v3.10.0.1
Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2018
Feb. 22, 2019
Jun. 29, 2018
DEI [Abstract]      
Entity Registrant Name FORTINET INC    
Entity Central Index Key 0001262039    
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
Amendment Flag false    
Document Fiscal Year Focus 2018    
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   170,633,671  
Entity Public Float     $ 7,073,006,351