Income Taxes	3 Months Ended
Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The effective tax rate was 20.8% and 38.7% for the three months ended December 31, 2018 and 2017, respectively. The effective tax rate for the three months ended December 31, 2018 and December 31, 2017 includes various impacts from the Tax Cuts and Jobs Act enacted on December 22, 2017. Significant impacts for the three months ended December 31, 2018 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 three months ended December 31, 2017 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. In accordance with SEC Staff Accounting Bulletin (“SAB”) No. 118, the Company recorded a provisional tax expense of $7.0 million related to the deemed repatriation of undistributed foreign earnings in the period ended December 31, 2017. SAB No. 118 provided a one-year measurement period to complete the accounting. During the period ended December 31, 2018, the Company finalized the computations for the deemed repatriation of undistributed foreign earnings and did not record any adjustments to the provisional amounts previously recorded. At December 31, 2018, the Company had $33.5 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. The Company is currently under audit by the IRS for fiscal year 2016 and by various states for fiscal years 2014 through 2017. Major jurisdictions where there are wholly owned subsidiaries of F5 Networks, Inc. which require income tax filings include the United Kingdom, Japan, Singapore, and Australia. The earliest periods open for review by local taxing authorities are fiscal years 2017 for the United Kingdom, 2013 for Japan, 2014 for Singapore, and 2014 for Australia. 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.

Income Taxes

The effective tax rate was 20.8% and 38.7% for the three months ended December 31, 2018 and 2017, respectively. The effective tax rate for the three months ended December 31, 2018 and December 31, 2017 includes various impacts from the Tax Cuts and Jobs Act enacted on December 22, 2017. Significant impacts for the three months ended December 31, 2018 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 three months ended December 31, 2017 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.

In accordance with SEC Staff Accounting Bulletin (“SAB”) No. 118, the Company recorded a provisional tax expense of $7.0 million related to the deemed repatriation of undistributed foreign earnings in the period ended December 31, 2017. SAB No. 118 provided a one-year measurement period to complete the accounting. During the period ended December 31, 2018, the Company finalized the computations for the deemed repatriation of undistributed foreign earnings and did not record any adjustments to the provisional amounts previously recorded.

At December 31, 2018, the Company had $33.5 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. The Company is currently under audit by the IRS for fiscal year 2016 and by various states for fiscal years 2014 through 2017. Major jurisdictions where there are wholly owned subsidiaries of F5 Networks, Inc. which require income tax filings include the United Kingdom, Japan, Singapore, and Australia. The earliest periods open for review by local taxing authorities are fiscal years 2017 for the United Kingdom, 2013 for Japan, 2014 for Singapore, and 2014 for Australia. 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.