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Document and Entity Information - USD ($)
$ in Billions
 12 Months Ended
Oct. 01, 2017
Nov. 01, 2017
Apr. 02, 2017
Document and Entity Information      
Entity Registrant Name TETRA TECH INC    
Entity Central Index Key 0000831641    
Current Fiscal Year End Date --10-01    
Entity Filer Category Large Accelerated Filer    
Document Type 10-K    
Document Period End Date Oct. 01, 2017    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Common Stock, Shares Outstanding   55,722,592  
Entity Public Float     $ 2.3