v3.3.1.900
Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2015
Feb. 19, 2016
Jun. 30, 2015
Document Information [Line Items]      
Entity Registrant Name FORTINET INC    
Entity Central Index Key 0001262039    
Document Type 10-K    
Document Period End Date Dec. 31, 2015    
Amendment Flag false    
Document Fiscal Year Focus 2015    
Document Fiscal Period Focus FY    
Current Fiscal Year End Date --12-31    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Well Known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Common Stock, Shares Outstanding   171,603,611  
Entity Public Float     $ 5,300,165,895