Represents the earn out period for operating income projection used for fair value measurement.

No definition available.

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

- Definition

Business Combination, Consideration Transferred, Working Capital Adjustments

+ References

No definition available.

+ Details

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

- Definition

Business Combination, Contingent Consideration Arrangements, Earn Out Period

+ References

No definition available.

+ Details

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Namespace Prefix:	ttek_
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- 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

Name:	us-gaap_BusinessAcquisitionLineItems
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 8
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-SubTopic 30
-Section 30
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-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6578-128477

+ Details

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

- Definition

For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

+ Details

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

- Definition

Amount of liability recognized arising from contingent consideration in a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
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-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6911189&loc=d3e6408-128476

+ Details

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

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

+ Details

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

- Details

Name:	srt_RangeAxis=srt_MinimumMember
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Data Type:	na
Balance Type:
Period Type:

- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TradeNamesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
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Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ttek_SegueTechnologiesIncMember
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Balance Type:
Period Type:

- Details

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Acquisitions - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Acquisitions - Narrative (Details) - USD ($) $ in Millions	Dec. 27, 2020	Sep. 27, 2020	Mar. 29, 2020
Business Acquisition [Line Items]
Contingent earn-out liability	$ 26.0
Aggregate maximum of contingent consideration	$ 63.9
Minimum
Business Acquisition [Line Items]
Significant unobservable input, earn-out period	2 years
Minimum \| Existing customer contracts
Business Acquisition [Line Items]
Useful life of intangible assets	1 year
Minimum \| Trade names
Business Acquisition [Line Items]
Useful life of intangible assets	3 years
Maximum
Business Acquisition [Line Items]
Significant unobservable input, earn-out period	3 years
Maximum \| Existing customer contracts
Business Acquisition [Line Items]
Useful life of intangible assets	10 years
Maximum \| Trade names
Business Acquisition [Line Items]
Useful life of intangible assets	5 years
SEG
Business Acquisition [Line Items]
Fair value of acquisition purchase price			$ 40.9
Initial cash payments			29.6
Contingent earn-out liability			11.3
Aggregate maximum of contingent consideration			$ 20.0
Earn-out period			3 years
BWF
Business Acquisition [Line Items]
Fair value of acquisition purchase price		$ 47.7
Initial cash payments		41.8
Contingent earn-out liability		5.2
Aggregate maximum of contingent consideration		$ 8.0
Earn-out period		3 years
Payables related to estimated post-closing adjustments		$ 0.7