Revenue and Contract Balances - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
Jan. 02, 2022	Dec. 27, 2020	Oct. 03, 2021
Revenue from Contract with Customer [Abstract]
Contract liability revenue recognized during the period	$ 63,000	$ 60,000
Net favorable (unfavorable) revenue and operating income adjustments	2,800	$ 0
Liabilities for anticipated losses	14,200		$ 12,700
Estimated cost to complete the related contracts	$ 116,000		104,000
Period for billing and collecting unbilled receivables	12 months
Unbilled accounts receivable related to claims and requests for equitable adjustment on contracts	$ 11,000		$ 11,000
Remaining unsatisfied performance obligation	$ 3,435,383
Remaining performance obligation, termination notice period one	30 days
Remaining performance obligation, termination notice period two	60 days
Remaining performance obligation, termination notice period three	90 days

Represents the unbilled accounts receivable related to claims and requests for equitable adjustment on contracts.

No definition available.

Name:	ttek_GovernmentContractReceivableUnbilledAmountsRelatedToClaimsAndRequestsForEquitableAdjustmentOnContract
Namespace Prefix:	ttek_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the amount of remaining estimated costs to complete the contracts.

+ References

No definition available.

+ Details

Name:	ttek_RevenueRecognitionEstimatedCostToCompleteRelatedContracts
Namespace Prefix:	ttek_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Revenue, Remaining Performance Obligation, Termination Notice Period One

+ References

No definition available.

+ Details

Name:	ttek_RevenueRemainingPerformanceObligationTerminationNoticePeriodOne
Namespace Prefix:	ttek_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Revenue, Remaining Performance Obligation, Termination Notice Period Three

+ References

No definition available.

+ Details

Name:	ttek_RevenueRemainingPerformanceObligationTerminationNoticePeriodThree
Namespace Prefix:	ttek_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Revenue, Remaining Performance Obligation, Termination Notice Period Two

+ References

No definition available.

+ Details

Name:	ttek_RevenueRemainingPerformanceObligationTerminationNoticePeriodTwo
Namespace Prefix:	ttek_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the period within which substantially all unbilled receivables are expected to be billed and collected.

+ References

No definition available.

+ Details

Name:	ttek_UnbilledContractsReceivablePeriodForBillingAndCollection
Namespace Prefix:	ttek_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase (decrease) in revenue recognized for cumulative catch-up adjustment from change in measure of progress which (increases) decreases obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130551-203045

+ Details

Name:	us-gaap_ContractWithCustomerLiabilityCumulativeCatchUpAdjustmentToRevenueChangeInMeasureOfProgress
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130549-203045

+ Details

Name:	us-gaap_ContractWithCustomerLiabilityRevenueRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Cumulative provision for contract losses not offset against related costs accumulated on the balance sheet.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 605
-SubTopic 35
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=123355788&loc=d3e57777-111642

+ Details

Name:	us-gaap_ProvisionForLossOnContracts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130556-203045

+ Details

Name:	us-gaap_RevenueRemainingPerformanceObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant