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Reportable Segments (Details 2) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Mar. 29, 2015
Mar. 30, 2014
Mar. 29, 2015
Mar. 30, 2014
Revenue by client sector        
Threshold percentage for disclosure of revenue from a single client     10.00%ttek_ConcentrationOfRiskRevenuePercentageMinimum  
Revenue $ 564,763us-gaap_SalesRevenueServicesNet $ 586,285us-gaap_SalesRevenueServicesNet $ 1,145,819us-gaap_SalesRevenueServicesNet $ 1,232,133us-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 173,680us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_InternationalMember		 181,570us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_InternationalMember		 320,741us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_InternationalMember		 345,504us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_InternationalMember
U.S. commercial        
Revenue by client sector        
Revenue 158,156us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_CommercialMember		 147,854us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_CommercialMember		 333,339us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_CommercialMember		 334,150us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_CommercialMember
U.S. federal government        
Revenue by client sector        
Revenue 168,250us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_FederalGovernmentMember		 178,341us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_FederalGovernmentMember		 352,436us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_FederalGovernmentMember		 373,525us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_FederalGovernmentMember
U.S. state and local government        
Revenue by client sector        
Revenue $ 64,677us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_StateAndLocalGovernmentMember		 $ 78,520us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_StateAndLocalGovernmentMember		 $ 139,303us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_StateAndLocalGovernmentMember		 $ 178,954us-gaap_SalesRevenueServicesNet
/ us-gaap_MajorCustomersAxis
= ttek_StateAndLocalGovernmentMember