Cover Page - shares	3 Months Ended	9 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Jul. 31, 2020
Cover [Abstract]
Document Type		10-Q
Document Quarterly Report		true
Document Period End Date		Jun. 30, 2020
Document Transition Report		false
Entity File Number		000-26041
Entity Registrant Name		F5 NETWORKS, INC.
Entity Incorporation, State or Country Code		WA
Entity Tax Identification Number		91-1714307
Entity Address, Address Line One		801 5th Avenue
Entity Address, City or Town		Seattle
Entity Address, State or Province		WA
Entity Address, Postal Zip Code		98104
City Area Code		206
Local Phone Number		272-5555
Title of 12(b) Security		Common stock, no par value
Trading Symbol		FFIV
Security Exchange Name		NASDAQ
Entity Current Reporting Status		Yes
Entity Interactive Data Current		Yes
Entity Filer Category		Large Accelerated Filer
Entity Small Business		false
Entity Emerging Growth Company		false
Entity Shell Company		false
Entity Common Stock, Shares Outstanding			61,173,109
Entity Central Index Key		0001048695
Current Fiscal Year End Date		--09-30
Document Fiscal Year Focus		2020
Document Fiscal Period Focus	Q3
Amendment Flag		false