Commitments and Contingencies	6 Months Ended
Mar. 31, 2017
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, 2017 and March 31, 2016 were not considered material. Commitments As of March 31, 2017, 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 2025. There have been no material changes in the Company’s principal lease commitments 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 May 1, 2013, Radware, Ltd. and Radware, Inc. (Radware), filed a complaint for patent infringement against the Company in the United States District Court for the Northern District of California seeking an unspecified amount of monetary damages, as well as interest, costs, and injunctive relief, based upon allegations that the Company infringed three Radware patents related to ISP link load balancing. The Company answered the Complaint, denied the substantive allegations, and sought a declaratory judgment that the asserted claims were invalid. The Company also counter-claimed alleging that Radware had infringed certain Company patents relating to cookie persistence. F5 was granted summary judgment of non-infringement of one of Radware’s three patents, and was also granted summary judgment of non-infringement with respect to the other two patents in connection with design-arounds. F5 also took an offer of judgment as to Radware’s infringement of its patents. Radware’s damages claims and F5’s invalidity claims proceeded to trial and on March 15, 2016, the jury returned a verdict finding that F5 had not proven that the asserted patents were invalid, that the Company willfully infringed one of the two patents and that Radware was due $6.4 million in damages. On August 22, 2016, the Court, in response to various post-trial motions, granted F5 judgment as a matter of law on Radware’s claim of willful infringement (and thus denied Radware’s motion for treble damages and attorneys’ fees), awarded prejudgment interest, and issued an injunction. On September 7, 2016, the Court entered judgment in the amount of $6,870,978.95 in favor of Radware on its claims, and $40,000 in favor of F5 on its counterclaim (pursuant to the offer and acceptance of judgment). On October 5, 2016, F5 moved for a new trial on invalidity. That motion was denied. F5 has appealed the denial of the motion and Radware has appealed the summary judgments of non-infringement, the district court’s exclusion of certain evidence, and the grant of judgment as a matter of law on the willfulness claim. Separately, Radware has requested an award of $192,000 in costs. F5 has objected to the request and it remains pending. F5 has posted security of $6,895,391 to stay execution of the judgment pending appeal. In addition, on April 4, 2016, the Company sued Radware, Inc. in the United States District Court for the Western District of Washington accusing Radware of infringing three other 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 has also filed a counterclaim asserting that F5 is infringing another ISP link load balancing patent related to the same patents involved in the California litigation. F5 has denied infringement. The parties have also filed petitions for inter partes reviews on all four patents in the litigation. Claim construction briefing is scheduled over the next three months, but no claim construction hearing or trial is scheduled at this point. In addition to the above referenced matters, the Company is subject to legal proceedings, claims, and litigation arising in the ordinary course of business, including intellectual property litigation. 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. In fiscal 2016, the Company recorded an accrual related to the Radware litigation mentioned above of $7.0 million for the verdict and recorded a credit of $0.1 million in the second quarter of fiscal 2017. The Company has not recorded any other accruals for loss contingencies associated with other legal proceedings.

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, 2017 and March 31, 2016 were not considered material.

Commitments

As of March 31, 2017, 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 2025. There have been no material changes in the Company’s principal lease commitments 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 May 1, 2013, Radware, Ltd. and Radware, Inc. (Radware), filed a complaint for patent infringement against the Company in the United States District Court for the Northern District of California seeking an unspecified amount of monetary damages, as well as interest, costs, and injunctive relief, based upon allegations that the Company infringed three Radware patents related to ISP link load balancing. The Company answered the Complaint, denied the substantive allegations, and sought a declaratory judgment that the asserted claims were invalid. The Company also counter-claimed alleging that Radware had infringed certain Company patents relating to cookie persistence. F5 was granted summary judgment of non-infringement of one of Radware’s three patents, and was also granted summary judgment of non-infringement with respect to the other two patents in connection with design-arounds. F5 also took an offer of judgment as to Radware’s infringement of its patents. Radware’s damages claims and F5’s invalidity claims proceeded to trial and on March 15, 2016, the jury returned a verdict finding that F5 had not proven that the asserted patents were invalid, that the Company willfully infringed one of the two patents and that Radware was due $6.4 million in damages. On August 22, 2016, the Court, in response to various post-trial motions, granted F5 judgment as a matter of law on Radware’s claim of willful infringement (and thus denied Radware’s motion for treble damages and attorneys’ fees), awarded prejudgment interest, and issued an injunction. On September 7, 2016, the Court entered judgment in the amount of $6,870,978.95 in favor of Radware on its claims, and $40,000 in favor of F5 on its counterclaim (pursuant to the offer and acceptance of judgment). On October 5, 2016, F5 moved for a new trial on invalidity. That motion was denied. F5 has appealed the denial of the motion and Radware has appealed the summary judgments of non-infringement, the district court’s exclusion of certain evidence, and the grant of judgment as a matter of law on the willfulness claim. Separately, Radware has requested an award of $192,000 in costs. F5 has objected to the request and it remains pending. F5 has posted security of $6,895,391 to stay execution of the judgment pending appeal.

In addition, on April 4, 2016, the Company sued Radware, Inc. in the United States District Court for the Western District of Washington accusing Radware of infringing three other 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 has also filed a counterclaim asserting that F5 is infringing another ISP link load balancing patent related to the same patents involved in the California litigation. F5 has denied infringement. The parties have also filed petitions for inter partes reviews on all four patents in the litigation. Claim construction briefing is scheduled over the next three months, but no claim construction hearing or trial is scheduled at this point.

In addition to the above referenced matters, the Company is subject to legal proceedings, claims, and litigation arising in the ordinary course of business, including intellectual property litigation. 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. In fiscal 2016, the Company recorded an accrual related to the Radware litigation mentioned above of $7.0 million for the verdict and recorded a credit of $0.1 million in the second quarter of fiscal 2017. The Company has not recorded any other accruals for loss contingencies associated with other legal proceedings.