Accounts Receivable - Net - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 29, 2019	Sep. 30, 2018
Financial information concerning reportable segments
Period for billing and collecting unbilled receivables	12 months
Unbilled accounts receivable related to claims and requests for equitable adjustment on contracts	$ 15,000	$ 74,000
Billed accounts receivable related to U.S. federal government contracts	129,300	89,300
Unbilled	300,035	267,739
CIG
Financial information concerning reportable segments
Reduction in the revenue related to the evaluation of the claims	26,700
Loss from claim settlement	28,200
RCM
Financial information concerning reportable segments
Reduction in the revenue related to the evaluation of the claims	4,600	10,600
Loss from claim settlement	5,700	12,500
U.S. government
Financial information concerning reportable segments
Unbilled	$ 59,600	$ 38,600

Contract Receivable Decrease In Revenue Related To Claims And Requests For Equitable Adjustment On Contract

No definition available.

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Namespace Prefix:	ttek_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of billed receivables that are derived from government contracts.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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- Definition

Amount of gain (loss) recognized in settlement of litigation and insurance claims. Excludes claims within an insurance entity's normal claims settlement process.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942415&loc=d3e14326-108349

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Period Type:	duration

- Definition

Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 912
-SubTopic 310
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118261985&loc=d3e55302-109406

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3(c)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Period Type:	instant

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- Details

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- Details

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