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Basis of Presentation and Preparation - Goodwill and Intangible Assets (Details) | 12 Months Ended
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Oct. 01, 2017 Level Step
Goodwill and Intangible Assets |
Number of levels below reportable segments at which goodwill impairment testing is performed | Level | 1
Number of steps involved in process of goodwill annual impairment test | Step | 2

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- References No definition available. + Details
Name: | ttek_GoodwillAndIntangibleAssetsAbstract
Namespace Prefix: | ttek_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of levels below reportable segments at which the reporting units reside for which goodwill impairment testing is performed. + References No definition available. + Details
Name: | ttek_NumberOfLevelsBelowReportableSegmentsForGoodwillImpairmentTesting
Namespace Prefix: | ttek_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of steps involved in the process of goodwill annual impairment test. + References No definition available. + Details
Name: | ttek_NumberOfStepsInvolvedInProcessOfGoodwillAnnualImpairmentTest
Namespace Prefix: | ttek_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration