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Leases - Reconciliation of Termination Costs (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2018 | Oct. 01, 2017
Reconciliation of the beginning and ending balances of liabilities related to contract termination costs |  |
Balance at the beginning of the period | $ 1,694 | $ 3,104
Adjustments | (774) | (1,410)
Balance at the end of the period | 920 | 1,694
GSG |  |
Reconciliation of the beginning and ending balances of liabilities related to contract termination costs |  |
Balance at the beginning of the period | 259 | 674
Adjustments | (259) | (415)
Balance at the end of the period | 0 | 259
CIG |  |
Reconciliation of the beginning and ending balances of liabilities related to contract termination costs |  |
Balance at the beginning of the period | 1,432 | 2,391
Adjustments | (512) | (959)
Balance at the end of the period | 920 | 1,432
RCM |  |
Reconciliation of the beginning and ending balances of liabilities related to contract termination costs |  |
Balance at the beginning of the period | 3 | 39
Adjustments | (3) | (36)
Balance at the end of the period | $ 0 | $ 3

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- Definition Amount of any reversal and other adjustment made during the period, to the amount of a previously accrued liability for lease contract termination costs associated with exit from business activities. + References No definition available. + Details
Name: | ttek_ContractTerminationAccrualAdjustment
Namespace Prefix: | ttek_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Represents the carrying amount of liabilities associated with lease contract termination. + References No definition available. + Details
Name: | ttek_ContractTerminationLiability
Namespace Prefix: | ttek_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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Data Type: | na
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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ttek_CommercialInternationalServicesGroupMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=ttek_RemediationAndConstructionManagementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
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