to retirement-eligible employees. Those awards vest on an accelerated basis. (2) These charges are primarily the result of the acceleration of the vesting of certain restricted stock units related to the CEB acquisition.
Schedule of fair value assumptions of SARs	The fair value of a SARs award is determined on the date of grant using the Black-Scholes-Merton valuation model with the following weighted average assumptions: Nine Months Ended September 30, 2017 2016 Expected dividend yield (1) — % — % Expected stock price volatility (2) 22 % 22 % Risk-free interest rate (3) 1.8 % 1.1 % Expected life in years (4) 4.5 4.4 (1) The expected dividend yield assumption was based on both the Company's historical and anticipated dividend payouts. Historically, the Company has not paid cash dividends on its Common Stock. (2) The determination of expected stock price volatility was based on both historical Common Stock prices and implied volatility from publicly traded options in the Common Stock. (3) The risk-free interest rate was based on the yield of a U.S. Treasury security with a maturity similar to the expected life of the award. (4) The expected life represents the Company’s estimate of the weighted average period of time the SARs are expected to be outstanding (that is, the period between the service inception date and the expected exercise date).
Stock appreciation rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of summary of the changes in SARS, RSUs, and CSEs outstanding	The following table summarizes changes in SARs outstanding during the nine months ended September 30, 2017 : Stock Appreciation Rights ("SARs") (in millions) Per Share Weighted Average Exercise Price Per Share Weighted Average Grant Date Fair Value Weighted Average Remaining Contractual Term (Years) Outstanding at December 31, 2016 1.3 $ 66.22 $ 15.77 4.40 Granted 0.3 99.07 22.02 6.35 Exercised (0.3 ) 51.25 14.74 n/a Outstanding at September 30, 2017 (1) (2) 1.3 76.64 17.36 4.57 Vested and exercisable at September 30, 2017 (2) 0.6 $ 64.71 $ 15.60 3.49 n/a=not applicable. (1) As of September 30, 2017 , 0.7 million of the total SARs outstanding were unvested. The Company expects that substantially all of those unvested awards will vest in future periods. (2) As of September 30, 2017 , the total SARs outstanding had an intrinsic value of $62.8 million . On such date, SARs vested and exercisable had an intrinsic value of $33.4 million .
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of summary of the changes in SARS, RSUs, and CSEs outstanding	The following table summarizes the changes in RSUs outstanding during the nine months ended September 30, 2017 : Restricted Stock Units ("RSUs") (in millions) Per Share Weighted Average Grant Date Fair Value Outstanding at December 31, 2016 1.3 $ 73.19 Granted (1) (2) 1.1 105.45 Vested and released (0.7 ) 78.44 Forfeited (0.1 ) 93.18 Outstanding at September 30, 2017 (3) (4) 1.6 $ 91.37 (1) The 1.1 million of RSUs granted during the nine months ended September 30, 2017 consisted of 0.2 million of performance-based RSUs awarded to executives and 0.9 million of service-based RSUs awarded to non-executive employees and non-management board members. The 0.2 million of performance-based RSUs represents the target amount of the grant for the year, which is tied to an increase in the Company's total contract value for 2017. Total contract value for this determination represents the value attributable to all of the Company's subscription-related contracts but not including CEB contract value. The final number of performance-based RSUs that will ultimately be awarded for 2017 ranges from 0% to 200% of the target amount, with the final number dependent on the actual increase in total contract value for 2017 as measured on December 31, 2017. If the specified minimum level of achievement is not met, the performance-based RSUs will be forfeited in their entirety and any previously recorded compensation expense will be reversed. (2) Includes 0.6 million of RSUs awarded to employees that joined Gartner as a result of the CEB acquisition. (3) The Company expects that substantially all of the RSUs outstanding will vest in future periods. (4) As of September 30, 2017, the weighted average remaining contractual term of the RSUs outstanding was approximately 1.4 years.
Common stock equivalents
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of summary of the changes in SARS, RSUs, and CSEs outstanding	The following table summarizes the changes in CSEs outstanding during the nine months ended September 30, 2017 : Common Stock Equivalents ("CSEs") Per Share Weighted Average Grant Date Fair Value Outstanding at December 31, 2016 107,338 $ 20.74 Granted 4,382 118.75 Converted to shares of Common Stock upon grant (3,177 ) 118.71 Outstanding at September 30, 2017 108,543 $ 21.82

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Schedule of segment reporting information, by segment	The following tables present operating information about the Company’s reportable segments for the periods indicated (in thousands). The segment results for the three and nine months ended September 30, 2017 include the results of CEB beginning on the acquisition date: Three Months Ended September 30, 2017 Research Consulting Events Talent Assessment & Other Consolidated Revenues $ 653,443 $ 72,117 $ 44,953 $ 57,572 $ 828,085 Gross contribution 436,222 16,188 16,011 31,215 499,636 Corporate and other expenses (523,985 ) Operating (loss) $ (24,349 ) Three Months Ended September 30, 2016 Research Consulting Events Talent Assessment & Other Consolidated Revenues (1) $ 466,877 $ 73,707 $ 33,475 $ — $ 574,059 Gross contribution (1) 322,646 18,215 14,529 — 355,390 Corporate and other expenses (306,664 ) Operating income $ 48,726 Nine Months Ended September 30, 2017 Research Consulting Events Talent Assessment & Other Consolidated Revenues $ 1,778,481 $ 242,404 $ 171,427 $ 104,673 $ 2,296,985 Gross contribution 1,187,906 71,558 79,313 48,512 1,387,289 Corporate and other expenses (1,456,512 ) Operating (loss) $ (69,223 ) Nine Months Ended September 30, 2016 Research Consulting Events Talent Assessment & Other Consolidated Revenues (1) $ 1,371,157 $ 237,876 $ 132,290 $ — $ 1,741,323 Gross contribution (1) 954,276 71,110 63,574 — 1,088,960 Corporate and other expenses (892,506 ) Operating income $ 196,454 (1) Effective June 30, 2017, the Company began reporting the results of its SAS business in Research whereas previously the SAS business was reported with Consulting. As a result, revenues of $5.4 million and $20.2 million pertaining to the three and nine months ended September 30, 2016, respectively, were reclassified from Consulting to Research to be comparable with the current year presentation. Gross contribution of $3.4 million and $13.2 million for the three and nine months ended September 30, 2016, respectively, was also reclassified from Consulting to Research.
Schedule of reconciliation of segment gross contribution to net income (loss)	The following table provides a reconciliation of total segment gross contribution to net (loss) income for the periods indicated (in thousands): Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Total segment gross contribution $ 499,636 $ 355,390 $ 1,387,289 $ 1,088,960 Costs and expenses: Cost of services and product development - unallocated (1) 3,758 4,453 12,124 14,222 Selling, general and administrative 421,163 269,902 1,133,633 799,322 Depreciation and amortization 68,564 15,752 168,651 46,004 Acquisition and integration charges 30,500 16,557 142,104 32,958 Operating (loss) income (24,349 ) 48,726 (69,223 ) 196,454 Interest expense and other, net 37,591 3,978 86,971 14,208 (Benefit) provision for income taxes (13,760 ) 14,264 (52,166 ) 55,149 Net (loss) income $ (48,180 ) $ 30,484 $ (104,028 ) $ 127,097 (1) The unallocated amounts consist of certain bonus and related fringe costs recorded in consolidated Cost of services and product development expense that are not allocated to segment expense. The Company's policy is to only allocate bonus and related fringe charges to segments for up to 100% of the segment employee's target bonus. Amounts above 100% are absorbed by corporate.

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of changes to the carrying amount of goodwill by reporting unit	The following table presents changes to the carrying amount of goodwill by reportable segment during the nine months ended September 30, 2017 (in thousands): Research Consulting Events Talent Assessment & Other Total Balance at December 31, 2016 (1) $ 595,450 $ 96,480 $ 46,523 $ — $ 738,453 Additions due to acquisitions (2) 1,717,750 — 123,423 535,693 2,376,866 Foreign currency translation impact 23,183 1,375 1,462 3,707 29,727 Balance at September 30, 2017 $ 2,336,383 $ 97,855 $ 171,408 $ 539,400 $ 3,145,046 (1) The Company does not have any accumulated goodwill impairment losses. (2) The goodwill additions are due to the acquisitions of CEB and L2 during April 2017 and March 2017, respectively (see Note 2 for additional information regarding our recent acquisitions).
Schedule of amortizable intangible assets	The following tables present reconciliations of the carrying amounts of the Company's finite-lived intangible assets as of the dates indicated (in thousands): September 30, 2017 Customer Software Content Other Total Gross cost at December 31, 2016 $ 63,369 $ 16,025 $ 3,728 $ 33,645 $ 116,767 Additions due to acquisitions (1) 1,251,000 181,000 143,500 14,490 1,589,990 Write-off of fully amortized intangible assets — — (4,227 ) — (4,227 ) Foreign currency translation impact 12,073 1,194 6,080 496 19,843 Gross cost 1,326,442 198,219 149,081 48,631 1,722,373 Accumulated amortization (2) (75,173 ) (24,419 ) (36,884 ) (21,432 ) (157,908 ) Balance at September 30, 2017 $ 1,251,269 $ 173,800 $ 112,197 $ 27,199 $ 1,564,465 December 31, 2016 Customer Software Content Other Total Gross cost $ 63,369 $ 16,025 $ 3,728 $ 33,645 $ 116,767 Accumulated amortization (2) (16,744 ) (8,904 ) (2,033 ) (12,285 ) (39,966 ) Balance at December 31, 2016 $ 46,625 $ 7,121 $ 1,695 $ 21,360 $ 76,801 (1) The additions were primarily due to the acquisitions of CEB and L2 during April 2017 and March 2017, respectively (see Note 2 for additional information regarding our recent acquisitions). (2) Finite-lived intangible assets are amortized using the straight-line method over the following periods: Customer relationships— 4 to 13 years ; Software— 3 to 7 years ; Content— 1.5 to 5 years ; and Other (consisting of trade names and non-competes) — 2 to 5 years .
Schedule of estimated future amortization expense by year	The estimated future amortization expense by year for finite-lived intangible assets will be as follows (in thousands): 2017 (remaining three months) $ 59,743 2018 222,102 2019 165,855 2020 158,981 2021 138,639 Thereafter 819,145 $ 1,564,465

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Schedule of debt	The following table summarizes the Company’s total outstanding borrowings (in thousands): Balance Balance September 30, December 31, Description: 2017 2016 2016 Credit Agreement - Term loan A facility (1) $ 1,447,875 $ 585,000 2016 Credit Agreement - Term loan B facility (1) 497,500 — 2016 Credit Agreement - Revolving credit facility (1), (2) 640,000 115,000 Senior notes (3) 800,000 — Other (4) 2,500 2,500 Principal amount outstanding (5) $ 3,387,875 $ 702,500 Less: deferred financing fees (6) (46,045 ) (8,109 ) Net balance sheet carrying amount $ 3,341,830 $ 694,391 (1) The contractual annualized interest rate as of September 30, 2017 on the Term loan A and B facilities was 3.23% , which consisted of a floating eurodollar base rate of 1.23% plus a margin of 2.00% . The contractual annualized interest rate on the revolving credit facility was 3.73% , which consisted of a floating eurodollar base rate of 1.23% plus a margin of 2.50% . However, the Company has interest rate swap contracts which effectively convert the floating eurodollar base rates on a portion of the amounts outstanding to a fixed base rate. (2) The Company had $534.0 million of available borrowing capacity on the revolver (not including the expansion feature) as of September 30, 2017 . (3) Consists of $800.0 million principal amount of Senior Notes outstanding, which the Company issued on March 30, 2017 to finance in part the CEB acquisition. The Senior Notes pay a fixed rate of 5.125% and have an eight year maturity. (4) Consists of a $2.5 million State of Connecticut economic development loan with a 3.00% fixed rate of interest. The loan was originated in 2012 and has a 10 year maturity. Principal payments are deferred for the first five years and the loan may be repaid at any point by the Company without penalty. (5) The average annual effective rates on the Company's total debt outstanding for the three and nine months ended September 30, 2017 , including the effect of its interest rate swaps discussed below, were 4.01% and 3.73% , respectively. (6) The deferred financing fees are being amortized to Interest expense, net over the term of the respective debt obligation. During the nine months ended September 30, 2017, the Company paid $51.2 million in additional deferred financing fees and recorded a charge of approximately $6.1 million for the write-off of deferred financing fees related to the prior financing arrangement.

Equity (Tables)

Equity (Tables)	9 Months Ended
	Sep. 30, 2017
Stockholders' Equity Note [Abstract]
Schedule of share repurchase activity	The Company’s recent share repurchase activity is presented in the following table: Three Months Ended Nine Months Ended September 30, September 30, 2017 2016 2017 2016 Number of shares repurchased (1) 26,725 9,882 348,236 542,792 Cash paid for repurchased shares (in thousands) (2) $ 3,275 $ 922 $ 37,188 $ 52,889 (1) The average purchase price for repurchased shares was $122.53 and $106.79 for the three and nine months ended September 30, 2017 , respectively, and $93.33 and $84.17 for the three and nine months ended September 30, 2016. (2) The cash paid for the nine months ended September 30, 2016 includes $7.2 million for share repurchases that were executed in late December 2015 and were settled in early January 2016.
Schedule of the changes in Accumulated Other Comprehensive (Loss) Income by component (net of tax)	The following tables disclose information about changes in AOCL/I by component and the related amounts reclassified out of AOCL/I to income during the periods indicated (net of tax, in thousands) (1): For the three months ended September 30, 2017 : Interest Rate Swaps Defined Benefit Pension Plans Foreign Currency Translation Adjustments Total Balance - June 30, 2017 $ (6,107 ) $ (5,700 ) $ (37,082 ) $ (48,889 ) Changes during the period: Change in AOCL/I before reclassifications to income 9 — 46,319 46,328 Reclassifications from AOCL/I to income during the period (2), (3) 1,293 52 — 1,345 Other comprehensive (loss) income for the period 1,302 52 46,319 47,673 Balance – September 30, 2017 $ (4,805 ) $ (5,648 ) $ 9,237 $ (1,216 ) For the three months ended September 30, 2016 : Interest Rate Defined Foreign Total Balance – June 30, 2016 $ (12,158 ) $ (4,757 ) $ (31,088 ) $ (48,003 ) Changes during the period: Change in AOCL/I before reclassifications to income 2,076 — (3,032 ) (956 ) Reclassifications from AOCL/I to income during the period (2), (3) 1,138 37 — 1,175 Other comprehensive (loss) income for the period 3,214 37 (3,032 ) 219 Balance – September 30, 2016 $ (8,944 ) $ (4,720 ) $ (34,120 ) $ (47,784 ) For the nine months ended September 30, 2017: Interest Rate Defined Foreign Total Balance – December 31, 2016 $ (1,409 ) $ (5,797 ) $ (42,477 ) $ (49,683 ) Changes during the period: Change in AOCL/I before reclassifications to income (6,912 ) — 51,714 44,802 Reclassifications from AOCL/I to income during the period (2), (3) 3,516 149 — 3,665 Other comprehensive (loss) income for the period (3,396 ) 149 51,714 48,467 Balance – September 30, 2017 $ (4,805 ) $ (5,648 ) $ 9,237 $ (1,216 ) For the nine months ended September 30, 2016: Interest Rate Defined Foreign Total Balance – December 31, 2015 $ (3,079 ) $ (4,832 ) $ (36,491 ) $ (44,402 ) Changes during the period: Change in AOCL/I before reclassifications to income (9,376 ) — 2,371 (7,005 ) Reclassifications from AOCL/I to income during the period (2), (3) 3,511 112 — 3,623 Other comprehensive (loss) income for the period (5,865 ) 112 2,371 (3,382 ) Balance – September 30, 2016 $ (8,944 ) $ (4,720 ) $ (34,120 ) $ (47,784 ) (1) Amounts in parentheses represent debits (deferred losses). (2) The reclassifications related to interest rate swaps (cash flow hedges) were recorded in Interest expense, net of tax effect. See Note 10 – Derivatives and Hedging for information regarding the hedges. (3) The reclassifications related to defined benefit pension plans were recorded in Selling, general and administrative expense, net of tax effect. See Note 12 – Employee Benefits for information regarding the Company’s defined benefit pension plans.

Derivatives and Hedging (Tables

Derivatives and Hedging (Tables)	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of information regarding outstanding derivative contracts	The following tables provide information regarding the Company’s outstanding derivatives contracts as of the dates indicated (in thousands, except for number of outstanding contracts): September 30, 2017 Derivative Contract Type Number of Outstanding Contracts Notional Amounts Fair Value Asset (Liability), Net (3) Balance Sheet Line Item Unrealized Loss Recorded in OCI Interest rate swaps (1) 5 $ 1,400,000 $ (8,009 ) Other liabilities $ (4,805 ) Foreign currency forwards (2) 29 102,262 (88 ) Accrued liabilities — Total 34 $ 1,502,262 $ (8,097 ) $ (4,805 ) December 31, 2016 Derivative Contract Type Number of Outstanding Contracts Notional Amounts Fair Value Asset (Liability), Net (3) Balance Sheet Line Item Unrealized Loss Recorded in OCI Interest rate swaps (1) 3 $ 700,000 $ (2,349 ) Other liabilities $ (1,409 ) Foreign currency forwards (2) 84 86,946 (320 ) Accrued liabilities — Total 87 $ 786,946 $ (2,669 ) $ (1,409 ) (1) The swaps have been designated and are accounted for as cash flow hedges of the forecasted interest payments on borrowings. As a result, changes in fair value of the swaps are deferred and are recorded in AOCL/I, net of tax effect (see Note 7 — Debt for additional information). (2) The Company has foreign exchange transaction risk since it typically enters into transactions in the normal course of business that are denominated in foreign currencies that differ from the local functional currency. The Company enters into short-term foreign currency forward exchange contracts to mitigate the cash flow risk associated with changes in foreign currency rates on forecasted foreign currency transactions. These contracts are accounted for at fair value with realized and unrealized gains and losses recognized in Other expense, net since the Company does not designate these contracts as hedges for accounting purposes. All of the contracts outstanding at September 30, 2017 matured by the end of October 2017. (3) See Note 11 — Fair Value Disclosures for the determination of the fair value of these instruments.
Schedule of amounts recognized in statement of operations	The following table provides information regarding amounts recognized in the Condensed Consolidated Statements of Operations for derivative contracts for the periods indicated (in thousands): Three Months Ended Nine Months Ended September 30, September 30, Amount recorded in: 2017 2016 2017 2016 Interest expense, net (1) $ 2,154 $ 1,895 $ 5,860 $ 5,851 Other (income), net (2) (1,764 ) (814 ) (960 ) (239 ) Total expense, net $ 390 $ 1,081 $ 4,900 $ 5,612 (1) Consists of interest expense from interest rate swap contracts. (2) Consists of net realized and unrealized gains and losses on foreign currency forward contracts.

Fair Value Disclosures (Tables)

Fair Value Disclosures (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Schedule of assets and liabilities that are remeasured to fair value	The following table presents the fair value of certain financial assets and liabilities (in thousands): Fair Value Fair Value Description: September 30, December 31, Assets: Values based on Level 1 inputs: Deferred compensation plan assets (1) $ 12,478 $ 10,247 Total Level 1 inputs 12,478 10,247 Values based on Level 2 inputs: Deferred compensation plan assets (1) 31,050 27,847 Foreign currency forward contracts (2) 64 165 Total Level 2 inputs 31,114 28,012 Total Assets $ 43,592 $ 38,259 Liabilities: Values based on Level 2 inputs: Deferred compensation plan liabilities (1) $ 48,793 $ 43,075 Foreign currency forward contracts (2) 152 485 Interest rate swap contracts (3) 8,009 2,349 Senior Notes due 2025 (4) 844,448 — Total Level 2 inputs 901,402 45,909 Total Liabilities $ 901,402 $ 45,909 (1) The Company has a deferred compensation plan for the benefit of certain highly compensated officers, managers and other key employees. The assets consist of investments in money market and mutual funds, and company-owned life insurance contracts, all of which are valued based on Level 1 or Level 2 valuation inputs. The related deferred compensation plan liabilities are recorded at fair value, or the estimated amount needed to settle the liability, which the Company considers to be a Level 2 input. (2) The Company enters into foreign currency forward exchange contracts to hedge the effects of adverse fluctuations in foreign currency exchange rates. Valuation of the foreign currency forward contracts is based on observable foreign currency exchange rates in active markets, which the Company considers a Level 2 input. (3) The Company has interest rate swap contracts which hedge the risk of variability from interest payments on its borrowings (see Note 7 — Debt). The fair value of the swaps is based on mark-to-market valuations prepared by a third-party broker. The valuations are based on observable interest rates from recently executed market transactions and other observable market data, which the Company considers Level 2 inputs. The Company independently corroborates the reasonableness of the valuations prepared by the third-party broker through the use of an electronic quotation service. (4) As discussed in Note 7 — Debt, the Company issued $800.0 million of principal amount fixed-rate Senior Notes due 2025 on March 30, 2017. The estimated fair value of the notes was derived from quoted market prices provided by an independent dealer which the Company considers to be a Level 2 input.

Business and Basis of Present31

Business and Basis of Presentation (Details) enterprise in Thousands, employee in Thousands	9 Months Ended
	Sep. 30, 2017employeesegmentcountryacquisitionenterprise
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Entity number of employees | employee	14
Number of enterprises served | enterprise	11
Number of countries in which entity operates | country	100
Number of reportable segments | segment	4
Number of acquisitions | acquisition	2

Acquisitions (Details)

Acquisitions (Details) enterprise in Thousands	Apr. 05, 2017USD ($)employeecountry	Mar. 09, 2017USD ($)employee	Apr. 05, 2017USD ($)employeecountry	Sep. 30, 2017USD ($)employeecountryenterprise	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)employeecountryacquisitionenterprise	Sep. 30, 2016USD ($)
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%		100.00%
Consideration transferred, aggregate purchase price			$ 3,653,028,000
Consideration transferred, cash paid			$ 2,821,903,000
Entity number of employees | employee				14,000		14,000
Number of corporations served | enterprise				11		11
Number of countries in which entity operates | country				100		100
Net cash paid for acquisition						$ 2,630,000,000
Number of acquisitions | acquisition						2
Acquisition and integration charges				$ 30,500,000	$ 16,557,000	$ 142,104,000	$ 32,958,000
CEB
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%		100.00%
Consideration transferred, aggregate purchase price	$ 3,518,829,000
Consideration transferred, cash paid	2,687,704,000			$ 12,500,000
Consideration transferred, fair value of common shares issued	$ 818,700,000		$ 818,700,000
Entity number of employees | employee	4,900		4,900
Number of countries in which entity operates | country	70		70
Proceeds from issuance of debt	$ 2,780,000,000
Goodwill, expected to be deductible for tax purposes	$ 0		$ 0
L2, Inc.
Business Acquisition [Line Items]
Percentage of voting interests acquired		100.00%
Consideration transferred, aggregate purchase price		$ 134,199,000
Consideration transferred, cash paid		$ 134,199,000
Entity number of employees | employee		150
Contingent consideration arrangements, maximum value		$ 20,800,000
Contingent consideration arrangements, maximum value, period of expense recognition		3 years
Goodwill, expected to be deductible for tax purposes		$ 0

Acquisitions - Summary of Consi

Acquisitions - Summary of Consideration Transferred (Details) - USD ($) $ in Thousands	Apr. 05, 2017	Mar. 09, 2017	Apr. 05, 2017	Sep. 30, 2017
Business Acquisition [Line Items]
Cash paid at close			$ 2,821,903
Additional cash paid			12,465
Fair value of Gartner equity awards		$ 0	818,660
Total aggregate consideration			$ 3,653,028
CEB
Business Acquisition [Line Items]
Cash paid at close	$ 2,687,704			$ 12,500
Additional cash paid	12,465
Fair value of Gartner equity awards	818,660
Total aggregate consideration	$ 3,518,829
L2, Inc.
Business Acquisition [Line Items]
Cash paid at close		134,199
Additional cash paid		0
Total aggregate consideration		$ 134,199

Acquisitions - Summary of the A

Acquisitions - Summary of the Allocation of the Purchase Price to the Fair Value of the Assets and Liabilities Assumed (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Apr. 05, 2017	Mar. 09, 2017	Dec. 31, 2016
Assets:
Cash		$ 199,558
Fees receivable		183,717
Prepaid expenses and other current assets		53,199
Property, equipment and leasehold improvements		52,414
Goodwill	$ 3,145,046	2,376,866		$ 738,453
Finite-lived intangible assets		1,589,990
Other assets		195,041
Total assets		4,650,785
Liabilities:
Accounts payable and accrued liabilities		133,594
Deferred revenues (current)		259,672
Other liabilities		604,491
Total liabilities		997,757
Net assets acquired		3,653,028
CEB
Assets:
Cash		194,706
Fees receivable		175,440
Prepaid expenses and other current assets		52,032
Property, equipment and leasehold improvements		51,751
Goodwill		2,267,087
Finite-lived intangible assets		1,574,100
Other assets		182,720
Total assets		4,497,836
Liabilities:
Accounts payable and accrued liabilities		130,544
Deferred revenues (current)		246,472
Other liabilities		601,991
Total liabilities		979,007
Net assets acquired		$ 3,518,829
L2, Inc.
Assets:
Cash			$ 4,852
Fees receivable			8,277
Prepaid expenses and other current assets			1,167
Property, equipment and leasehold improvements			663
Goodwill			109,779
Finite-lived intangible assets			15,890
Other assets			12,321
Total assets			152,949
Liabilities:
Accounts payable and accrued liabilities			3,050
Deferred revenues (current)			13,200
Other liabilities			2,500
Total liabilities			18,750
Net assets acquired			$ 134,199

Acquisitions - Proforma (Detail

Acquisitions - Proforma (Details) - CEB - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Business Acquisition, Pro Forma Information [Abstract]
Pro forma total revenue	$ 891,003	$ 756,083	$ 2,664,450	$ 2,233,942
Pro forma net (loss)	$ (7,283)	$ (40,825)	$ (71,122)	$ (133,379)
Pro forma basic earnings per share (in dollars per share)	$ (0.08)	$ (0.45)	$ (0.79)	$ (1.48)
Pro forma diluted earnings per share (in dollars per share)	$ (0.08)	$ (0.45)	$ (0.79)	$ (1.48)

Acquisitions Acquisitions - Sum

Acquisitions Acquisitions - Summary of Exited Space Liability (Details) - CEB - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Sep. 30, 2017	Jun. 30, 2017
Liability Related to Exited Space [Roll Forward]
Beginning liability balance	$ 20,149	$ 0
Charges and adjustments during the period	974	20,149
Ending liability balance	$ 21,123	$ 20,149

Net (Loss) Income per Share - C

Net (Loss) Income per Share - Calculations Of Basic And Diluted (Loss) Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net (loss) income used for calculating basic and diluted (loss) income per common share	$ (48,180)	$ 30,484	$ (104,028)	$ 127,097
Denominator:
Weighted average number of common shares used in the calculation of basic (loss) income per share (in shares)	90,624	82,638	87,585	82,549
Common stock equivalents associated with stock-based compensation plans (in shares)	0	1,165	0	1,212
Shares used in the calculation of diluted (loss) income per share (in shares)	90,624	83,803	87,585	83,761
Basic (loss) income per share (in dollars per share)	$ (0.53)	$ 0.37	$ (1.19)	$ 1.54
Diluted (loss) income per share (in dollars per share)	$ (0.53)	$ 0.36	$ (1.19)	$ 1.52

Net (Loss) Income per Share (De

Net (Loss) Income per Share (Details) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Antidilutive securities excluded from computation of (loss) income per share (less than) (in shares)	1.4	0.2	1.4	0.2

Stock-Based Compensation (Detai

Stock-Based Compensation (Details) - USD ($) $ / shares in Units, shares in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for grant (in shares)	5.5
Common stock par value (in dollars per share)	$ 0.0005		$ 0.0005
Total share-based compensation cost not yet recognized	$ 97,400,000
Total compensation cost not yet recognized, period for recognition	2 years 6 months
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee stock purchase plan, stock purchases as percentage of employee compensation maximum	10.00%
Maximum share value authorized for purchase under employee stock purchase plan	$ 23,750
Exercisable price percentage of closing price of another class of stock	95.00%
Maximum number of shares that may be purchased by eligible participants (in shares)	0.8
Proceeds from stock plans	$ 8,600,000	$ 6,900,000

Stock-Based Compensation - Stoc

Stock-Based Compensation - Stock-based Compensation Expense by Award Type and Expense Category (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 14.4	$ 9.5	$ 68	$ 36.1
Cost of services and product development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	6.1	4.6	21.5	17
Selling, general and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	7.1	4.9	30.8	19.1
Acquisition and integration charges
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	1.2	0	15.7	0
Stock appreciation rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	1.1	1.3	5.2	4.3
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	13.2	8	62.3	31.3
Common stock equivalents
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	0.1	0.2	0.5	0.5
Retirement eligible employees equity award
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 2.1	$ 2.5	$ 22.6	$ 16.5

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary Of Changes In SARs Outstanding (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation cost not yet recognized, period for recognition	2 years 6 months
Stock appreciation rights
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
Award expiration period from date of grant	7 years
Total compensation cost not yet recognized, period for recognition	4 years
SARs [Roll Forward]
Outstanding at December 31, 2016 (in shares)	1.3
Granted (in shares)	0.3
Exercised (in shares)	(0.3)
Outstanding at September 30, 2017 (in shares)	1.3	1.3
Vested and exercisable at September 30, 2017 (in shares)	0.6
Aggregate Intrinsic value of outstanding shares	$ 62.8
Aggregate intrinsic value of shares vested and exercisable	$ 33.4
Per Share Weighted Average Exercise Price [Roll Forward]
Outstanding at December 31, 2016 (in dollars per share)	$ 66.22
Granted (in dollars per share)	99.07
Exercised (in dollars per share)	51.25
Outstanding at September 30, 2017 (in dollars per share)	76.64	$ 66.22
Vested and exercisable at September 30, 2017 (in dollars per share)	64.71
Per Share Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding at December 31, 2016 (in dollars per share)	15.77
Granted (in dollars per share)	22.02
Exercised (in dollars per share)	14.74
Outstanding at September 30, 2017 (in dollars per share)	17.36	$ 15.77
Vested and exercisable at September 30, 2017 (in dollars per share)	$ 15.60
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Remaining Contractual Term [Abstract]
Granted	6 years 4 months 6 days
Outstanding weighted average remaining contractual term	4 years 6 months 26 days	4 years 4 months 24 days
Vested and exercisable at September 30, 2017	3 years 5 months 27 days
Unvested | Stock appreciation rights
SARs [Roll Forward]
Outstanding at September 30, 2017 (in shares)	0.7

Stock-Based Compensation - Weig

Stock-Based Compensation - Weighted Average Assumptions Used To Determine Fair Value Of SARs Grants On Date Of Grant Using Black-Scholes (Details) - Stock appreciation rights	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected dividend yield	0.00%	0.00%
Expected stock price volatility	22.00%	22.00%
Risk-free interest rate	1.80%	1.10%
Expected life in years	4 years 6 months	4 years 4 months 24 days

Stock-Based Compensation - Su43

Stock-Based Compensation - Summary Of Changes In RSUs Outstanding (Details) shares in Millions	9 Months Ended
	Sep. 30, 2017$ / sharesshares
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
Restricted Stock Units (RSUs) [Roll Forward]
Outstanding at December 31, 2016 (in shares)	1.3
Granted (in shares)	1.1
Vested and released (in shares)	(0.7)
Forfeited (in shares)	(0.1)
Outstanding at September 30, 2017 (in shares)	1.6
Per Share Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding at December 31, 2016 (in dollars per share) | $ / shares	$ 73.19
Granted (in dollars per share) | $ / shares	105.45
Vested and released (in dollars per share) | $ / shares	78.44
Forfeited (in dollars per share) | $ / shares	93.18
Outstanding at September 30, 2017 (in dollars per share) | $ / shares	$ 91.37
Weighted average remaining contractual terms	1 year 4 months 24 days
Restricted stock units | Executive officer
Restricted Stock Units (RSUs) [Roll Forward]
Granted (in shares)	0.2
Restricted stock units | Executive officer | Minimum | Target amount
Per Share Weighted Average Grant Date Fair Value [Roll Forward]
Percentage of shares authorized for grant	0.00%
Restricted stock units | Executive officer | Maximum | Target amount
Per Share Weighted Average Grant Date Fair Value [Roll Forward]
Percentage of shares authorized for grant	200.00%
Restricted stock units | Non executive
Restricted Stock Units (RSUs) [Roll Forward]
Granted (in shares)	0.9
Performance-based RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	4 years
CEB | Restricted stock units
Restricted Stock Units (RSUs) [Roll Forward]
Granted (in shares)	0.6

Stock-Based Compensation - Su44

Stock-Based Compensation - Summary Of Changes In CSEs Outstanding (Details) - Common stock equivalents	9 Months Ended
	Sep. 30, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Base fee percentage (up to)	50.00%
Common Stock Equivalents (CSEs) [Roll Forward]
Outstanding at December 31, 2016 (in shares) | shares	107,338
Granted (in shares) | shares	4,382
Converted to common shares upon grant (in shares) | shares	(3,177)
Outstanding at September 30, 2017 (in shares) | shares	108,543
Per Share Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding at December 31, 2016 (in dollars per share) | $ / shares	$ 20.74
Granted (in dollars per share) | $ / shares	118.75
Converted to common shares upon grant (in dollars per share) | $ / shares	118.71
Outstanding at September 30, 2017 (in dollars per share) | $ / shares	$ 21.82

Segment Information - Informati

Segment Information - Information About Reportable Segments (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)segment	Sep. 30, 2016USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			4
Segment Reporting Information [Line Items]
Revenues	$ 828,085,000	$ 574,059,000	$ 2,296,985,000	$ 1,741,323,000
Operating (loss) income	(24,349,000)	48,726,000	(69,223,000)	196,454,000
Intersegment revenues
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Reportable segments
Segment Reporting Information [Line Items]
Revenues	828,085,000	574,059,000	2,296,985,000	1,741,323,000
Gross contribution	499,636,000	355,390,000	1,387,289,000	1,088,960,000
Corporate and other expenses
Segment Reporting Information [Line Items]
Corporate and other expenses	(523,985,000)	(306,664,000)	(1,456,512,000)	(892,506,000)
Research
Segment Reporting Information [Line Items]
Revenues		5,400,000		20,200,000
Gross contribution		3,400,000		13,200,000
Research | Reportable segments
Segment Reporting Information [Line Items]
Revenues	653,443,000	466,877,000	1,778,481,000	1,371,157,000
Gross contribution	436,222,000	322,646,000	1,187,906,000	954,276,000
Consulting
Segment Reporting Information [Line Items]
Revenues		(5,400,000)		(20,200,000)
Gross contribution		(3,400,000)		(13,200,000)
Consulting | Reportable segments
Segment Reporting Information [Line Items]
Revenues	72,117,000	73,707,000	242,404,000	237,876,000
Gross contribution	16,188,000	18,215,000	71,558,000	71,110,000
Events | Reportable segments
Segment Reporting Information [Line Items]
Revenues	44,953,000	33,475,000	171,427,000	132,290,000
Gross contribution	16,011,000	14,529,000	79,313,000	63,574,000
Talent Assessment & Other | Reportable segments
Segment Reporting Information [Line Items]
Revenues	57,572,000	0	104,673,000	0
Gross contribution	$ 31,215,000	$ 0	$ 48,512,000	$ 0

Segment Information - Reconcili

Segment Information - Reconciliation of Segment Gross Contribution to Net Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Costs and expenses:
Cost of services and product development - unallocated	$ 332,207	$ 223,122	$ 921,820	$ 666,585
Selling, general and administrative	421,163	269,902	1,133,633	799,322
Acquisition and integration charges	30,500	16,557	142,104	32,958
Operating (loss) income	(24,349)	48,726	(69,223)	196,454
Interest expense and other, net	37,591	3,978	86,971	14,208
(Benefit) provision for income taxes	(13,760)	14,264	(52,166)	55,149
Net (loss) income	(48,180)	30,484	(104,028)	127,097
Reportable segments
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Total segment gross contribution	499,636	355,390	$ 1,387,289	1,088,960
Reportable segments | Maximum
Costs and expenses:
Percent of target bonus allocated to segments			100.00%
Corporate and other expenses
Costs and expenses:
Cost of services and product development - unallocated	3,758	4,453	$ 12,124	14,222
Selling, general and administrative	421,163	269,902	1,133,633	799,322
Depreciation and amortization	68,564	15,752	168,651	46,004
Acquisition and integration charges	$ 30,500	$ 16,557	$ 142,104	$ 32,958

Goodwill and Intangible Asset47

Goodwill and Intangible Assets - Changes to Carrying Amount of Goodwill by Reporting Unit (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill impairment	$ 0
Goodwill, accumulated impairment losses			$ 0
Goodwill [Roll Forward]
Balance, December 31, 2016		$ 738,453,000
Additions due to acquisitions		2,376,866,000
Foreign currency translation impact		29,727,000
Balance at September 30, 2017	3,145,046,000	3,145,046,000
Research
Goodwill [Roll Forward]
Balance, December 31, 2016		595,450,000
Additions due to acquisitions		1,717,750,000
Foreign currency translation impact		23,183,000
Balance at September 30, 2017	2,336,383,000	2,336,383,000
Consulting
Goodwill [Roll Forward]
Balance, December 31, 2016		96,480,000
Additions due to acquisitions		0
Foreign currency translation impact		1,375,000
Balance at September 30, 2017	97,855,000	97,855,000
Events
Goodwill [Roll Forward]
Balance, December 31, 2016		46,523,000
Additions due to acquisitions		123,423,000
Foreign currency translation impact		1,462,000
Balance at September 30, 2017	171,408,000	171,408,000
Talent Assessment & Other
Goodwill [Roll Forward]
Balance, December 31, 2016		0
Additions due to acquisitions		535,693,000
Foreign currency translation impact		3,707,000
Balance at September 30, 2017	$ 539,400,000	$ 539,400,000

Goodwill and Intangible Asset48

Goodwill and Intangible Assets - Carrying Amounts of Amortizable Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Finite-lived Intangible Assets [Roll Forward]
Gross cost, beginning balance			$ 116,767
Additions due to acquisitions			1,589,990
Write-off of fully amortized intangible assets			(4,227)
Foreign currency translation impact			19,843
Gross cost, ending balance	$ 1,722,373		1,722,373
Accumulated amortization	(157,908)		(157,908)		$ (39,966)
Finite-lived intangible assets, net	1,564,465		1,564,465		76,801
Amortization of intangibles	51,224	$ 6,221	123,014	$ 18,614
Customer Relationships
Finite-lived Intangible Assets [Roll Forward]
Gross cost, beginning balance			63,369
Additions due to acquisitions			1,251,000
Write-off of fully amortized intangible assets			0
Foreign currency translation impact			12,073
Gross cost, ending balance	1,326,442		1,326,442
Accumulated amortization	(75,173)		(75,173)		(16,744)
Finite-lived intangible assets, net	1,251,269		$ 1,251,269		46,625
Customer Relationships | Minimum
Finite-lived Intangible Assets [Roll Forward]
Finite-lived intangible assets useful life			4 years
Customer Relationships | Maximum
Finite-lived Intangible Assets [Roll Forward]
Finite-lived intangible assets useful life			13 years
Software
Finite-lived Intangible Assets [Roll Forward]
Gross cost, beginning balance			$ 16,025
Additions due to acquisitions			181,000
Write-off of fully amortized intangible assets			0
Foreign currency translation impact			1,194
Gross cost, ending balance	198,219		198,219
Accumulated amortization	(24,419)		(24,419)		(8,904)
Finite-lived intangible assets, net	173,800		$ 173,800		7,121
Software | Minimum
Finite-lived Intangible Assets [Roll Forward]
Finite-lived intangible assets useful life			3 years
Software | Maximum
Finite-lived Intangible Assets [Roll Forward]
Finite-lived intangible assets useful life			7 years
Content
Finite-lived Intangible Assets [Roll Forward]
Gross cost, beginning balance			$ 3,728
Additions due to acquisitions			143,500
Write-off of fully amortized intangible assets			(4,227)
Foreign currency translation impact			6,080
Gross cost, ending balance	149,081		149,081
Accumulated amortization	(36,884)		(36,884)		(2,033)
Finite-lived intangible assets, net	112,197		$ 112,197		1,695
Content | Minimum
Finite-lived Intangible Assets [Roll Forward]
Finite-lived intangible assets useful life			1 year 6 months
Content | Maximum
Finite-lived Intangible Assets [Roll Forward]
Finite-lived intangible assets useful life			5 years
Other
Finite-lived Intangible Assets [Roll Forward]
Gross cost, beginning balance			$ 33,645
Additions due to acquisitions			14,490
Write-off of fully amortized intangible assets			0
Foreign currency translation impact			496
Gross cost, ending balance	48,631		48,631
Accumulated amortization	(21,432)		(21,432)		(12,285)
Finite-lived intangible assets, net	$ 27,199		$ 27,199		$ 21,360
Other | Minimum
Finite-lived Intangible Assets [Roll Forward]
Finite-lived intangible assets useful life			2 years
Other | Maximum
Finite-lived Intangible Assets [Roll Forward]
Finite-lived intangible assets useful life			5 years

Goodwill and Intangible Asset49

Goodwill and Intangible Assets - Estimated Future Amortization Expense By Year From Amortizable Intangibles (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2017 (remaining three months)	$ 59,743
2,018	222,102
2,019	165,855
2,020	158,981
2,021	138,639
Thereafter	819,145
Finite-lived intangible assets, net	$ 1,564,465	$ 76,801

Debt (Details)

Debt (Details)	Apr. 05, 2017USD ($)	Mar. 30, 2017USD ($)	Mar. 20, 2017installment	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Sep. 30, 2017USD ($)amendment	Dec. 31, 2012USD ($)	Dec. 31, 2016USD ($)
Bank Term Loan - A Facility
Debt Instrument [Line Items]
Long-term debt, gross				$ 1,447,875,000		$ 1,447,875,000		$ 585,000,000
Bank Term Loan - B Facility
Debt Instrument [Line Items]
Long-term debt, gross				497,500,000		497,500,000		0
Senior Notes
Debt Instrument [Line Items]
Proceeds from issuance of debt		$ 800,000,000
Long-term debt, gross		$ 800,000,000		800,000,000		800,000,000		0
Debt instrument term		8 years
Debt instrument, fixed interest rate		5.125%
Connecticut Economic Development Program
Debt Instrument [Line Items]
Long-term debt, gross				2,500,000		2,500,000	$ 2,500,000	2,500,000
Debt instrument term							10 years
Debt instrument, fixed interest rate							3.00%
Revolving credit facility
Debt Instrument [Line Items]
Long-term debt, gross				640,000,000		$ 640,000,000		$ 115,000,000
Interest rate swaps
Debt Instrument [Line Items]
Derivative, term of contract						30 days
Interest rate, fair value hedge derivative at fair value, net (negative fair value)				8,000,000		$ 8,000,000
2016 Agreement
Debt Instrument [Line Items]
Number of debt agreement amendments | amendment						3
Debt instrument, interest, additional interest above eurodollar rate			1.00%
2016 Agreement | Federal Funds Rate
Debt Instrument [Line Items]
Debt instrument interest additional interest above federal fund rate			0.50%
Minimum applicable margin rate			0.125%
Maximum applicable margin rate			1.50%
2016 Agreement | Euro Dollar rate
Debt Instrument [Line Items]
Minimum applicable margin rate			1.125%
Maximum applicable margin rate			2.50%
2016 Agreement | Term loans
Debt Instrument [Line Items]
Line of credit facility, frequency of payments, number of quarterly installments | installment			16
2016 Agreement | Term loans | Bank Term Loan - A Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				1,500,000,000		$ 1,500,000,000
2016 Agreement | Revolving credit facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity				1,200,000,000		1,200,000,000
CEB
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 2,780,000,000
CEB | Senior Notes
Debt Instrument [Line Items]
Proceeds from issuance of debt	800,000,000	$ 800,000,000
Debt instrument, fixed interest rate		5.125%
Debt instrument, issue price, percent		100.00%
Debt instrument, redemption, prince, percentage of principal amount that may be redeemed		40.00%
CEB | Revolving credit facility
Debt Instrument [Line Items]
Proceeds from issuance of debt	275,000,000
CEB | Debt instrument, redemption with the proceeds of certain equity offerings | Senior Notes
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage		105.125%
CEB | Debt instrument, redemption excluding make whole premium | Senior Notes
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage		100.00%
CEB | Debt instrument, redemption due to specific kinds of change of control | Senior Notes
Debt Instrument [Line Items]
Debt instrument, redemption price, percentage		101.00%
CEB | 2016 Agreement
Debt Instrument [Line Items]
Proceeds from issuance of debt	1,675,000,000
CEB | 2016 Agreement | Term loans | Bank Term Loan - A Facility
Debt Instrument [Line Items]
Proceeds from issuance of debt	900,000,000
Line of credit facility, additional borrowing capacity	900,000,000
CEB | 2016 Agreement | Term loans | Bank Term Loan - B Facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	500,000,000			500,000,000		$ 500,000,000
Proceeds from issuance of debt	$ 500,000,000
CEB | Bank Term Loan - B Facility | Term loans | LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.00%
CEB | Bank Term Loan - B Facility | Term loans | Base Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.00%
Bridge loan | CEB
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 300,000,000
Debt instrument term	364 days				364 days
Repayments of short-term debt				$ 200,000,000	$ 100,000,000

Debt - Borrowings (Details)

Debt - Borrowings (Details) - USD ($)	Apr. 05, 2017	Mar. 30, 2017	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2012	Dec. 31, 2016
Debt Instrument [Line Items]
Principal amount outstanding			$ 3,387,875,000		$ 3,387,875,000			$ 702,500,000
Less: deferred financing fees			(46,045,000)		(46,045,000)			(8,109,000)
Net balance sheet carrying amount			$ 3,341,830,000		$ 3,341,830,000			694,391,000
Average annual effective rate			4.01%		3.73%
Payments of deferred financing fees					$ 51,171,000	$ 4,975,000
Write off of deferred finance fees					6,100,000
Bank Term Loan - A Facility
Debt Instrument [Line Items]
Long-term debt, gross			$ 1,447,875,000		$ 1,447,875,000			585,000,000
Contractual annualized interest rate			3.23%		3.23%
Floating eurodollar base rate			1.23%		1.23%
Debt instrument interest additional interest above base rate					2.00%
Bank Term Loan - B Facility
Debt Instrument [Line Items]
Long-term debt, gross			$ 497,500,000		$ 497,500,000			0
Contractual annualized interest rate			3.23%		3.23%
Floating eurodollar base rate			1.23%		1.23%
Debt instrument interest additional interest above base rate					2.00%
Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross		$ 800,000,000	$ 800,000,000		$ 800,000,000			0
Debt instrument term		8 years
Proceeds from issuance of debt		$ 800,000,000
Debt instrument, fixed interest rate		5.125%
Connecticut Economic Development Program
Debt Instrument [Line Items]
Long-term debt, gross			2,500,000		2,500,000		$ 2,500,000	2,500,000
Debt instrument term							10 years
Debt instrument, fixed interest rate							3.00%
Debt instrument, period principal payments are deferred							5 years
Revolving credit facility
Debt Instrument [Line Items]
Long-term debt, gross			$ 640,000,000		$ 640,000,000			$ 115,000,000
Contractual annualized interest rate			3.73%		3.73%
Floating eurodollar base rate			1.23%		1.23%
Debt instrument interest additional interest above base rate					2.50%
Line of credit facility, remaining borrowing capacity			$ 534,000,000		$ 534,000,000
CEB
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 2,780,000,000
CEB | Senior Notes
Debt Instrument [Line Items]
Proceeds from issuance of debt	800,000,000	$ 800,000,000
Debt instrument, fixed interest rate		5.125%
CEB | Revolving credit facility
Debt Instrument [Line Items]
Proceeds from issuance of debt	$ 275,000,000
CEB | Bridge loan
Debt Instrument [Line Items]
Debt instrument term	364 days			364 days
Proceeds from issuance of debt	$ 300,000,000
Repayments of short-term debt			$ 200,000,000	$ 100,000,000

Equity - Share Repurchase Progr

Equity - Share Repurchase Program and Share Repurchase Activity (Details) - USD ($)	Apr. 05, 2017	Jan. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Class of Stock [Line Items]
Stock repurchase program, authorized amount			$ 1,200,000,000		$ 1,200,000,000
Stock repurchase program, remaining authorized repurchase amount			$ 1,100,000,000		$ 1,100,000,000
Number of shares repurchased (in shares)			26,725	9,882	348,236	542,792
Cash paid for repurchased shares (in thousands)		$ 7,200,000	$ 3,275,000	$ 922,000	$ 37,188,000	$ 52,889,000
Treasury stock, average price paid per share (in dollars per share)			$ 122.53	$ 93.33	$ 106.79	$ 84.17
CEB
Class of Stock [Line Items]
Business acquisition, equity interest issued, number of shares (in shares)	7,400,000
Business combination, equity interest issued, per share fair value (in dollars per share)	$ 109.65

Equity - Changes in AOCL_I by C

Equity - Changes in AOCL/I by Component (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 60,878
Other comprehensive (loss) income, net of tax	$ 47,673	$ 219	48,467	$ (3,382)
Ending balance	863,410		863,410
Interest Rate Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(6,107)	(12,158)	(1,409)	(3,079)
Change in AOCL/I before reclassifications to income	9	2,076	(6,912)	(9,376)
Reclassifications from AOCL/I to income during the period	1,293	1,138	3,516	3,511
Other comprehensive (loss) income, net of tax	1,302	3,214	(3,396)	(5,865)
Ending balance	(4,805)	(8,944)	(4,805)	(8,944)
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(5,700)	(4,757)	(5,797)	(4,832)
Change in AOCL/I before reclassifications to income	0	0	0	0
Reclassifications from AOCL/I to income during the period	52	37	149	112
Other comprehensive (loss) income, net of tax	52	37	149	112
Ending balance	(5,648)	(4,720)	(5,648)	(4,720)
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(37,082)	(31,088)	(42,477)	(36,491)
Change in AOCL/I before reclassifications to income	46,319	(3,032)	51,714	2,371
Reclassifications from AOCL/I to income during the period	0	0	0	0
Other comprehensive (loss) income, net of tax	46,319	(3,032)	51,714	2,371
Ending balance	9,237	(34,120)	9,237	(34,120)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(48,889)	(48,003)	(49,683)	(44,402)
Change in AOCL/I before reclassifications to income	46,328	(956)	44,802	(7,005)
Reclassifications from AOCL/I to income during the period	1,345	1,175	3,665	3,623
Other comprehensive (loss) income, net of tax	47,673	219	48,467	(3,382)
Ending balance	$ (1,216)	$ (47,784)	$ (1,216)	$ (47,784)

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Effective income tax rate	22.20%	31.90%	33.40%	30.30%
(Benefit) provision for income taxes	$ (13,760)	$ 14,264	$ (52,166)	$ 55,149
(Loss) income before income taxes	(61,940)	$ 44,748	(156,194)	$ 182,246
Unrecognized tax benefits	72,200		72,200		$ 37,100
Decrease in unrecognized tax benefits is reasonably possible	$ 8,000		$ 8,000

Derivatives and Hedging - Outst

Derivatives and Hedging - Outstanding Derivatives Contracts (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017USD ($)outstanding_contract	Dec. 31, 2016USD ($)outstanding_contract
Derivatives, Fair Value [Line Items]
Number of Outstanding Contracts | outstanding_contract	34	87
Notional Amounts	$ 1,502,262	$ 786,946
Fair Value Asset (Liability), Net	(8,097)	(2,669)
Unrealized Loss Recorded in OCI	$ (4,805)	$ (1,409)
Designated as hedging instrument | Other liabilities | Interest rate swaps
Derivatives, Fair Value [Line Items]
Number of Outstanding Contracts | outstanding_contract	5	3
Notional Amounts	$ 1,400,000	$ 700,000
Fair Value Asset (Liability), Net	(8,009)	(2,349)
Unrealized Loss Recorded in OCI	$ (4,805)	$ (1,409)
Not Designated as Hedging Instrument | Accrued liabilities | Foreign currency forwards
Derivatives, Fair Value [Line Items]
Number of Outstanding Contracts | outstanding_contract	29	84
Notional Amounts	$ 102,262	$ 86,946
Fair Value Asset (Liability), Net	(88)	(320)
Unrealized Loss Recorded in OCI	$ 0	$ 0

Derivatives and Hedging - Amoun

Derivatives and Hedging - Amounts Recognized in the Condensed Consolidated Statements of Operations for Derivative Contracts (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Total expense, net	$ 390	$ 1,081	$ 4,900	$ 5,612
Interest expense, net
Derivative Instruments, Gain (Loss) [Line Items]
Total expense, net	2,154	1,895	5,860	5,851
Other (income), net
Derivative Instruments, Gain (Loss) [Line Items]
Total expense, net	$ (1,764)	$ (814)	$ (960)	$ (239)

Fair Value Disclosures - Assets

Fair Value Disclosures - Assets And Liabilities Measured At Fair Value On Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Assets:
Assets measured at fair value on a recurring basis		$ 43,592	$ 38,259
Fair Value, Inputs, Level 1 | Deferred compensation plan assets
Assets:
Assets measured at fair value on a recurring basis		12,478	10,247
Fair Value, Inputs, Level 2
Assets:
Assets measured at fair value on a recurring basis		31,114	28,012
Liabilities:
Liabilities measured at fair value on a recurring basis		901,402	45,909
Fair Value, Inputs, Level 2 | Foreign currency forward contracts
Assets:
Assets measured at fair value on a recurring basis		64	165
Liabilities:
Liabilities measured at fair value on a recurring basis		152	485
Fair Value, Inputs, Level 2 | Interest rate swap contracts
Liabilities:
Liabilities measured at fair value on a recurring basis		8,009	2,349
Fair Value, Inputs, Level 2 | Deferred compensation plan assets
Assets:
Assets measured at fair value on a recurring basis		31,050	27,847
Fair Value, Inputs, Level 2 | Deferred compensation plan liabilities
Liabilities:
Liabilities measured at fair value on a recurring basis		48,793	43,075
Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proceeds from issuance of debt	$ 800,000
Senior Notes | Fair Value, Inputs, Level 2 | Senior notes
Liabilities:
Liabilities measured at fair value on a recurring basis		$ 844,448	$ 0

Employee Benefits (Details)

Employee Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Retirement Benefits [Abstract]
Net periodic pension expense	$ 0.9	$ 0.9	$ 2.4	$ 2.4

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Thousands	Sep. 30, 2017USD ($)
Commitments and Contingencies Disclosure [Abstract]
Payment obligations under indemnification agreements	$ 0