Document And Entity Information (USD $)	12 Months Ended
	Sep. 30, 2014	Nov. 20, 2014	Mar. 31, 2014
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-K
Document Period End Date	Sep. 30, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		73,783,302
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 7,990,507,572