F5 (FFIV) 10-Q 3 May 19 2019 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Mar. 31, 2019	Apr. 29, 2019
Document And Entity Information [Abstract]
Entity Registrant Name	F5 NETWORKS INC
Entity Central Index Key	0001048695
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Amendment Flag	false
Entity Common Stock, Shares Outstanding		59,695,236
Entity Emerging Growth Company	false
Entity Small Business	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Current assets
Cash and cash equivalents	$ 726,662	$ 424,707
Short-term investments	587,115	614,705
Accounts receivable, net of allowances of $2,326 and $2,040	321,484	295,352
Inventories	33,463	30,568
Other current assets	138,736	52,326
Total current assets	1,807,460	1,417,658
Property and equipment, net	208,221	145,042
Long-term investments	301,357	411,184
Deferred tax assets	21,551	33,441
Goodwill	555,965	555,965
Other assets, net	95,682	42,186
Total assets	2,990,236	2,605,476
Current liabilities
Accounts payable	68,156	57,757
Accrued liabilities	197,295	180,979
Deferred revenue	809,336	715,697
Total current liabilities	1,074,787	954,433
Other long-term liabilities	95,530	65,892
Deferred revenue, long-term	352,109	299,624
Deferred tax liabilities	402	35
Total long-term liabilities	448,041	365,551
Commitments and contingencies (Note 6)
Shareholders’ equity
Preferred stock, no par value; 10,000 shares authorized, no shares outstanding	0	0
Common stock, no par value; 200,000 shares authorized, 59,695 and 60,215 shares issued and outstanding	29,401	20,427
Accumulated other comprehensive loss	(19,341)	(22,178)
Retained earnings	1,457,348	1,287,243
Total shareholders’ equity	1,467,408	1,285,492
Total liabilities and shareholders’ equity	$ 2,990,236	$ 2,605,476

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Statement of Financial Position [Abstract]
Accounts receivable, allowances	$ 2,326	$ 2,040
Preferred stock, par value (dollars per share)	$ 0	$ 0
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares outstanding	0	0
Common stock, par value (dollars per share)	$ 0	$ 0
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	59,695,000	60,215,000
Common stock, shares outstanding	59,695,000	60,215,000

Consolidated Income Statements

Consolidated Income Statements - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Net revenues
Net revenues	$ 544,895	$ 533,304	$ 1,088,665	$ 1,056,495
Cost of net revenues
Cost of net revenues	88,178	89,645	174,892	177,032
Gross profit	456,717	443,659	913,773	879,463
Operating expenses
Sales and marketing	170,954	169,970	335,213	337,904
Research and development	96,314	91,056	188,352	176,945
General and administrative	46,656	39,276	89,199	79,260
Total	313,924	300,302	612,764	594,109
Income from operations	142,793	143,357	301,009	285,354
Other income, net	7,434	2,790	14,529	4,935
Income before income taxes	150,227	146,147	315,538	290,289
Income Tax Expense (Benefit)	34,140	36,511	68,546	92,224
Net income	$ 116,087	$ 109,636	$ 246,992	$ 198,065
Net income per share — basic (dollars per share)	$ 1.94	$ 1.79	$ 4.12	$ 3.20
Weighted average shares — basic (shares)	59,686	61,420	59,954	61,812
Net income per share — diluted (dollars per share)	$ 1.93	$ 1.77	$ 4.09	$ 3.18
Weighted average shares — diluted (shares)	60,029	62,059	60,374	62,351
Product [Member]
Net revenues
Net revenues	$ 237,859	$ 237,558	$ 471,736	$ 464,861
Cost of net revenues
Cost of net revenues	43,547	44,127	85,957	87,392
Service [Member]
Net revenues
Net revenues	307,036	295,746	616,929	591,634
Cost of net revenues
Cost of net revenues	$ 44,631	$ 45,518	$ 88,935	$ 89,640

Consolidated Statements Of Comp

Consolidated Statements Of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 116,087	$ 109,636	$ 246,992	$ 198,065
Other comprehensive income (loss):
Foreign currency translation adjustment	(197)	384	(56)	545
Available-for-sale securities:
Unrealized gains (losses) on securities, net of taxes of $679 and $(612) for the three months ended March 31, 2019 and 2018, respectively, and $743 and $(1,186) for the six months ended March 31, 2019 and 2018, respectively	2,849	(1,886)	2,893	(3,536)
Reclassification adjustment for realized losses included in net income, net of taxes of $0 and $0 for the three months ended March 31, 2019 and 2018, respectively, and $0 and $(3) for the six months ended March 31, 2019 and 2018, respectively	0	0	0	8
Net change in unrealized gains (losses) on available-for-sale securities, net of tax	2,849	(1,886)	2,893	(3,528)
Total other comprehensive income (loss)	2,652	(1,502)	2,837	(2,983)
Comprehensive income	$ 118,739	$ 108,134	$ 249,829	$ 195,082

Consolidated Statements Of Co_2

Consolidated Statements Of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Tax effect of unrealized gain (loss) on securities	$ 679	$ (612)	$ 743	$ (1,186)
Tax effect of reclassification adjustment for realized (gains) losses	$ 0	$ 0	$ 0	$ (3)

Consolidated Statements Of Cash

Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Cash Flows [Abstract]
Other Significant Noncash Transaction, Value of Consideration Received	$ 28,814	$ 0
Operating activities
Net income	246,992	198,065
Adjustments to reconcile net income to net cash provided by operating activities:
Realized loss on disposition of assets and investments	47	52
Stock-based compensation	78,183	82,268
Provisions for doubtful accounts and sales returns	(90)	691
Depreciation and amortization	28,246	30,049
Deferred income taxes	3,606	17,642
Changes in operating assets and liabilities:
Accounts receivable	(24,419)	314
Inventories	(2,895)	322
Other current assets	(35,735)	5,493
Other assets	2,683	(1,111)
Accounts payable and accrued liabilities	16,746	(5,308)
Deferred revenue	78,046	46,235
Net cash provided by operating activities	391,410	374,712
Investing activities
Purchases of investments	(211,087)	(353,414)
Maturities of investments	351,600	186,961
Sales of investments	2,499	9,248
Cash provided by sale of fixed asset	0	1,000
Purchases of property and equipment	(50,056)	(16,246)
Net cash provided by (used in) investing activities	92,956	(172,451)
Financing activities
Proceeds from the exercise of stock options and purchases of stock under employee stock purchase plan	18,900	19,917
Repurchase of common stock	(201,045)	(300,046)
Net cash used in financing activities	(182,145)	(280,129)
Net increase (decrease) in cash, cash equivalents and restricted cash	302,221	(77,868)
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(265)	576
Cash, cash equivalents and restricted cash, beginning of period	425,894	$ 674,452
Cash, cash equivalents and restricted cash, end of period	$ 726,662

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity Statement - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]
Common stock, shares outstanding	62,594,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,229,392	$ 17,627	$ 1,229,762	$ (17,997)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	1,000
Stock Issued During Period, Value, Stock Options Exercised	$ 3	3
Stock Issued During Period, Shares, Employee Stock Purchase Plans	193,000
Stock Issued During Period, Value, Employee Stock Purchase Plan	$ 19,915	19,915
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures	635,000
Stock Repurchased and Retired During Period, Shares	(2,308,000)
Stock Repurchased and Retired During Period, Value	$ (300,046)	(98,693)	(201,353)
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures	82,268	82,268
Net income	198,065		198,065
Other Comprehensive Income (Loss), Net of Tax	$ (2,983)			(2,983)
Common stock, shares outstanding	61,844,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,227,178	20,029	1,226,627	(19,478)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	1,000
Stock Issued During Period, Value, Stock Options Exercised		3
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures	335,000
Stock Repurchased and Retired During Period, Shares	(1,065,000)
Stock Repurchased and Retired During Period, Value	$ (150,021)	(40,232)	(109,789)
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures	41,320	41,320
Net income	109,636		109,636
Other Comprehensive Income (Loss), Net of Tax	$ (1,502)			(1,502)
Common stock, shares outstanding	61,115,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,226,614	21,120	1,226,474	(20,980)
Common stock, shares outstanding	60,215,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,285,492	20,427	1,287,243	(22,178)
Stock Issued During Period, Shares, Employee Stock Purchase Plans	135,000
Stock Issued During Period, Value, Employee Stock Purchase Plan	$ 18,901	18,901
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures	531,000
Stock Repurchased and Retired During Period, Shares	(1,185,881)
Stock Repurchased and Retired During Period, Value	$ (201,045)	(88,110)	(112,935)
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures	78,183	78,183
Net income	246,992		246,992
Other Comprehensive Income (Loss), Net of Tax	$ 2,837			2,837
Common stock, shares outstanding	60,058,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,409,188	13,277	1,417,904	(21,993)
Stock Issued During Period, Shares, Restricted Stock Award, Net of Forfeitures	254,000
Stock Repurchased and Retired During Period, Shares	(617,000)
Stock Repurchased and Retired During Period, Value	$ (100,013)	(23,370)	(76,643)
Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures	39,494	39,494
Net income	116,087		116,087
Other Comprehensive Income (Loss), Net of Tax	$ 2,652			2,652
Common stock, shares outstanding	59,695,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,467,408	$ 29,401	$ 1,457,348	$ (19,341)

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Description of Business F5 Networks, Inc. (the “Company”) is the leading developer and provider of application services. The Company’s core technology is a full-proxy, programmable, highly-scalable software platform called TMOS, which supports a broad array of features and functions designed to ensure that applications delivered over Internet Protocol (IP) networks are secure, fast and available. The Company’s offerings include software products for local and global traffic management, network and application security, identity and access management, web acceleration and a number of other network and application services. F5 offerings are available via software that can run individually or as part of an integrated solution on the Company’s high-performance, scalable, purpose-built BIG-IP appliances and VIPRION chassis-based hardware, as software-only Virtual Editions, or as software-based services available through a number of leading cloud marketplaces. The Company also offers distributed denial-of-service ( DDoS) protection, application security and other application services by subscription on its cloud-based Silverline managed service platform. In connection with its products, the Company offers a broad range of support and services including consulting, training, installation and maintenance. Pending Merger On March 9, 2019, the Company announced a definitive agreement under which it will acquire all issued and outstanding shares of Nginx, Inc. (“NGINX”), a privately held company headquartered in San Francisco, California, for a total enterprise value of approximately $670 million , subject to certain adjustments. The merger is expected to close later in the second calendar quarter of 2019. Basis of Presentation The year-end consolidated balance sheet data was derived from audited financial statements, but does not include all disclosures required by accounting principles generally accepted in the United States of America. In the opinion of management, the unaudited consolidated financial statements reflect all adjustments, consisting only of normal recurring adjustments, necessary for their fair statement in conformity with accounting principles generally accepted in the United States of America. Certain information and footnote disclosures normally included in annual financial statements have been condensed or omitted in accordance with the rules and regulations of the Securities and Exchange Commission. The information included in this Form 10-Q should be read in conjunction with Management’s Discussion and Analysis of Financial Condition and Results of Operations and financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the fiscal year ended September 30, 2018 . There have been no material changes to our significant accounting policies as of and for the three and six months ended March 31, 2019 , except for the accounting policies for revenue recognition and deferred contract costs that were updated as a result of adopting Accounting Standards Update 2014-09, Revenue from Contracts with Customers (Topic 606) (ASU 2014-09). For more information, refer to the "Revenue Recognition" and "Recently Adopted Accounting Standards" sections of Note 1 and Note 2 - Revenue from Contracts with Customers. Revenue Recognition On October 1, 2018, the Company adopted the new revenue recognition standard by applying the modified retrospective approach to those contracts which were not completed as of October 1, 2018. Results for reporting periods beginning after October 1, 2018 are presented under the new revenue recognition standard, while prior period amounts are not adjusted and continue to be reported under the accounting standards in effect for the prior periods. The Company sells products through distributors, resellers, and directly to end users. Revenue related to the Company's contracts with customers is recognized by following a five-step process: • Identify the contract(s) with a customer. Evidence of a contract generally consists of a purchase order issued pursuant to the terms and conditions of a distributor, reseller or end user agreement. • Identify the performance obligations in the contract. Performance obligations are explicitly identified in the Company's contracts and include hardware-based software, software-only solutions, cloud-based subscription services as well as a broad range of service performance obligations including consulting, training, installation and maintenance. • Determine the transaction price. The purchase price stated in an agreed upon purchase order is generally representative of the transaction price. The Company offers several programs in which customers are eligible for certain levels of rebates if certain conditions are met. When determining the transaction price, the Company considers the effects of any variable consideration. • Allocate the transaction price to the performance obligations in the contract. The transaction price in a contract is allocated based upon the relative standalone selling price of each distinct performance obligation identified in the contract. • Recognize revenue when (or as) the entity satisfies a performance obligation. The Company satisfies performance obligations either over time or at a point in time as discussed in further detail below. Revenue is recognized at the time the related performance obligation is satisfied by transferring control of promised products and services to a customer. The following is a description of the principal activities from which the Company generates revenue: Product Revenue from the sale of the Company's hardware and perpetual software products is generally recognized at a point in time when the product has been delivered and the customer is obligated to pay for the product. When rights of return are present and the Company cannot estimate returns, revenue is recognized when such rights of return lapse. Payment terms to domestic customers are generally net 30 days to net 45 days. Payment terms to international customers range from net 30 days to net 120 days based on normal and customary trade practices in the individual markets. Subscription The Company also offers several products by subscription, either through term-based license agreements or as a service through its cloud-based Silverline platform. Revenue for term-based license agreements is recognized at a point in time, when the Company delivers the software license to the customer and the subscription term has commenced. For the Company's software-as-a-service Silverline offerings, revenue is recognized ratably as the services are provided. Support and professional services Revenues for post-contract customer support (PCS) are recognized on a straight-line basis over the service contract term. PCS includes a limited period of telephone support, updates, repair or replacement of any failed product or component that fails during the term of the agreement, bug fixes and rights to upgrades, when and if available. Consulting services are customarily billed at fixed hourly rates, plus out-of-pocket expenses, and revenues are recognized as the consulting is completed. Similarly, training revenue is recognized as the training is completed. Contract acquisition costs Sales commissions earned by the Company's sales force are considered incremental and recoverable costs of obtaining a contract with a customer. Sales commissions for initial service contracts and subscription offerings are deferred and then amortized as an expense on a straight-line basis over the period of benefit which management has determined to be 4.5 years and 3 years , respectively. Significant Judgments The Company’s contracts with customers often include promises to transfer multiple products and services. Determining whether products and services are considered distinct performance obligations that should be accounted for separately versus together may require significant judgment. Judgment is also required to determine the standalone selling price (SSP) for each distinct performance obligation. The Company sells post-contract customer support (PCS), subscriptions and professional services on a standalone basis and can therefore use a population of historical standalone sales to determine fair value. For distinct performance obligations that the Company does not generally sell on a standalone basis (hardware and perpetual software), a combination of the adjusted market assessment approach and the expected cost plus a margin approach are used to estimate the SSP. Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments with original maturities of three months or less to be cash equivalents. The Company invests its cash and cash equivalents in deposits with five major financial institutions, which, at times, may exceed federally insured limits. The Company has not experienced any losses on its cash and cash equivalents. Amounts included in restricted cash represent those required to be set aside by a contractual agreement. The following table provides a reconciliation of the Company’s cash and cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total cash, cash equivalents and restricted cash shown in the Company’s consolidated statements of cash flows for the periods presented (in thousands): March 31, September 30, 2019 2018 Cash and cash equivalents $ 726,662 $ 424,707 Restricted cash included in other assets, net 1,188 1,187 Total cash, cash equivalents and restricted cash $ 727,850 $ 425,894 Goodwill and Acquired Intangible Assets Goodwill represents the excess purchase price over the estimated fair value of net assets acquired as of the acquisition date. The Company tests goodwill for impairment on an annual basis and between annual tests when impairment indicators are identified, and goodwill is written down when impaired. Goodwill was recorded in connection with various acquisitions in prior years. For its annual goodwill impairment test in all periods to date, the Company has operated under one reporting unit and the fair value of its reporting unit has been determined by the Company’s enterprise value. The Company performs its annual goodwill impairment test during the second fiscal quarter. As part of the annual goodwill impairment test, the Company has the option to perform a qualitative assessment to determine whether further impairment testing is necessary. Examples of events and circumstances that might indicate that the reporting unit’s fair value is less than its carrying amount include macro-economic conditions such as deterioration in the entity’s operating environment or industry or market considerations; entity-specific events such as increasing costs, declining financial performance, or loss of key personnel; or other events such as a sustained decrease in the stock price on either an absolute basis or relative to peers. If, as a result of its qualitative assessment, it is more-likely-than-not (i.e. greater than 50% chance) that the fair value of the Company’s reporting unit is less than its carrying amount, the quantitative impairment test will be required. Otherwise, no further testing will be required. For its annual impairment test performed in the second quarter of fiscal 2019, the Company completed a quantitative assessment and determined that there was no impairment of goodwill. The Company's intangible assets subject to amortization are amortized using the straight-line method over their estimated useful lives, ranging from three to ten years. The Company evaluates the recoverability of intangible assets periodically by taking into account events or circumstances that may warrant revised estimates of useful lives or that indicate the asset may be impaired. The Company considered potential impairment indicators of acquired intangible assets at March 31, 2019 and noted no indicators of impairment. Software Development Costs The authoritative guidance requires certain internal software development costs related to software to be sold to be capitalized upon the establishment of technological feasibility. The Company's software development costs incurred subsequent to achieving technological feasibility have not been significant and, as a result all software development costs have been expensed as research and development activities as incurred. Internal Use Software In accordance with the authoritative guidance, the Company capitalizes application development stage costs associated with the development of internal-use software and software developed related to its SaaS-based product offerings. The capitalized costs are then amortized over the estimated useful life of the software, which is generally three to five years, and are included in property and equipment in the accompanying consolidated balance sheets. Stock-Based Compensation The Company accounts for stock-based compensation using the straight-line attribution method for recognizing compensation expense. The Company recognized $39.5 million and $41.3 million of stock-based compensation expense for the three months ended March 31, 2019 and 2018 , respectively and $78.2 million and $82.3 million of stock-based compensation expense for the six months ended March 31, 2019 and 2018 , respectively. As of March 31, 2019 , there was $174.5 million of total unrecognized stock-based compensation cost, the majority of which will be recognized over the next two years. Going forward, stock-based compensation expenses may increase as the Company issues additional equity-based awards to continue to attract and retain key employees. The Company issues incentive awards to its employees through stock-based compensation consisting of restricted stock units (RSUs). On October 31, 2018 , the Company’s Board of Directors and Compensation Committee approved 774,313 RSUs to employees and executive officers pursuant to the Company’s annual equity awards program. The value of RSUs is determined using the fair value method, which in this case, is based on the number of shares granted and the quoted price of the Company’s common stock on the date of grant. The Company recognizes compensation expense for only the portion of restricted stock units that are expected to vest. Therefore, the Company applies estimated forfeiture rates that are derived from historical employee termination behavior. Based on historical differences with forfeitures of stock-based awards granted to the Company’s executive officers and Board of Directors versus grants awarded to all other employees, the Company has developed separate forfeiture expectations for these two groups. In determining the fair value of shares issued under the Employee Stock Purchase Plan (ESPP), the Company uses the Black-Scholes option pricing model. Compensation expense related to the shares issued pursuant to the ESPP is recognized on a straight-line basis over the offering period. The Company issues incentive awards to certain current executive officers as part of its annual equity awards program. Fifty percent of the aggregate number of RSUs issued to executive officers vest in equal quarterly increments, and 50% are subject to the Company achieving specified performance goals. For performance stock awards granted prior to fiscal 2018, attainment is based on the Company achieving specific quarterly revenue and EBITDA targets. In each case, 70% of the quarterly performance stock grant is based on achieving at least 80% of the quarterly revenue goal set by the Company's Board of Directors, and the other 30% is based on achieving at least 80% of the quarterly EBITDA goal set by the Company's Board of Directors. The quarterly performance stock grant is paid linearly over 80% of the targeted goals. At least 100% of both goals must be attained in order for the quarterly performance stock grant to be awarded over 100% . Each goal is evaluated individually and subject to the 80% achievement threshold and the 100% over-achievement threshold. Each goal is also capped at achievement of 200% above target. For the fiscal 2018 and 2019 performance stock awards, the Company's Compensation Committee adopted a new set of metrics that are differentiated from the quarterly revenue and EBITDA measures, including (1) 50% of the annual performance stock grant is based on achieving 80% of the annual revenue goal set by the Company’s Board of Directors; (2) 25% of the annual performance stock grant is based on achieving at least a 18% increase in annual software revenue compared to the prior year; and (3) 25% of the annual performance stock grant is based on relative total shareholder return benchmarked to the S&P 500 index. In each case, no vesting or payment with respect to a performance goal shall occur unless a minimum threshold is met for the applicable goal. Vesting and payment with respect to the performance goal is linear above the threshold of the applicable goal and is capped at achievement of 200% above target. As of March 31, 2019 , the following annual equity grants for executive officers or a portion thereof are outstanding: Grant Date RSUs Granted Vesting Schedule Vesting Period Date Fully Vested November 1, 2018 144,066 Quarterly, Annually 1 3 years November 1, 2021 November 1, 2017 140,135 Quarterly, Annually 1 4 years November 1, 2021 November 1, 2016 115,347 Quarterly 4 years November 1, 2020 November 2, 2015 145,508 Quarterly 4 years November 1, 2019 (1) 50% of the annual equity grant vests in equal quarterly increments and 50% is subject to the Company achieving specified annual performance goals. The Company recognizes compensation costs for awards with performance conditions when it concludes it is probable that the performance condition will be achieved. The Company reassesses the probability of vesting at each balance sheet date and adjusts compensation costs based on the probability assessment. Common Stock Repurchase On October 31, 2018 , the Company announced that its Board of Directors authorized an additional $1.0 billion for its common stock share repurchase program. This new authorization is incremental to the existing $4.4 billion program, initially approved in October 2010 and expanded in each fiscal year. Acquisitions for the share repurchase programs will be made from time to time in private transactions or open market purchases as permitted by securities laws and other legal requirements. For the six months ended March 31, 2019 , the Company repurchased and retired 1,185,881 shares at an average price of $169.53 per share and the Company had $1.4 billion remaining authorized to purchase shares at March 31, 2019 . In the second quarter of fiscal 2019, the Company announced that, in connection with its acquisition of NGINX, it is suspending its common stock share repurchase program. Management will continue to evaluate market conditions and other factors including the Company’s capital requirements in determining when and whether to continue such program and the levels of such program. The program does not require the purchase of any minimum number of shares and the program may be modified, suspended or discontinued at any time. Earnings Per Share Basic net income per share is computed by dividing net income by the weighted average number of common shares outstanding during the period. Diluted net income per share is computed by dividing net income by the weighted average number of common and dilutive common stock equivalent shares outstanding during the period. The Company’s nonvested restricted stock awards and restricted stock units do not have nonforfeitable rights to dividends or dividend equivalents and are not considered participating securities that should be included in the computation of earnings per share under the two-class method. The following table sets forth the computation of basic and diluted net income per share (in thousands, except per share data): Three months ended Six months ended 2019 2018 2019 2018 Numerator Net income $ 116,087 $ 109,636 $ 246,992 $ 198,065 Denominator Weighted average shares outstanding — basic 59,686 61,420 59,954 61,812 Dilutive effect of common shares from stock options and restricted stock units 343 639 420 539 Weighted average shares outstanding — diluted 60,029 62,059 60,374 62,351 Basic net income per share $ 1.94 $ 1.79 $ 4.12 $ 3.20 Diluted net income per share $ 1.93 $ 1.77 $ 4.09 $ 3.18 Anti-dilutive stock-based awards excluded from the calculations of diluted earnings per share were immaterial for the three and six months ended March 31, 2019 and 2018. Comprehensive Income Comprehensive income includes certain changes in equity that are excluded from net income. Specifically, unrealized gains or losses on securities and foreign currency translation adjustments are included in accumulated other comprehensive income or loss. Recently Adopted Accounting Standards In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) (ASU 2014-09). ASU 2014-09 and the related amendments outline a new, single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. The new model requires revenue recognition to depict the transfer of promised goods or services to customers in an amount that reflects the consideration a company expects to receive in exchange for those goods or services. The Company adopted this new accounting standard and the related amendments on October 1, 2018 using the modified retrospective method. See Note 2 - Revenue from Contracts with Customers for further details. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (ASU 2016-15), which clarifies how companies present and classify certain cash receipts and cash payments in the statement of cash flows. The Company adopted this new accounting standard on October 1, 2018. The adoption did not have an impact on the Company's condensed consolidated financial statements. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (ASU 2016-18), which requires a company’s cash flow statement to explain the changes during a reporting period of the totals for cash, cash equivalents, restricted cash and restricted cash equivalents. Additionally, amounts for restricted cash and restricted cash equivalents are to be included with cash and cash equivalents if the cash flow statement includes a reconciliation of the total cash balances for a reporting period. The Company adopted this new accounting standard on October 1, 2018, which resulted in an immaterial reclassification of beginning and ending cash, cash equivalents and restricted cash for the periods presented within the statement of cash flows. Upon adoption of this new standard, restricted cash activity is no longer separately presented within the investing activities in the statement of cash flows. The overall adoption of the standard did not have a material impact to the consolidated financial statements. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business (ASU 2017-01), which provides a more robust framework to use in determining when a set of assets and activities is considered a business. The Company adopted this new accounting standard on October 1, 2018. The adoption did not have an impact on the Company's condensed consolidated financial statements. In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (ASU 2017-09), which provides clarification on when modification accounting should be used for changes to the terms or conditions of a share-based payment award. This ASU does not change the accounting for modifications but clarifies that modification accounting guidance should only be applied if there is a change to the value, vesting conditions or award classification and would not be required if the changes are considered non-substantive. The Company adopted this new accounting standard on October 1, 2018. The adoption did not have an impact on the Company's condensed consolidated financial statements. Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) (ASU 2016-02), which requires lessees to recognize on the balance sheet a right-of-use asset, representing its right to use the underlying asset for the lease term, and a corresponding lease liability for all leases with terms greater than twelve months. The Company’s leases consist of operating leases for its offices and lab spaces. The guidance also requires qualitative and quantitative disclosures designed to assess the amount, timing and uncertainty of cash flows arising from leases. Adoption of the new lease standard requires measurement of leases at the beginning of the earliest period presented on a modified retrospective basis. The new standard will be effective for the Company beginning October 1, 2019. The Company anticipates that its long-term leases for office space will be recognized as lease liabilities and corresponding right-of-use assets, and will have a material impact on its consolidated balance sheets upon adoption. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842) (ASU 2018-11), which provides an optional transition method to adopt ASU 2016-02, Leases (Topic 842) by allowing lessees to apply the new standard at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. Comparative periods will still be presented under current GAAP (ASC 840), along with the applicable Topic 840 disclosures for those comparative periods. The new standard is effective at the same time as adoption of ASU 2016-02, Leases (Topic 842), which the Company is planning to adopt in the first quarter of fiscal year 2020. The Company plans to adopt the transitional provisions allowed under ASU 2018-11. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (ASU 2016-13), which replaces the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company does not anticipate that the adoption of this standard will have a material impact on its consolidated financial statements. In August 2018, the FASB issued ASU 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40) (ASU 2018-15), which aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software, and hosting arrangements that include an internal use software license. The accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendments in this update. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company does not anticipate that the adoption of this standard will have a material impact on its consolidated financial statements.

Revenue from Contracts with Cus

Revenue from Contracts with Customers (Notes)	6 Months Ended
	Mar. 31, 2019
Revenue from Contract with Customer [Abstract]
Revenue from Contracts with Customers	Revenue from Contracts with Customers The following table summarizes the impact of adopting ASC 606 on the Company's consolidated balance sheet as of the date of adoption, October 1, 2018. This table does not represent the full consolidated balance sheet as it only reflects the accounts impacted by the adoption of ASC 606. Ending Balance as of September 30, 2018 (ASC 605) ASC 606 Adjustments Beginning Balance as of October 1, 2018 (ASC 606) Assets Other current assets 1 $ 52,326 $ 50,558 $ 102,884 Other assets, net 1 $ 42,186 $ 59,676 $ 101,862 Deferred tax assets $ 33,441 $ (7,902 ) $ 25,539 Liabilities and Shareholders' Equity Deferred revenue $ 715,697 $ 35,464 $ 751,161 Deferred revenue, long-term $ 299,624 $ 32,614 $ 332,238 Retained earnings 2 $ 1,287,243 $ 36,048 $ 1,323,291 (1) Upon the adoption of ASC 606, contract assets and unbilled accounts receivable are reported as part of other current assets and other assets, net. (2) The net increase to retained earnings of $36.0 million was primarily related to the capitalization of contract acquisition costs of $54.6 million , partially offset by a decrease of $8.8 million due to changes in deferred revenue and a decrease of $7.9 million from the impact on deferred income taxes. The adoption of ASC 606 had an immaterial impact to the Company's consolidated income statements and statements of cash flows for the three and six months ended March 31, 2019 . Capitalized Contract Acquisition Costs The table below shows significant movements in capitalized contract acquisition costs (current and noncurrent) for the six months ended March 31, 2019 (in thousands): Balance, September 30, 2018 $ — Impacts from adoption of ASC 606 $ 54,608 Additional capitalized contract acquisition costs deferred $ 11,287 Amortization of capitalized contract acquisition costs $ (14,444 ) Balance, March 31, 2019 $ 51,451 Amortization of capitalized contract acquisition costs is recorded in Sales and Marketing expense in the accompanying consolidated income statements. There was no impairment loss in relation to costs capitalized. Remaining Performance Obligations Remaining performance obligations represent the amount of the transaction price under contracts with customers that are attributable to performance obligations that are unsatisfied or partially satisfied at the reporting date. The Company expects to recognize this amount as revenue over the following time periods (in thousands): 2019 2020 Thereafter Total Revenue expected to be recognized on remaining performance obligations $ 525,006 $ 402,034 $ 234,405 $ 1,161,445 Contract Balances Timing may differ between the satisfaction of performance obligations and the invoicing and collection of amounts related to our contracts with customers. We record assets for amounts related to performance obligations that are satisfied but not yet billed and/or collected, in addition to contracts that have started, but not yet been fully billed. These assets are recorded as contract assets rather than receivables when receipt of the consideration is conditional on something other than the passage of time. Liabilities are recorded for amounts that are collected in advance of the satisfaction of performance obligations. These liabilities are classified as current and non-current deferred revenue. The table below shows significant movements in contract assets (current and noncurrent) for the six months ended March 31, 2019 (in thousands): Balance, September 30, 2018 $ — Impacts from adoption of ASC 606 $ 57,499 Revenue recognized during period but not yet billed $ 3,733 Contract asset net additions $ 29,900 Contract assets reclassified to accounts receivable $ (16,854 ) Balance, March 31, 2019 $ 74,278 As of December 31, 2018, contract assets that are expected to be reclassified to receivables within the next 12 months are included in other current assets, with those expected to be transferred to receivables in more than 12 months included in other assets. There were no impairments of contract assets during the six months ended March 31, 2019 . The impacts to deferred revenue from the adoption of ASC 606 are primarily related to unbilled accounts receivable now being capitalized and included as contract assets on the balance sheet. The table below shows significant movements in the deferred revenue balances (current and noncurrent) for the six months ended March 31, 2019 (in thousands): Balance, September 30, 2018 $ 1,015,321 Impacts from adoption of ASC 606 $ 68,078 Amounts billed but not recognized as revenues $ 567,681 Revenues recognized related to the opening balance of deferred revenue $ (489,635 ) Balance, March 31, 2019 $ 1,161,445 Our contract assets and liabilities are reported in a net position on a contract by contract basis at the end of each reporting period. See Note 8, Geographic Sales and Significant Customers, for disaggregated revenue by significant customer and geographic region.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements In accordance with the authoritative guidance on fair value measurements and disclosure under GAAP, the Company determines fair value using a fair value hierarchy that distinguishes between market participant assumptions developed based on market data obtained from sources independent of the reporting entity, and the reporting entity’s own assumptions about market participant assumptions developed based on the best information available in the circumstances and expands disclosure about fair value measurements. Fair value is the price that would be received to sell an asset or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants at the measurement date, essentially the exit price. The levels of fair value hierarchy are: Level 1: Quoted prices in active markets for identical assets and liabilities at the measurement date that the Company has the ability to access. Level 2: Observable inputs other than quoted prices included in Level 1, such as quoted prices for similar assets and liabilities in active markets; quoted prices for identical or similar assets and liabilities in markets that are not active; or other inputs that are observable or can be corroborated by observable market data. Level 3: Unobservable inputs for which there is little or no market data available. These inputs reflect management’s assumptions of what market participants would use in pricing the asset or liability. Level 1 investments are valued based on quoted market prices in active markets and include the Company’s cash equivalent investments. Level 2 investments, which include investments that are valued based on quoted prices in markets that are not active, broker or dealer quotations, actual trade data, benchmark yields or alternative pricing sources with reasonable levels of price transparency, include the Company’s certificates of deposit, corporate bonds and notes, municipal bonds and notes, U.S. government securities, U.S. government agency securities and international government securities. Fair values for the Company’s level 2 investments are based on similar assets without applying significant judgments. In addition, all of the Company’s level 2 investments have a sufficient level of trading volume to demonstrate that the fair values used are appropriate for these investments. A financial instrument’s level within the fair value hierarchy is based upon the lowest level of any input that is significant to the fair value measurement. However, the determination of what constitutes “observable” requires significant judgment by the Company. The Company considers observable data to be market data which is readily available, regularly distributed or updated, reliable and verifiable, not proprietary, and provided by independent sources that are actively involved in the relevant market. The Company’s financial assets measured at fair value on a recurring basis subject to the disclosure requirements at March 31, 2019 , were as follows (in thousands): Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Securities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Fair Value at March 31, Cash equivalents $ 234,666 $ 179,687 $ — $ 414,353 Short-term investments Available-for-sale securities — certificates of deposits — 2,228 — 2,228 Available-for-sale securities — corporate bonds and notes — 462,459 — 462,459 Available-for-sale securities — municipal bonds and notes — 21,712 — 21,712 Available-for-sale securities — U.S. government securities — 71,757 — 71,757 Available-for-sale securities — U.S. government agency securities — 28,959 — 28,959 Long-term investments Available-for-sale securities — corporate bonds and notes — 277,421 — 277,421 Available-for-sale securities — municipal bonds and notes — 14,491 — 14,491 Available-for-sale securities — U.S. government securities — 2,967 — 2,967 Available-for-sale securities — U.S. government agency securities — 6,478 — 6,478 Total $ 234,666 $ 1,068,159 $ — $ 1,302,825 The Company’s financial assets measured at fair value on a recurring basis subject to the disclosure requirements at September 30, 2018 , were as follows (in thousands): Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Securities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Fair Value at September 30, 2018 Cash equivalents $ 41,468 $ 13,118 $ — $ 54,586 Short-term investments Available-for-sale securities — certificates of deposits — 2,970 — 2,970 Available-for-sale securities — corporate bonds and notes — 393,750 — 393,750 Available-for-sale securities — municipal bonds and notes — 22,524 — 22,524 Available-for-sale securities — U.S. government securities — 120,078 — 120,078 Available-for-sale securities — U.S. government agency securities — 75,383 — 75,383 Long-term investments Available-for-sale securities — corporate bonds and notes — 367,710 — 367,710 Available-for-sale securities — municipal bonds and notes — 24,286 — 24,286 Available-for-sale securities — U.S. government securities — 12,771 — 12,771 Available-for-sale securities — U.S. government agency securities — 6,417 — 6,417 Total $ 41,468 $ 1,039,007 $ — $ 1,080,475 The Company uses the fair value hierarchy for financial assets and liabilities. The Company’s non-financial assets and liabilities, which include goodwill, intangible assets, and long-lived assets, are not required to be carried at fair value on a recurring basis. These non-financial assets and liabilities are measured at fair value on a non-recurring basis when there is an indicator of impairment, and they are recorded at fair value only when impairment is recognized. The Company reviews goodwill and intangible assets for impairment annually, during the second quarter of each fiscal year, or as circumstances indicate the possibility of impairment. The Company monitors the carrying value of long-lived assets for impairment whenever events or changes in circumstances indicate its carrying amount may not be recoverable. During the three and six months ended March 31, 2019 and 2018, the Company did not recognize any impairment charges related to goodwill, intangible assets, or long-lived assets. The carrying amounts of other current financial assets and other current financial liabilities approximate fair value due to their short-term nature.

Short-Term and Long-Term Invest

Short-Term and Long-Term Investments	6 Months Ended
	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Short-Term and Long-Term Investments	Short-Term and Long-Term Investments Short-term investments consist of the following (in thousands): March 31, 2019 Cost or Cost Gross Gains Gross Losses Fair Value Certificates of deposits $ 2,228 $ — $ — $ 2,228 Corporate bonds and notes 463,014 134 (689 ) 462,459 Municipal bonds and notes 21,761 2 (51 ) 21,712 U.S. government securities 71,827 15 (85 ) 71,757 U.S. government agency securities 29,009 — (50 ) 28,959 $ 587,839 $ 151 $ (875 ) $ 587,115 September 30, 2018 Cost or Cost Gross Gains Gross Losses Fair Value Certificates of deposits $ 2,970 $ — $ — $ 2,970 Corporate bonds and notes 394,684 9 (943 ) 393,750 Municipal bonds and notes 22,588 1 (65 ) 22,524 U.S. government securities 120,283 — (205 ) 120,078 U.S. government agency securities 75,587 — (204 ) 75,383 $ 616,112 $ 10 $ (1,417 ) $ 614,705 Long-term investments consist of the following (in thousands): March 31, 2019 Cost or Cost Gross Gains Gross Losses Fair Value Corporate bonds and notes $ 277,526 $ 633 $ (738 ) $ 277,421 Municipal bonds and notes 14,491 23 (23 ) 14,491 U.S. government securities 2,997 — (30 ) 2,967 U.S. government agency securities 6,500 1 (23 ) 6,478 $ 301,514 $ 657 $ (814 ) $ 301,357 September 30, 2018 Cost or Cost Gross Gains Gross Losses Fair Value Corporate bonds and notes $ 370,377 $ 25 $ (2,692 ) $ 367,710 Municipal bonds and notes 24,468 — (182 ) 24,286 U.S. government securities 12,956 — (185 ) 12,771 U.S. government agency securities 6,500 — (83 ) 6,417 $ 414,301 $ 25 $ (3,142 ) $ 411,184 The following table summarizes investments that have been in a continuous unrealized loss position for less than 12 months and those that have been in a continuous unrealized loss position for more than 12 months as of March 31, 2019 (in thousands): Less Than 12 Months 12 Months or Greater Total March 31, 2019 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Corporate bonds and notes $ 248,518 $ (168 ) $ 245,916 $ (1,259 ) $ 494,434 $ (1,427 ) Municipal bonds and notes 6,004 (1 ) 18,419 (73 ) 24,423 (74 ) U.S. government securities 28,007 (6 ) 20,863 (109 ) 48,870 (115 ) U.S. government agency securities 9,124 (2 ) 15,428 (71 ) 24,552 (73 ) Total $ 291,653 $ (177 ) $ 300,626 $ (1,512 ) $ 592,279 $ (1,689 ) The following table summarizes investments that have been in a continuous unrealized loss position for less than 12 months and those that have been in a continuous unrealized loss position for more than 12 months as of September 30, 2018 (in thousands): Less Than 12 Months 12 Months or Greater Total September 30, 2018 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Corporate bonds and notes $ 543,729 $ (1,800 ) $ 152,097 $ (1,835 ) $ 695,826 $ (3,635 ) Municipal bonds and notes 26,846 (123 ) 14,363 (124 ) 41,209 (247 ) U.S. government securities 103,470 (281 ) 29,379 (109 ) 132,849 (390 ) U.S. government agency securities 44,812 (110 ) 36,987 (177 ) 81,799 (287 ) Total $ 718,857 $ (2,314 ) $ 232,826 $ (2,245 ) $ 951,683 $ (4,559 ) The Company invests in securities that are rated investment grade or better. The Company reviews the individual securities in its portfolio to determine whether a decline in a security's fair value below the amortized cost basis is other-than-temporary. The Company determined that as of March 31, 2019 , there were no investments in its portfolio that were other-than-temporarily impaired.

Inventories

Inventories	6 Months Ended
	Mar. 31, 2019
Inventory Disclosure [Abstract]
Inventories	Inventories The Company outsources the manufacturing of its pre-configured hardware platforms to contract manufacturers, who assemble each product to the Company’s specifications. As protection against component shortages and to provide replacement parts for its service teams, the Company also stocks limited supplies of certain key product components. The Company reduces inventory to net realizable value based on excess and obsolete inventories determined primarily by historical usage and forecasted demand. Inventories consist of hardware and related component parts and are recorded at the lower of cost and net realizable value (as determined by the first-in, first-out method). Inventories consist of the following (in thousands): March 31, September 30, Finished goods $ 22,305 $ 21,339 Raw materials 11,158 9,229 $ 33,463 $ 30,568

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Guarantees and Product Warranties In the normal course of business to facilitate sales of its products, the Company indemnifies other parties, including customers, resellers, lessors, and parties to other transactions with the Company, with respect to certain matters. The Company has agreed to hold the other party harmless against losses arising from a breach of representations or covenants, or out of intellectual property infringement or other claims made against certain parties. These agreements may limit the time within which an indemnification claim can be made and the amount of the claim. The Company has entered into indemnification agreements with its officers and directors and certain other employees, and the Company’s bylaws contain similar indemnification obligations to the Company’s agents. It is not possible to determine the maximum potential amount under these indemnification agreements due to the limited history of prior indemnification claims and the unique facts and circumstances involved in each particular agreement. The Company generally offers warranties of one year for hardware for those customers without service contracts, with the option of purchasing additional warranty coverage in yearly increments. The Company accrues for warranty costs as part of its cost of sales based on associated material product costs and technical support labor costs. Accrued warranty costs as of March 31, 2019 and September 30, 2018 were not considered material. Commitments As of March 31, 2019 , the Company’s principal commitments consisted of obligations outstanding under operating leases. The Company leases its facilities under operating leases that expire at various dates through 2033. There have been no material changes in the Company's lease obligations compared to those discussed in Note 7 to its annual consolidated financial statements. The Company currently has arrangements with contract manufacturers and other suppliers for the manufacturing of its products. The arrangement with the primary contract manufacturer allows them to procure component inventory on the Company’s behalf based on a rolling production forecast provided by the Company. The Company is obligated to the purchase of component inventory that the contract manufacturer procures in accordance with the forecast, unless it gives notice of order cancellation in advance of applicable lead times. There have been no material changes in the Company's inventory purchase obligations compared to those discussed in Note 7 to its annual consolidated financial statements. Legal Proceedings On April 4, 2016, the Company sued Radware, Inc. in the United States District Court for the Western District of Washington (the case was subsequently moved to the Northern District of California) accusing Radware of infringing three Company patents. The Company’s complaint seeks a jury trial and an unspecified amount of monetary damages, as well as interest, costs, and injunctive relief. Radware moved to dismiss the allegations of one patent, but the motion was denied. Radware filed inter partes review (IPR) petitions on all of the asserted Company patents, but the US Patent Office dismissed all of the petitions. Radware also filed a counterclaim asserting that the Company infringed one of its now-expired patents. The Company filed an IPR petition against Radware’s counterclaim patent that resulted in cancellation of all but four of the patent’s claims by the Patent Office. The Court then held that F5 does not infringe the four remaining claims. Accordingly, the case will proceed solely on F5's infringement claims against Radware. The Court has entered a schedule for trial in February of 2020. In addition to the above referenced matters, the Company is subject to other legal proceedings, claims, and litigation arising in the ordinary course of business. Management believes that the Company has meritorious defenses to the allegations made in its pending cases and intends to vigorously defend these lawsuits; however, the Company is unable currently to determine the ultimate outcome of these or similar matters or the potential exposure to loss, if any. There are many uncertainties associated with any litigation and these actions or other third-party claims against the Company may cause it to incur costly litigation and/or substantial settlement charges that could have a material adverse effect on the Company's business, financial condition, results of operations, and cash flows. The Company records an accrual for loss contingencies for legal proceedings when it believes that an unfavorable outcome is both (a) probable and (b) the amount or range of any possible loss is reasonably estimable. The Company has not recorded an accrual for loss contingencies associated with the legal proceedings or the investigations discussed above.

Income Taxes

Income Taxes	6 Months Ended
	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The effective tax rate was 22.7% and 21.7% for the three and six months ended March 31, 2019 , respectively, compared to 25.0% and 31.8% for the three and six months ended March 31, 2018 , respectively. The effective tax rate for the three and six months ended March 31, 2019 and March 31, 2018 includes various impacts from the Tax Cuts and Jobs Act enacted on December 22, 2017. Significant impacts for the three and six months ended March 31, 2019 include a further reduction in the U.S. federal income tax rate to 21%, a deduction for foreign derived intangible income, a tax on global intangible low taxed income, and repeal of the deduction for income attributable to domestic production activities. The six months ended March 31, 2018 included one-time charges for the deemed repatriation of undistributed foreign earnings, and for the remeasurement of the Company’s net deferred tax assets to reflect the change in the U.S. federal income tax rate when temporary differences are expected to reverse. At March 31, 2019 , the Company had $35.2 million of unrecognized tax benefits that, if recognized, would affect the effective tax rate. It is anticipated that the Company’s existing liabilities for unrecognized tax benefits will change within the next twelve months due to audit settlements or the expiration of statutes of limitations. The Company does not expect these changes to be material to the consolidated financial statements. The Company recognizes interest and, if applicable, penalties for any uncertain tax positions as a component of income tax expense. The Company and its subsidiaries are subject to U.S. federal income tax as well as the income tax of multiple state and foreign jurisdictions. The Company has concluded all U.S. federal income tax matters for fiscal years through September 30, 2014. Major jurisdictions where there are wholly owned subsidiaries of F5 Networks, Inc. which require income tax filings include the United Kingdom, Japan, Singapore, Australia, and Israel. The earliest periods open for review by local taxing authorities are fiscal years 2017 for the United Kingdom, 2013 for Japan, 2014 for Singapore, 2014 for Australia, and 2013 for Israel. The Company is currently under audit by the IRS for fiscal year 2016, by various states for fiscal years 2014 through 2017, and by Israel for fiscal years 2013 to 2015. Within the next four fiscal quarters, the statute of limitations will begin to close on the fiscal year 2015 federal income tax return, fiscal years 2014 and 2015 state income tax returns, and fiscal years 2013 to 2017 foreign income tax returns.

Geographic Sales and Significan

Geographic Sales and Significant Customers	6 Months Ended
	Mar. 31, 2019
Segment Reporting [Abstract]
Geographic Sales and Significant Customers	Geographic Sales and Significant Customers Operating segments are defined as components of an enterprise for which separate financial information is available and evaluated regularly by the chief operating decision-maker, or decision-making group, in deciding how to allocate resources and in assessing performance. Management has determined that the Company is organized as, and operates in, one reportable operating segment: the development, marketing and sale of application services that optimize the security, performance and availability of network applications, servers and storage systems. The Company does business in three main geographic regions: the Americas (primarily the United States); Europe, the Middle East, and Africa (EMEA); and the Asia Pacific region (APAC). The Company’s chief operating decision-maker reviews financial information presented on a consolidated basis accompanied by information about revenues by geographic region. The Company’s foreign offices conduct sales, marketing and support activities. Revenues are attributed by geographic location based on the location of the customer. The following presents revenues by geographic region (in thousands): Three months ended Six months ended 2019 2018 2019 2018 Americas: United States $ 282,037 $ 266,440 $ 546,788 $ 530,583 Other 25,375 29,538 52,268 56,906 Total Americas 307,412 295,978 599,056 587,489 EMEA 136,549 136,903 285,533 275,891 Asia Pacific 1 100,934 100,423 204,076 193,115 $ 544,895 $ 533,304 $ 1,088,665 $ 1,056,495 (1) Beginning with the first quarter of fiscal 2019, revenue from Japan is now included with the APAC region. This change has been applied to all periods presented for comparability purposes. The following distributors of the Company's products accounted for more than 10% of total net revenue: Three months ended Six months ended 2019 2018 2019 2018 Ingram Micro, Inc. 20.0 % 16.6 % 18.4 % 16.0 % Tech Data 10.0 % 12.0 % — 12.2 % Arrow ECS 10.3 % 10.7 % 10.6 % 11.1 % Synnex Corporation — 11.2 % — 11.1 % Westcon Group, Inc. — 10.8 % 10.5 % 10.5 % The Company tracks assets by physical location. Long-lived assets consist of property and equipment, net, and are shown below (in thousands): March 31, September 30, United States $ 183,987 $ 126,790 EMEA 12,460 12,538 Other countries 11,774 5,714 $ 208,221 $ 145,042

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policy)	6 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy [Policy Text Block]	Cash, Cash Equivalents and Restricted Cash The Company considers all highly liquid investments with original maturities of three months or less to be cash equivalents. The Company invests its cash and cash equivalents in deposits with five major financial institutions, which, at times, may exceed federally insured limits. The Company has not experienced any losses on its cash and cash equivalents. Amounts included in restricted cash represent those required to be set aside by a contractual agreement.
Description of Business	Description of Business F5 Networks, Inc. (the “Company”) is the leading developer and provider of application services. The Company’s core technology is a full-proxy, programmable, highly-scalable software platform called TMOS, which supports a broad array of features and functions designed to ensure that applications delivered over Internet Protocol (IP) networks are secure, fast and available. The Company’s offerings include software products for local and global traffic management, network and application security, identity and access management, web acceleration and a number of other network and application services. F5 offerings are available via software that can run individually or as part of an integrated solution on the Company’s high-performance, scalable, purpose-built BIG-IP appliances and VIPRION chassis-based hardware, as software-only Virtual Editions, or as software-based services available through a number of leading cloud marketplaces. The Company also offers distributed denial-of-service ( DDoS) protection, application security and other application services by subscription on its cloud-based Silverline managed service platform. In connection with its products, the Company offers a broad range of support and services including consulting, training, installation and maintenance. Pending Merger On March 9, 2019, the Company announced a definitive agreement under which it will acquire all issued and outstanding shares of Nginx, Inc. (“NGINX”), a privately held company headquartered in San Francisco, California, for a total enterprise value of approximately $670 million , subject to certain adjustments. The merger is expected to close later in the second calendar quarter of 2019.
Basis of Presentation	Basis of Presentation The year-end consolidated balance sheet data was derived from audited financial statements, but does not include all disclosures required by accounting principles generally accepted in the United States of America. In the opinion of management, the unaudited consolidated financial statements reflect all adjustments, consisting only of normal recurring adjustments, necessary for their fair statement in conformity with accounting principles generally accepted in the United States of America. Certain information and footnote disclosures normally included in annual financial statements have been condensed or omitted in accordance with the rules and regulations of the Securities and Exchange Commission. The information included in this Form 10-Q should be read in conjunction with Management’s Discussion and Analysis of Financial Condition and Results of Operations and financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the fiscal year ended September 30, 2018 . There have been no material changes to our significant accounting policies as of and for the three and six months ended March 31, 2019 , except for the accounting policies for revenue recognition and deferred contract costs that were updated as a result of adopting Accounting Standards Update 2014-09, Revenue from Contracts with Customers (Topic 606) (ASU 2014-09). For more information, refer to the "Revenue Recognition" and "Recently Adopted Accounting Standards" sections of Note 1 and Note 2 - Revenue from Contracts with Customers.
Revenue Recognition	Revenue Recognition On October 1, 2018, the Company adopted the new revenue recognition standard by applying the modified retrospective approach to those contracts which were not completed as of October 1, 2018. Results for reporting periods beginning after October 1, 2018 are presented under the new revenue recognition standard, while prior period amounts are not adjusted and continue to be reported under the accounting standards in effect for the prior periods. The Company sells products through distributors, resellers, and directly to end users. Revenue related to the Company's contracts with customers is recognized by following a five-step process: • Identify the contract(s) with a customer. Evidence of a contract generally consists of a purchase order issued pursuant to the terms and conditions of a distributor, reseller or end user agreement. • Identify the performance obligations in the contract. Performance obligations are explicitly identified in the Company's contracts and include hardware-based software, software-only solutions, cloud-based subscription services as well as a broad range of service performance obligations including consulting, training, installation and maintenance. • Determine the transaction price. The purchase price stated in an agreed upon purchase order is generally representative of the transaction price. The Company offers several programs in which customers are eligible for certain levels of rebates if certain conditions are met. When determining the transaction price, the Company considers the effects of any variable consideration. • Allocate the transaction price to the performance obligations in the contract. The transaction price in a contract is allocated based upon the relative standalone selling price of each distinct performance obligation identified in the contract. • Recognize revenue when (or as) the entity satisfies a performance obligation. The Company satisfies performance obligations either over time or at a point in time as discussed in further detail below. Revenue is recognized at the time the related performance obligation is satisfied by transferring control of promised products and services to a customer. The following is a description of the principal activities from which the Company generates revenue: Product Revenue from the sale of the Company's hardware and perpetual software products is generally recognized at a point in time when the product has been delivered and the customer is obligated to pay for the product. When rights of return are present and the Company cannot estimate returns, revenue is recognized when such rights of return lapse. Payment terms to domestic customers are generally net 30 days to net 45 days. Payment terms to international customers range from net 30 days to net 120 days based on normal and customary trade practices in the individual markets. Subscription The Company also offers several products by subscription, either through term-based license agreements or as a service through its cloud-based Silverline platform. Revenue for term-based license agreements is recognized at a point in time, when the Company delivers the software license to the customer and the subscription term has commenced. For the Company's software-as-a-service Silverline offerings, revenue is recognized ratably as the services are provided. Support and professional services Revenues for post-contract customer support (PCS) are recognized on a straight-line basis over the service contract term. PCS includes a limited period of telephone support, updates, repair or replacement of any failed product or component that fails during the term of the agreement, bug fixes and rights to upgrades, when and if available. Consulting services are customarily billed at fixed hourly rates, plus out-of-pocket expenses, and revenues are recognized as the consulting is completed. Similarly, training revenue is recognized as the training is completed. Contract acquisition costs Sales commissions earned by the Company's sales force are considered incremental and recoverable costs of obtaining a contract with a customer. Sales commissions for initial service contracts and subscription offerings are deferred and then amortized as an expense on a straight-line basis over the period of benefit which management has determined to be 4.5 years and 3 years , respectively. Significant Judgments The Company’s contracts with customers often include promises to transfer multiple products and services. Determining whether products and services are considered distinct performance obligations that should be accounted for separately versus together may require significant judgment. Judgment is also required to determine the standalone selling price (SSP) for each distinct performance obligation. The Company sells post-contract customer support (PCS), subscriptions and professional services on a standalone basis and can therefore use a population of historical standalone sales to determine fair value. For distinct performance obligations that the Company does not generally sell on a standalone basis (hardware and perpetual software), a combination of the adjusted market assessment approach and the expected cost plus a margin approach are used to estimate the SSP.
Goodwill and Acquired Intangible Assets	Goodwill and Acquired Intangible Assets Goodwill represents the excess purchase price over the estimated fair value of net assets acquired as of the acquisition date. The Company tests goodwill for impairment on an annual basis and between annual tests when impairment indicators are identified, and goodwill is written down when impaired. Goodwill was recorded in connection with various acquisitions in prior years. For its annual goodwill impairment test in all periods to date, the Company has operated under one reporting unit and the fair value of its reporting unit has been determined by the Company’s enterprise value. The Company performs its annual goodwill impairment test during the second fiscal quarter. As part of the annual goodwill impairment test, the Company has the option to perform a qualitative assessment to determine whether further impairment testing is necessary. Examples of events and circumstances that might indicate that the reporting unit’s fair value is less than its carrying amount include macro-economic conditions such as deterioration in the entity’s operating environment or industry or market considerations; entity-specific events such as increasing costs, declining financial performance, or loss of key personnel; or other events such as a sustained decrease in the stock price on either an absolute basis or relative to peers. If, as a result of its qualitative assessment, it is more-likely-than-not (i.e. greater than 50% chance) that the fair value of the Company’s reporting unit is less than its carrying amount, the quantitative impairment test will be required. Otherwise, no further testing will be required. For its annual impairment test performed in the second quarter of fiscal 2019, the Company completed a quantitative assessment and determined that there was no impairment of goodwill. The Company's intangible assets subject to amortization are amortized using the straight-line method over their estimated useful lives, ranging from three to ten years. The Company evaluates the recoverability of intangible assets periodically by taking into account events or circumstances that may warrant revised estimates of useful lives or that indicate the asset may be impaired. The Company considered potential impairment indicators of acquired intangible assets at March 31, 2019 and noted no indicators of impairment.
Software Development Costs	Software Development Costs The authoritative guidance requires certain internal software development costs related to software to be sold to be capitalized upon the establishment of technological feasibility. The Company's software development costs incurred subsequent to achieving technological feasibility have not been significant and, as a result all software development costs have been expensed as research and development activities as incurred.
Internal Use Software	Internal Use Software In accordance with the authoritative guidance, the Company capitalizes application development stage costs associated with the development of internal-use software and software developed related to its SaaS-based product offerings. The capitalized costs are then amortized over the estimated useful life of the software, which is generally three to five years, and are included in property and equipment in the accompanying consolidated balance sheets.
Stock-Based Compensation	Stock-Based Compensation The Company accounts for stock-based compensation using the straight-line attribution method for recognizing compensation expense. The Company recognized $39.5 million and $41.3 million of stock-based compensation expense for the three months ended March 31, 2019 and 2018 , respectively and $78.2 million and $82.3 million of stock-based compensation expense for the six months ended March 31, 2019 and 2018 , respectively. As of March 31, 2019 , there was $174.5 million of total unrecognized stock-based compensation cost, the majority of which will be recognized over the next two years. Going forward, stock-based compensation expenses may increase as the Company issues additional equity-based awards to continue to attract and retain key employees. The Company issues incentive awards to its employees through stock-based compensation consisting of restricted stock units (RSUs). On October 31, 2018 , the Company’s Board of Directors and Compensation Committee approved 774,313 RSUs to employees and executive officers pursuant to the Company’s annual equity awards program. The value of RSUs is determined using the fair value method, which in this case, is based on the number of shares granted and the quoted price of the Company’s common stock on the date of grant. The Company recognizes compensation expense for only the portion of restricted stock units that are expected to vest. Therefore, the Company applies estimated forfeiture rates that are derived from historical employee termination behavior. Based on historical differences with forfeitures of stock-based awards granted to the Company’s executive officers and Board of Directors versus grants awarded to all other employees, the Company has developed separate forfeiture expectations for these two groups. In determining the fair value of shares issued under the Employee Stock Purchase Plan (ESPP), the Company uses the Black-Scholes option pricing model. Compensation expense related to the shares issued pursuant to the ESPP is recognized on a straight-line basis over the offering period. The Company issues incentive awards to certain current executive officers as part of its annual equity awards program. Fifty percent of the aggregate number of RSUs issued to executive officers vest in equal quarterly increments, and 50% are subject to the Company achieving specified performance goals. For performance stock awards granted prior to fiscal 2018, attainment is based on the Company achieving specific quarterly revenue and EBITDA targets. In each case, 70% of the quarterly performance stock grant is based on achieving at least 80% of the quarterly revenue goal set by the Company's Board of Directors, and the other 30% is based on achieving at least 80% of the quarterly EBITDA goal set by the Company's Board of Directors. The quarterly performance stock grant is paid linearly over 80% of the targeted goals. At least 100% of both goals must be attained in order for the quarterly performance stock grant to be awarded over 100% . Each goal is evaluated individually and subject to the 80% achievement threshold and the 100% over-achievement threshold. Each goal is also capped at achievement of 200% above target. For the fiscal 2018 and 2019 performance stock awards, the Company's Compensation Committee adopted a new set of metrics that are differentiated from the quarterly revenue and EBITDA measures, including (1) 50% of the annual performance stock grant is based on achieving 80% of the annual revenue goal set by the Company’s Board of Directors; (2) 25% of the annual performance stock grant is based on achieving at least a 18% increase in annual software revenue compared to the prior year; and (3) 25% of the annual performance stock grant is based on relative total shareholder return benchmarked to the S&P 500 index. In each case, no vesting or payment with respect to a performance goal shall occur unless a minimum threshold is met for the applicable goal. Vesting and payment with respect to the performance goal is linear above the threshold of the applicable goal and is capped at achievement of 200% above target. As of March 31, 2019 , the following annual equity grants for executive officers or a portion thereof are outstanding: Grant Date RSUs Granted Vesting Schedule Vesting Period Date Fully Vested November 1, 2018 144,066 Quarterly, Annually 1 3 years November 1, 2021 November 1, 2017 140,135 Quarterly, Annually 1 4 years November 1, 2021 November 1, 2016 115,347 Quarterly 4 years November 1, 2020 November 2, 2015 145,508 Quarterly 4 years November 1, 2019 (1) 50% of the annual equity grant vests in equal quarterly increments and 50% is subject to the Company achieving specified annual performance goals. The Company recognizes compensation costs for awards with performance conditions when it concludes it is probable that the performance condition will be achieved. The Company reassesses the probability of vesting at each balance sheet date and adjusts compensation costs based on the probability assessment.
Common Stock Repurchase	Common Stock Repurchase On October 31, 2018 , the Company announced that its Board of Directors authorized an additional $1.0 billion for its common stock share repurchase program. This new authorization is incremental to the existing $4.4 billion program, initially approved in October 2010 and expanded in each fiscal year. Acquisitions for the share repurchase programs will be made from time to time in private transactions or open market purchases as permitted by securities laws and other legal requirements. For the six months ended March 31, 2019 , the Company repurchased and retired 1,185,881 shares at an average price of $169.53 per share and the Company had $1.4 billion remaining authorized to purchase shares at March 31, 2019 . In the second quarter of fiscal 2019, the Company announced that, in connection with its acquisition of NGINX, it is suspending its common stock share repurchase program. Management will continue to evaluate market conditions and other factors including the Company’s capital requirements in determining when and whether to continue such program and the levels of such program. The program does not require the purchase of any minimum number of shares and the program may be modified, suspended or discontinued at any time.
Earnings Per Share	Earnings Per Share Basic net income per share is computed by dividing net income by the weighted average number of common shares outstanding during the period. Diluted net income per share is computed by dividing net income by the weighted average number of common and dilutive common stock equivalent shares outstanding during the period. The Company’s nonvested restricted stock awards and restricted stock units do not have nonforfeitable rights to dividends or dividend equivalents and are not considered participating securities that should be included in the computation of earnings per share under the two-class method.
Comprehensive Income	Comprehensive Income Comprehensive income includes certain changes in equity that are excluded from net income. Specifically, unrealized gains or losses on securities and foreign currency translation adjustments are included in accumulated other comprehensive income or loss.
Recent Accounting Pronouncements	Recently Adopted Accounting Standards In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606) (ASU 2014-09). ASU 2014-09 and the related amendments outline a new, single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. The new model requires revenue recognition to depict the transfer of promised goods or services to customers in an amount that reflects the consideration a company expects to receive in exchange for those goods or services. The Company adopted this new accounting standard and the related amendments on October 1, 2018 using the modified retrospective method. See Note 2 - Revenue from Contracts with Customers for further details. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (ASU 2016-15), which clarifies how companies present and classify certain cash receipts and cash payments in the statement of cash flows. The Company adopted this new accounting standard on October 1, 2018. The adoption did not have an impact on the Company's condensed consolidated financial statements. In November 2016, the FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230): Restricted Cash (ASU 2016-18), which requires a company’s cash flow statement to explain the changes during a reporting period of the totals for cash, cash equivalents, restricted cash and restricted cash equivalents. Additionally, amounts for restricted cash and restricted cash equivalents are to be included with cash and cash equivalents if the cash flow statement includes a reconciliation of the total cash balances for a reporting period. The Company adopted this new accounting standard on October 1, 2018, which resulted in an immaterial reclassification of beginning and ending cash, cash equivalents and restricted cash for the periods presented within the statement of cash flows. Upon adoption of this new standard, restricted cash activity is no longer separately presented within the investing activities in the statement of cash flows. The overall adoption of the standard did not have a material impact to the consolidated financial statements. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business (ASU 2017-01), which provides a more robust framework to use in determining when a set of assets and activities is considered a business. The Company adopted this new accounting standard on October 1, 2018. The adoption did not have an impact on the Company's condensed consolidated financial statements. In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (ASU 2017-09), which provides clarification on when modification accounting should be used for changes to the terms or conditions of a share-based payment award. This ASU does not change the accounting for modifications but clarifies that modification accounting guidance should only be applied if there is a change to the value, vesting conditions or award classification and would not be required if the changes are considered non-substantive. The Company adopted this new accounting standard on October 1, 2018. The adoption did not have an impact on the Company's condensed consolidated financial statements. Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) (ASU 2016-02), which requires lessees to recognize on the balance sheet a right-of-use asset, representing its right to use the underlying asset for the lease term, and a corresponding lease liability for all leases with terms greater than twelve months. The Company’s leases consist of operating leases for its offices and lab spaces. The guidance also requires qualitative and quantitative disclosures designed to assess the amount, timing and uncertainty of cash flows arising from leases. Adoption of the new lease standard requires measurement of leases at the beginning of the earliest period presented on a modified retrospective basis. The new standard will be effective for the Company beginning October 1, 2019. The Company anticipates that its long-term leases for office space will be recognized as lease liabilities and corresponding right-of-use assets, and will have a material impact on its consolidated balance sheets upon adoption. In July 2018, the FASB issued ASU 2018-11, Leases (Topic 842) (ASU 2018-11), which provides an optional transition method to adopt ASU 2016-02, Leases (Topic 842) by allowing lessees to apply the new standard at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. Comparative periods will still be presented under current GAAP (ASC 840), along with the applicable Topic 840 disclosures for those comparative periods. The new standard is effective at the same time as adoption of ASU 2016-02, Leases (Topic 842), which the Company is planning to adopt in the first quarter of fiscal year 2020. The Company plans to adopt the transitional provisions allowed under ASU 2018-11. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (ASU 2016-13), which replaces the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company does not anticipate that the adoption of this standard will have a material impact on its consolidated financial statements. In August 2018, the FASB issued ASU 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40) (ASU 2018-15), which aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software, and hosting arrangements that include an internal use software license. The accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendments in this update. The new standard is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company does not anticipate that the adoption of this standard will have a material impact on its consolidated financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Schedule of Cash and Cash Equivalents	The following table provides a reconciliation of the Company’s cash and cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total cash, cash equivalents and restricted cash shown in the Company’s consolidated statements of cash flows for the periods presented (in thousands): March 31, September 30, 2019 2018 Cash and cash equivalents $ 726,662 $ 424,707 Restricted cash included in other assets, net 1,188 1,187 Total cash, cash equivalents and restricted cash $ 727,850 $ 425,894
Restrictions on Cash and Cash Equivalents	The following table provides a reconciliation of the Company’s cash and cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total cash, cash equivalents and restricted cash shown in the Company’s consolidated statements of cash flows for the periods presented (in thousands): March 31, September 30, 2019 2018 Cash and cash equivalents $ 726,662 $ 424,707 Restricted cash included in other assets, net 1,188 1,187 Total cash, cash equivalents and restricted cash $ 727,850 $ 425,894
Schedule of Nonvested Restricted Stock Units Activity	As of March 31, 2019 , the following annual equity grants for executive officers or a portion thereof are outstanding: Grant Date RSUs Granted Vesting Schedule Vesting Period Date Fully Vested November 1, 2018 144,066 Quarterly, Annually 1 3 years November 1, 2021 November 1, 2017 140,135 Quarterly, Annually 1 4 years November 1, 2021 November 1, 2016 115,347 Quarterly 4 years November 1, 2020 November 2, 2015 145,508 Quarterly 4 years November 1, 2019 (1) 50% of the annual equity grant vests in equal quarterly increments and 50% is subject to the Company achieving specified annual performance goals.
Schedule of Computation of Basic and Diluted Net Income Per Share	The following table sets forth the computation of basic and diluted net income per share (in thousands, except per share data): Three months ended Six months ended 2019 2018 2019 2018 Numerator Net income $ 116,087 $ 109,636 $ 246,992 $ 198,065 Denominator Weighted average shares outstanding — basic 59,686 61,420 59,954 61,812 Dilutive effect of common shares from stock options and restricted stock units 343 639 420 539 Weighted average shares outstanding — diluted 60,029 62,059 60,374 62,351 Basic net income per share $ 1.94 $ 1.79 $ 4.12 $ 3.20 Diluted net income per share $ 1.93 $ 1.77 $ 4.09 $ 3.18

Revenue from Contracts with C_2

Revenue from Contracts with Customers (Tables)	6 Months Ended
	Mar. 31, 2019
Revenue from Contract with Customer [Abstract]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	The following table summarizes the impact of adopting ASC 606 on the Company's consolidated balance sheet as of the date of adoption, October 1, 2018. This table does not represent the full consolidated balance sheet as it only reflects the accounts impacted by the adoption of ASC 606. Ending Balance as of September 30, 2018 (ASC 605) ASC 606 Adjustments Beginning Balance as of October 1, 2018 (ASC 606) Assets Other current assets 1 $ 52,326 $ 50,558 $ 102,884 Other assets, net 1 $ 42,186 $ 59,676 $ 101,862 Deferred tax assets $ 33,441 $ (7,902 ) $ 25,539 Liabilities and Shareholders' Equity Deferred revenue $ 715,697 $ 35,464 $ 751,161 Deferred revenue, long-term $ 299,624 $ 32,614 $ 332,238 Retained earnings 2 $ 1,287,243 $ 36,048 $ 1,323,291 (1) Upon the adoption of ASC 606, contract assets and unbilled accounts receivable are reported as part of other current assets and other assets, net. (2) The net increase to retained earnings of $36.0 million was primarily related to the capitalization of contract acquisition costs of $54.6 million , partially offset by a decrease of $8.8 million due to changes in deferred revenue and a decrease of $7.9 million from the impact on deferred income taxes.
Capitalized Contract Cost	The table below shows significant movements in capitalized contract acquisition costs (current and noncurrent) for the six months ended March 31, 2019 (in thousands): Balance, September 30, 2018 $ — Impacts from adoption of ASC 606 $ 54,608 Additional capitalized contract acquisition costs deferred $ 11,287 Amortization of capitalized contract acquisition costs $ (14,444 ) Balance, March 31, 2019 $ 51,451
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction	Remaining performance obligations represent the amount of the transaction price under contracts with customers that are attributable to performance obligations that are unsatisfied or partially satisfied at the reporting date. The Company expects to recognize this amount as revenue over the following time periods (in thousands): 2019 2020 Thereafter Total Revenue expected to be recognized on remaining performance obligations $ 525,006 $ 402,034 $ 234,405 $ 1,161,445
Contract with Customer, Asset and Liability	The table below shows significant movements in contract assets (current and noncurrent) for the six months ended March 31, 2019 (in thousands): Balance, September 30, 2018 $ — Impacts from adoption of ASC 606 $ 57,499 Revenue recognized during period but not yet billed $ 3,733 Contract asset net additions $ 29,900 Contract assets reclassified to accounts receivable $ (16,854 ) Balance, March 31, 2019 $ 74,278 The table below shows significant movements in the deferred revenue balances (current and noncurrent) for the six months ended March 31, 2019 (in thousands): Balance, September 30, 2018 $ 1,015,321 Impacts from adoption of ASC 606 $ 68,078 Amounts billed but not recognized as revenues $ 567,681 Revenues recognized related to the opening balance of deferred revenue $ (489,635 ) Balance, March 31, 2019 $ 1,161,445

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Mar. 31, 2019
Fair Value Disclosures [Abstract]
Schedule of Financial Assets Measured at Fair Value on a Recurring Basis	The Company’s financial assets measured at fair value on a recurring basis subject to the disclosure requirements at March 31, 2019 , were as follows (in thousands): Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Securities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Fair Value at March 31, Cash equivalents $ 234,666 $ 179,687 $ — $ 414,353 Short-term investments Available-for-sale securities — certificates of deposits — 2,228 — 2,228 Available-for-sale securities — corporate bonds and notes — 462,459 — 462,459 Available-for-sale securities — municipal bonds and notes — 21,712 — 21,712 Available-for-sale securities — U.S. government securities — 71,757 — 71,757 Available-for-sale securities — U.S. government agency securities — 28,959 — 28,959 Long-term investments Available-for-sale securities — corporate bonds and notes — 277,421 — 277,421 Available-for-sale securities — municipal bonds and notes — 14,491 — 14,491 Available-for-sale securities — U.S. government securities — 2,967 — 2,967 Available-for-sale securities — U.S. government agency securities — 6,478 — 6,478 Total $ 234,666 $ 1,068,159 $ — $ 1,302,825 The Company’s financial assets measured at fair value on a recurring basis subject to the disclosure requirements at September 30, 2018 , were as follows (in thousands): Fair Value Measurements at Reporting Date Using Quoted Prices in Active Markets for Identical Securities (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Fair Value at September 30, 2018 Cash equivalents $ 41,468 $ 13,118 $ — $ 54,586 Short-term investments Available-for-sale securities — certificates of deposits — 2,970 — 2,970 Available-for-sale securities — corporate bonds and notes — 393,750 — 393,750 Available-for-sale securities — municipal bonds and notes — 22,524 — 22,524 Available-for-sale securities — U.S. government securities — 120,078 — 120,078 Available-for-sale securities — U.S. government agency securities — 75,383 — 75,383 Long-term investments Available-for-sale securities — corporate bonds and notes — 367,710 — 367,710 Available-for-sale securities — municipal bonds and notes — 24,286 — 24,286 Available-for-sale securities — U.S. government securities — 12,771 — 12,771 Available-for-sale securities — U.S. government agency securities — 6,417 — 6,417 Total $ 41,468 $ 1,039,007 $ — $ 1,080,475

Short-Term and Long-Term Inve_2

Short-Term and Long-Term Investments (Tables)	6 Months Ended
	Mar. 31, 2019
Schedule of Investments [Line Items]
Schedule of Short-Term and Long-Term Investments	Short-term investments consist of the following (in thousands): March 31, 2019 Cost or Cost Gross Gains Gross Losses Fair Value Certificates of deposits $ 2,228 $ — $ — $ 2,228 Corporate bonds and notes 463,014 134 (689 ) 462,459 Municipal bonds and notes 21,761 2 (51 ) 21,712 U.S. government securities 71,827 15 (85 ) 71,757 U.S. government agency securities 29,009 — (50 ) 28,959 $ 587,839 $ 151 $ (875 ) $ 587,115 September 30, 2018 Cost or Cost Gross Gains Gross Losses Fair Value Certificates of deposits $ 2,970 $ — $ — $ 2,970 Corporate bonds and notes 394,684 9 (943 ) 393,750 Municipal bonds and notes 22,588 1 (65 ) 22,524 U.S. government securities 120,283 — (205 ) 120,078 U.S. government agency securities 75,587 — (204 ) 75,383 $ 616,112 $ 10 $ (1,417 ) $ 614,705 Long-term investments consist of the following (in thousands): March 31, 2019 Cost or Cost Gross Gains Gross Losses Fair Value Corporate bonds and notes $ 277,526 $ 633 $ (738 ) $ 277,421 Municipal bonds and notes 14,491 23 (23 ) 14,491 U.S. government securities 2,997 — (30 ) 2,967 U.S. government agency securities 6,500 1 (23 ) 6,478 $ 301,514 $ 657 $ (814 ) $ 301,357 September 30, 2018 Cost or Cost Gross Gains Gross Losses Fair Value Corporate bonds and notes $ 370,377 $ 25 $ (2,692 ) $ 367,710 Municipal bonds and notes 24,468 — (182 ) 24,286 U.S. government securities 12,956 — (185 ) 12,771 U.S. government agency securities 6,500 — (83 ) 6,417 $ 414,301 $ 25 $ (3,142 ) $ 411,184
Schedule of Investments That Have Been in a Continuous Unrealized Loss Position	The following table summarizes investments that have been in a continuous unrealized loss position for less than 12 months and those that have been in a continuous unrealized loss position for more than 12 months as of March 31, 2019 (in thousands): Less Than 12 Months 12 Months or Greater Total March 31, 2019 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Corporate bonds and notes $ 248,518 $ (168 ) $ 245,916 $ (1,259 ) $ 494,434 $ (1,427 ) Municipal bonds and notes 6,004 (1 ) 18,419 (73 ) 24,423 (74 ) U.S. government securities 28,007 (6 ) 20,863 (109 ) 48,870 (115 ) U.S. government agency securities 9,124 (2 ) 15,428 (71 ) 24,552 (73 ) Total $ 291,653 $ (177 ) $ 300,626 $ (1,512 ) $ 592,279 $ (1,689 ) The following table summarizes investments that have been in a continuous unrealized loss position for less than 12 months and those that have been in a continuous unrealized loss position for more than 12 months as of September 30, 2018 (in thousands): Less Than 12 Months 12 Months or Greater Total September 30, 2018 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Corporate bonds and notes $ 543,729 $ (1,800 ) $ 152,097 $ (1,835 ) $ 695,826 $ (3,635 ) Municipal bonds and notes 26,846 (123 ) 14,363 (124 ) 41,209 (247 ) U.S. government securities 103,470 (281 ) 29,379 (109 ) 132,849 (390 ) U.S. government agency securities 44,812 (110 ) 36,987 (177 ) 81,799 (287 ) Total $ 718,857 $ (2,314 ) $ 232,826 $ (2,245 ) $ 951,683 $ (4,559 )

Inventories (Tables)

Inventories (Tables)	6 Months Ended
	Mar. 31, 2019
Inventory Disclosure [Abstract]
Schedule Of Inventories	Inventories consist of the following (in thousands): March 31, September 30, Finished goods $ 22,305 $ 21,339 Raw materials 11,158 9,229 $ 33,463 $ 30,568

Geographic Sales and Signific_2

Geographic Sales and Significant Customers (Tables)	6 Months Ended
	Mar. 31, 2019
Segment Reporting [Abstract]
Schedule of Revenues by Geographic Region	The following presents revenues by geographic region (in thousands): Three months ended Six months ended 2019 2018 2019 2018 Americas: United States $ 282,037 $ 266,440 $ 546,788 $ 530,583 Other 25,375 29,538 52,268 56,906 Total Americas 307,412 295,978 599,056 587,489 EMEA 136,549 136,903 285,533 275,891 Asia Pacific 1 100,934 100,423 204,076 193,115 $ 544,895 $ 533,304 $ 1,088,665 $ 1,056,495 (1) Beginning with the first quarter of fiscal 2019, revenue from Japan is now included with the APAC region. This change has been applied to all periods presented for comparability purposes.
Schedule of Revenue by Major Customers by Reporting Segments	The following distributors of the Company's products accounted for more than 10% of total net revenue: Three months ended Six months ended 2019 2018 2019 2018 Ingram Micro, Inc. 20.0 % 16.6 % 18.4 % 16.0 % Tech Data 10.0 % 12.0 % — 12.2 % Arrow ECS 10.3 % 10.7 % 10.6 % 11.1 % Synnex Corporation — 11.2 % — 11.1 % Westcon Group, Inc. — 10.8 % 10.5 % 10.5 %
Long-lived Assets by Geographic Areas	The Company tracks assets by physical location. Long-lived assets consist of property and equipment, net, and are shown below (in thousands): March 31, September 30, United States $ 183,987 $ 126,790 EMEA 12,460 12,538 Other countries 11,774 5,714 $ 208,221 $ 145,042

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Narrative) (Details)	Oct. 31, 2018USD ($)shares	Mar. 31, 2019USD ($)	Mar. 31, 2019USD ($)shares	Mar. 31, 2018USD ($)shares	Mar. 31, 2019USD ($)	Mar. 31, 2019USD ($)	Mar. 31, 2019USD ($)shares	Mar. 31, 2019USD ($)segment	Mar. 31, 2019USD ($)$ / shares	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)shares	Sep. 30, 2018USD ($)	Oct. 25, 2017USD ($)	Sep. 30, 2017USD ($)
Schedule Of Summary Of Significant Accounting Policies [Line Items]
Approximate Value of Pending Merger		$ 670,000,000
Cash and Cash Equivalents, at Carrying Value		726,662,000	$ 726,662,000		$ 726,662,000	$ 726,662,000	$ 726,662,000	$ 726,662,000	$ 726,662,000	$ 726,662,000		$ 424,707,000
Revenue Recognition
Number of reportable segments								1		1
Stock-Based Compensation
Share-based compensation expense			39,500,000	$ 41,300,000		78,200,000					$ 82,300,000
Unrecognized stock-based compensation cost		174,500,000	$ 174,500,000		$ 174,500,000	174,500,000	$ 174,500,000	$ 174,500,000	$ 174,500,000	$ 174,500,000
Unrecognized stock-based compensation cost, period for recognition					2 years
Common Stock Repurchase
Stock Repurchased and Retired During Period, Shares | shares			617,000	1,065,000			1,185,881				2,308,000
Treasury Stock Acquired, Average Cost Per Share | $ / shares									$ 169.53
Stock Repurchase Program, Remaining Authorized Repurchase Amount		1,400,000,000	$ 1,400,000,000		$ 1,400,000,000	1,400,000,000	$ 1,400,000,000	1,400,000,000	$ 1,400,000,000	1,400,000,000
Restricted Cash		1,188,000	1,188,000		1,188,000	1,188,000	1,188,000	1,188,000	1,188,000	1,188,000		1,187,000
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		$ 727,850,000	$ 727,850,000	$ 597,160,000	$ 727,850,000	$ 727,850,000	$ 727,850,000	$ 727,850,000	$ 727,850,000	$ 727,850,000	$ 597,160,000	$ 425,894,000		$ 674,452,000
Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Approved RSUs to employees and executive officers pursuant to the Company's annual equity awards program | shares	774,313
Minimum [Member]
Revenue Recognition
Domestic accounts receivable terms of payment					30 days
International accounts receivable terms of payment					30 days
Finite-Lived Intangible Asset, Useful Life					3 years
Maximum [Member]
Revenue Recognition
Domestic accounts receivable terms of payment					45 days
International accounts receivable terms of payment					120 days
Finite-Lived Intangible Asset, Useful Life					10 years
October Twenty Six Two Thousand Ten Program [Member]
Common Stock Repurchase
Stock Repurchase Program, Authorized Amount	$ 1,000,000,000												$ 4,400,000,000
Annual Equity Program [Member] | Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Percentage of the aggregate number of RSUs granted that vest in equal quarterly increments										50.00%
Portion of RSU grant subject to Company achieving specified quarterly revenue and EBITDA goals										50.00%
Percentage of quarterly performance stock grant based on achieving quarterly revenue goal										70.00%
Percentage of quarterly revenue goal to be achieved for performance stock grant										80.00%
Percentage of quarterly performance stock grant based on achieving EBITDA goal										30.00%
Percentage Of Quarterly Ebitda Goal To Be Achieved For Performance Stock Grant										80.00%
Threshold percentage of targeted goals above which quarterly performance stock grant is paid linearly										80.00%
Percentage of over-achievement threshold to which the goals are entitled										100.00%
Percentage Of Over-Achievement To Which Quarterly Performance Based Vesting Cannot Exceed										200.00%
Percentage Of Annual Performance Stock Grant Based On Achieving Annual Revenue Goal										50.00%
Percentage Of Annual Revenue Goal To Be Achieved For Performance Stock Grant										80.00%
Percentage Of Annual Performance Stock Grant Based On Achieving Annual Increase in Stand-alone Software Goal										25.00%
Percentage Of Annual Increase in Stand-alone Software Goal To Be Achieved For Performance Stock Grant										18.00%
Percentage Of Annual Performance Stock Grant Based On Achieving Relative Total Shareholder Return Benchmarked to the S&P 500 Index Goal										25.00%
Percentage Of Over-Achievement To Which Annual Performance Based Vesting Cannot Exceed										200.00%
Annual Equity Program [Member] | November 1, 2018 [Member] | Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Approved RSUs to employees and executive officers pursuant to the Company's annual equity awards program | shares							144,066
Annual equity awards program vesting period					3 years
Annual Equity Program [Member] | November 1, 2017 [Member] | Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Approved RSUs to employees and executive officers pursuant to the Company's annual equity awards program | shares							140,135
Annual equity awards program vesting period					4 years
Annual Equity Program [Member] | November 1, 2016 [Member] | Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Approved RSUs to employees and executive officers pursuant to the Company's annual equity awards program | shares							115,347
Annual equity awards program vesting period					4 years
Annual Equity Program [Member] | November 2, 2015 [Member] | Restricted Stock Units (RSUs) [Member]
Stock-Based Compensation
Approved RSUs to employees and executive officers pursuant to the Company's annual equity awards program | shares							145,508
Annual equity awards program vesting period					4 years
Software and Software Development Costs [Member] | Minimum [Member]
Revenue Recognition
Finite-Lived Intangible Asset, Useful Life					3 years
Software and Software Development Costs [Member] | Maximum [Member]
Revenue Recognition
Finite-Lived Intangible Asset, Useful Life					5 years
Maintenance [Member]
Common Stock Repurchase
Capitalized contract cost, amortization period		4 years 6 months	4 years 6 months		4 years 6 months	4 years 6 months	4 years 6 months	4 years 6 months	4 years 6 months	4 years 6 months
Subscription and Circulation [Member]
Common Stock Repurchase
Capitalized contract cost, amortization period		3 years	3 years		3 years	3 years	3 years	3 years	3 years	3 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies (Schedule Of Computation Of Basic And Diluted Net Income Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Numerator
Net income	$ 116,087	$ 109,636	$ 246,992	$ 198,065
Denominator
Weighted average shares outstanding - basic	59,686	61,420	59,954	61,812
Dilutive effect of common shares from stock options and restricted stock units	343	639	420	539
Weighted average shares outstanding - diluted	60,029	62,059	60,374	62,351
Basic net income per share (dollars per share)	$ 1.94	$ 1.79	$ 4.12	$ 3.20
Diluted net income per share (dollars per share)	$ 1.93	$ 1.77	$ 4.09	$ 3.18

Revenue from Contracts with C_3

Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Oct. 01, 2018	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets	$ 138,736	$ 102,884	$ 52,326
Other assets, net	95,682	101,862	42,186
Deferred tax assets	21,551	25,539	33,441
Deferred revenue	809,336	751,161	715,697
Deferred revenue, long-term	352,109	332,238	299,624
Retained earnings	1,457,348	1,323,291	1,287,243
Capitalized contract cost, net	51,451		0
Deferred revenue	$ 1,161,445		$ 1,015,321
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets		50,558
Other assets, net		59,676
Deferred tax assets		(7,902)
Deferred revenue		35,464
Deferred revenue, long-term		32,614
Retained earnings		36,048
Capitalized contract cost, net		54,600
Deferred revenue		$ (8,800)

Revenue from Contracts with C_4

Revenue from Contracts with Customers - Capitalized contract acquisition costs (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2019USD ($)
Changes In Capitalized Contract Cost [Roll Forward]
Balance, September 30, 2018	$ 0
Impacts from adoption of ASC 606	54,608
Additional capitalized contract acquisition costs deferred	11,287
Amortization of capitalized contract acquisition costs	(14,444)
Balance, March 31, 2019	$ 51,451

Revenue from Contracts with C_5

Revenue from Contracts with Customers - Contract Assets and Liabilities (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2019USD ($)
Change in Contract with Customer, Asset [Roll Forward]
Balance, September 30, 2018	$ 0
Impacts from adoption of ASC 606	57,499
Revenue recognized during period but not yet billed	3,733
Contract asset net additions	29,900
Contract assets reclassified to accounts receivable	16,854
Balance, March 31, 2019	74,278
Change in Contract with Customer, Liability [Roll Forward]
Balance, September 30, 2018	1,015,321
Impacts from adoption of ASC 606	68,078
Amounts billed but not recognized as revenues	567,681
Revenues recognized related to the opening balance of deferred revenue	(489,635)
Balance, March 31, 2019	$ 1,161,445

Revenue from Contracts with C_6

Revenue from Contracts with Customers Remaining Performance Obligations (Details) $ in Thousands	Mar. 31, 2019USD ($)
Revenue from Contract with Customer [Abstract]
Revenue, remaining performance obligation, amount	$ 1,161,445
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01
Revenue from Contract with Customer [Abstract]
Revenue, remaining performance obligation, amount	$ 525,006
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period	6 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-10-01
Revenue from Contract with Customer [Abstract]
Revenue, remaining performance obligation, amount	$ 402,034
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-10-01
Revenue from Contract with Customer [Abstract]
Revenue, remaining performance obligation, amount	$ 234,405
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation, expected timing of satisfaction, period

Fair Value Measurements (Schedu

Fair Value Measurements (Schedule of Financial Assets Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	$ 414,353	$ 54,586
Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	234,666	41,468
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	179,687	13,118
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Cash equivalents, fair value	0	0
Short-Term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	587,115	614,705
Short-Term Investments [Member] | Certificates of Deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	2,228	2,970
Short-Term Investments [Member] | Certificates of Deposit [Member] | Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Short-Term Investments [Member] | Certificates of Deposit [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	2,228	2,970
Short-Term Investments [Member] | Certificates of Deposit [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Short-Term Investments [Member] | Corporate Bonds and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	462,459	393,750
Short-Term Investments [Member] | Corporate Bonds and Notes [Member] | Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Short-Term Investments [Member] | Corporate Bonds and Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	462,459	393,750
Short-Term Investments [Member] | Corporate Bonds and Notes [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Short-Term Investments [Member] | Municipal Bonds and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	21,712	22,524
Short-Term Investments [Member] | Municipal Bonds and Notes [Member] | Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Short-Term Investments [Member] | Municipal Bonds and Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	21,712	22,524
Short-Term Investments [Member] | Municipal Bonds and Notes [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Short-Term Investments [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	71,757	120,078
Short-Term Investments [Member] | U.S. Government Securities [Member] | Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Short-Term Investments [Member] | U.S. Government Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	71,757	120,078
Short-Term Investments [Member] | U.S. Government Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Short-Term Investments [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	28,959	75,383
Short-Term Investments [Member] | U.S. Government Agency Securities [Member] | Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Short-Term Investments [Member] | U.S. Government Agency Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	28,959	75,383
Short-Term Investments [Member] | U.S. Government Agency Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Long-Term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	301,357	411,184
Long-Term Investments [Member] | Corporate Bonds and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	277,421	367,710
Long-Term Investments [Member] | Corporate Bonds and Notes [Member] | Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Long-Term Investments [Member] | Corporate Bonds and Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	277,421	367,710
Long-Term Investments [Member] | Corporate Bonds and Notes [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Long-Term Investments [Member] | Municipal Bonds and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	14,491	24,286
Long-Term Investments [Member] | Municipal Bonds and Notes [Member] | Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Long-Term Investments [Member] | Municipal Bonds and Notes [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	14,491	24,286
Long-Term Investments [Member] | Municipal Bonds and Notes [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Long-Term Investments [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	2,967	12,771
Long-Term Investments [Member] | U.S. Government Securities [Member] | Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Long-Term Investments [Member] | U.S. Government Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	2,967	12,771
Long-Term Investments [Member] | U.S. Government Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Long-Term Investments [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	6,478	6,417
Long-Term Investments [Member] | U.S. Government Agency Securities [Member] | Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Long-Term Investments [Member] | U.S. Government Agency Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	6,478	6,417
Long-Term Investments [Member] | U.S. Government Agency Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Investments, fair value	0	0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Assets, fair value disclosure	1,302,825	1,080,475
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Securities (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Assets, fair value disclosure	234,666	41,468
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Assets, fair value disclosure	1,068,159	1,039,007
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis, Valuation Techniques [Line Items]
Assets, fair value disclosure	$ 0	$ 0

Short-Term and Long-Term Inve_3

Short-Term and Long-Term Investments (Schedule of Short-Term and Long-Term Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Short-term Investments [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	$ 587,839	$ 616,112
Gross Unrealized Gains	151	10
Gross Unrealized Losses	(875)	(1,417)
Fair Value	587,115	614,705
Short-term Investments [Member] | Certificates of Deposit [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	2,228	2,970
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	2,228	2,970
Short-term Investments [Member] | Corporate bonds and notes
Schedule of Investments [Line Items]
Cost or Amortized Cost	463,014	394,684
Gross Unrealized Gains	134	9
Gross Unrealized Losses	(689)	(943)
Fair Value	462,459	393,750
Short-term Investments [Member] | Municipal bonds and notes
Schedule of Investments [Line Items]
Cost or Amortized Cost	21,761	22,588
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(51)	(65)
Fair Value	21,712	22,524
Short-term Investments [Member] | U.S. government securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	71,827	120,283
Gross Unrealized Gains	15	0
Gross Unrealized Losses	(85)	(205)
Fair Value	71,757	120,078
Short-term Investments [Member] | U.S. government agency securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	29,009	75,587
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(50)	(204)
Fair Value	28,959	75,383
Long Term Investments [Member]
Schedule of Investments [Line Items]
Cost or Amortized Cost	301,514	414,301
Gross Unrealized Gains	657	25
Gross Unrealized Losses	(814)	(3,142)
Fair Value	301,357	411,184
Long Term Investments [Member] | Corporate bonds and notes
Schedule of Investments [Line Items]
Cost or Amortized Cost	277,526	370,377
Gross Unrealized Gains	633	25
Gross Unrealized Losses	(738)	(2,692)
Fair Value	277,421	367,710
Long Term Investments [Member] | Municipal bonds and notes
Schedule of Investments [Line Items]
Cost or Amortized Cost	14,491	24,468
Gross Unrealized Gains	23	0
Gross Unrealized Losses	(23)	(182)
Fair Value	14,491	24,286
Long Term Investments [Member] | U.S. government securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	2,997	12,956
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(30)	(185)
Fair Value	2,967	12,771
Long Term Investments [Member] | U.S. government agency securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	6,500	6,500
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(23)	(83)
Fair Value	$ 6,478	$ 6,417

Short-Term and Long-Term Inve_4

Short-Term and Long-Term Investments (Schedule of Investments That Have Been in a Continuous Unrealized Loss Position) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Fair Value
Less Than 12 Months	$ 291,653	$ 718,857
12 Months or Greater	300,626	232,826
Total	592,279	951,683
Gross Unrealized Losses
Less Than 12 Months	(177)	(2,314)
12 Months or Greater	(1,512)	(2,245)
Total	(1,689)	(4,559)
Corporate Bonds and Notes [Member]
Fair Value
Less Than 12 Months	248,518	543,729
12 Months or Greater	245,916	152,097
Total	494,434	695,826
Gross Unrealized Losses
Less Than 12 Months	(168)	(1,800)
12 Months or Greater	(1,259)	(1,835)
Total	(1,427)	(3,635)
Municipal Bonds and Notes [Member]
Fair Value
Less Than 12 Months	6,004	26,846
12 Months or Greater	18,419	14,363
Total	24,423	41,209
Gross Unrealized Losses
Less Than 12 Months	(1)	(123)
12 Months or Greater	(73)	(124)
Total	(74)	(247)
U.S. Government Securities [Member]
Fair Value
Less Than 12 Months	28,007	103,470
12 Months or Greater	20,863	29,379
Total	48,870	132,849
Gross Unrealized Losses
Less Than 12 Months	(6)	(281)
12 Months or Greater	(109)	(109)
Total	(115)	(390)
U.S. Government Agency Securities [Member]
Fair Value
Less Than 12 Months	9,124	44,812
12 Months or Greater	15,428	36,987
Total	24,552	81,799
Gross Unrealized Losses
Less Than 12 Months	(2)	(110)
12 Months or Greater	(71)	(177)
Total	$ (73)	$ (287)

Inventories (Schedule of Invent

Inventories (Schedule of Inventories) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Inventory Disclosure [Abstract]
Finished goods	$ 22,305	$ 21,339
Raw materials	11,158	9,229
Inventories, total	$ 33,463	$ 30,568

Commitments And Contingencies (

Commitments And Contingencies (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2019USD ($)	Sep. 30, 2018USD ($)
Loss Contingencies [Line Items]
Standard product warranty accrual	$ 0	$ 0
Product warranty period	1 year
F5 Counterclaim of Radware, Ltd. and Radware, Inc. Patent Infringement Complaint [Member] | Pending Litigation [Member]
Loss Contingencies [Line Items]
Gain contingency, patents allegedly infringed upon, number	3

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Effective tax rate	22.70%	25.00%	21.70%	31.80%
Unrecognized tax benefit	$ 35.2		$ 35.2

Geographic Sales and Signific_3

Geographic Sales and Significant Customers (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Mar. 31, 2019USD ($)	Mar. 31, 2019USD ($)segment	Mar. 31, 2019USD ($)geographic_region	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)	Sep. 30, 2018USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments				1		1
Number of geographic regions | geographic_region					3
Net revenues	$ 544,895	$ 533,304	$ 1,088,665				$ 1,056,495
Long-lived assets	208,221		208,221	$ 208,221	$ 208,221	$ 208,221		$ 145,042
Americas [Member]
Segment Reporting Information [Line Items]
Net revenues	307,412	295,978	599,056				587,489
United States [Member]
Segment Reporting Information [Line Items]
Net revenues	282,037	266,440	546,788				530,583
Long-lived assets	183,987		183,987	183,987	183,987	183,987		126,790
Other [Member]
Segment Reporting Information [Line Items]
Net revenues	25,375	29,538	52,268				56,906
EMEA [Member]
Segment Reporting Information [Line Items]
Net revenues	136,549	136,903	285,533				275,891
Long-lived assets	12,460		12,460	12,460	12,460	12,460		12,538
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Net revenues	100,934	$ 100,423	204,076				$ 193,115
Other International Countries [Member]
Segment Reporting Information [Line Items]
Long-lived assets	$ 11,774		$ 11,774	$ 11,774	$ 11,774	$ 11,774		$ 5,714
Worldwide Distributor 1 [Member] | Net Revenue [Member] | Geographic Concentration [Member]
Segment Reporting Information [Line Items]
Concentration risk percentage	20.00%	16.60%				18.40%	16.00%
Worldwide Distributor 2 [Member] | Net Revenue [Member] | Geographic Concentration [Member]
Segment Reporting Information [Line Items]
Concentration risk percentage	10.00%	12.00%				0.00%	12.20%
Worldwide Distributor 3 [Member] | Net Revenue [Member] | Geographic Concentration [Member]
Segment Reporting Information [Line Items]
Concentration risk percentage	10.30%	10.70%				10.60%	11.10%
Worldwide Distributor4 [Member] | Net Revenue [Member] | Geographic Concentration [Member]
Segment Reporting Information [Line Items]
Concentration risk percentage	0.00%	11.20%				0.00%	11.10%
Worldwide Distributor5 [Member] | Net Revenue [Member] | Geographic Concentration [Member]
Segment Reporting Information [Line Items]
Concentration risk percentage	0.00%	10.80%				10.50%	10.50%

Uncategorized Items - ffiv-2019

Label	Element	Value
Accounting Standards Update 2014-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 36,048,000
Accounting Standards Update 2014-09 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 36,048,000