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Goodwill and Intangible Assets (Details 2) (USD $)
6 Months Ended
Mar. 29, 2015
Sep. 28, 2014
Goodwill 671,397,000us-gaap_Goodwill $ 714,190,000us-gaap_Goodwill
RME    
Goodwill 448,839,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
476,104,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
RME | GMP    
Fair value of business 102,800,000us-gaap_FairValueNetAssetLiability
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
/ us-gaap_SubsegmentsAxis
= ttek_GlobalMiningPracticeMember
 
Carrying value of business 91,800,000us-gaap_AssetsNet
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
/ us-gaap_SubsegmentsAxis
= ttek_GlobalMiningPracticeMember
 
Goodwill 61,400,000us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
/ us-gaap_SubsegmentsAxis
= ttek_GlobalMiningPracticeMember
 
RME | GMP | US treasury bonds    
Impairment of goodwill    
Maturity of investments whose yield is considered in determination of impairment 20 years  
Goodwill. | RME | GMP    
Discounted cash flow rate (as a percent) 14.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_GoodwillMember
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
/ us-gaap_SubsegmentsAxis
= ttek_GlobalMiningPracticeMember
 
Revenue multiple 0.7us-gaap_FairValueInputsRevenueMultiple
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_GoodwillMember
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
/ us-gaap_SubsegmentsAxis
= ttek_GlobalMiningPracticeMember
 
Impairment of goodwill    
Significant contraction in mining activity period 2 years  
Goodwill. | RME | GMP | Minimum    
Annual revenue growth rate (as a percent) 3.00%us-gaap_FairValueInputsLongTermRevenueGrowthRate
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_GoodwillMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
/ us-gaap_SubsegmentsAxis
= ttek_GlobalMiningPracticeMember
 
Impairment of goodwill    
Decline in future revenue leading to goodwill impairment (as a percent) 20.00%ttek_GoodwillDecreaseInRevenueThatWouldLeadToImpairment
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_GoodwillMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
/ us-gaap_SubsegmentsAxis
= ttek_GlobalMiningPracticeMember
 
Decline in future market prices of similar businesses leading to goodwill impairment (as a percent) 15.00%ttek_GoodwillDecreaseInMarketPricesForSimilarBusinessesThatWouldLeadToImpairment
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_GoodwillMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
/ us-gaap_SubsegmentsAxis
= ttek_GlobalMiningPracticeMember
 
Discounted cash flow rate leading to impairment of goodwill (as a percent) 15.50%ttek_GoodwillDiscountedCashFlowThatWouldLeadToImpairment
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_GoodwillMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
/ us-gaap_SubsegmentsAxis
= ttek_GlobalMiningPracticeMember
 
Goodwill. | RME | GMP | Maximum    
Annual revenue growth rate (as a percent) 5.00%us-gaap_FairValueInputsLongTermRevenueGrowthRate
/ us-gaap_FairValueByAssetClassAxis
= us-gaap_GoodwillMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementBusinessSegmentsAxis
= ttek_ResourceManagementAndEnergyMember
/ us-gaap_SubsegmentsAxis
= ttek_GlobalMiningPracticeMember
 
Income approach    
Weighted rate used in fair value of goodwill (as a percent) 70.00%ttek_GoodwillFairValueWeightedPercentage
/ us-gaap_ValuationTechniqueAxis
= us-gaap_IncomeApproachValuationTechniqueMember
 
Discounted cash flow and market approach    
Weighted rate used in fair value of goodwill (as a percent) 30.00%ttek_GoodwillFairValueWeightedPercentage
/ us-gaap_ValuationTechniqueAxis
= us-gaap_MarketApproachValuationTechniqueMember