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Basis of Presentation and Preparation - Fiscal Year (Details) | 12 Months Ended
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Sep. 30, 2018 | Oct. 01, 2017 | Oct. 02, 2016
Principles of Consolidation and Presentation |  |  |
Number of weeks in a fiscal year, low end of range | 364 days |  |
Number of weeks in a fiscal year, high end of range | 371 days |  |
Number of weeks in a fiscal year | 364 days | 364 days | 371 days

X
- Definition Represents the length of fiscal year. + References No definition available. + Details
Name: | ttek_LengthOfFiscalYear
Namespace Prefix: | ttek_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the high end of the range of length of fiscal year. + References No definition available. + Details
Name: | ttek_LengthOfFiscalYearHighEndOfRange
Namespace Prefix: | ttek_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the low end of the range of length of fiscal year. + References No definition available. + Details
Name: | ttek_LengthOfFiscalYearLowEndOfRange
Namespace Prefix: | ttek_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | ttek_PrinciplesOfConsolidationAndPresentationAbstract
Namespace Prefix: | ttek_
Data Type: | xbrli:stringItemType
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