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Reportable Segments (Details 2) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Dec. 28, 2014
Dec. 29, 2013
Revenue by client sector    
Threshold percentage for disclosure of revenue from a single client 10.00%ttek_ConcentrationOfRiskRevenuePercentageMinimum  
Revenue $ 581,056us-gaap_SalesRevenueServicesNet $ 645,848us-gaap_SalesRevenueServicesNet
Revenue    
Revenue by client sector    
Number of clients exceeding threshold 0ttek_ConcentrationRiskNumberOfClients
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueServicesNetMember		  
International    
Revenue by client sector    
Revenue 147,061us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_InternationalMember		 163,933us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_InternationalMember
U.S. commercial    
Revenue by client sector    
Revenue 175,183us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_CommercialMember		 186,296us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_CommercialMember
U.S. federal government    
Revenue by client sector    
Revenue 184,186us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_FederalGovernmentMember		 195,184us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_FederalGovernmentMember
U.S. state and local government    
Revenue by client sector    
Revenue $ 74,626us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_StateAndLocalGovernmentMember		 $ 100,435us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_StateAndLocalGovernmentMember