Retirement Plans (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2018	Oct. 01, 2017	Oct. 02, 2016
Retirement Benefits [Abstract]
Minimum service period for employee to participate in the defined contribution plans	1 year
Employer contributions to the plans	$ 22.4	$ 11.4	$ 10.7
Assets related to deferred compensation plans	29.4	25.0
Liabilities related to deferred compensation plans	$ 30.2	$ 25.2

Represents the minimum service period for employees to be eligible to participate in the entity's defined benefit contribution plans other than 401(k) plans.

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements payable beyond one year (or the operating cycle, if longer).

+ References

No definition available.

+ Details

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

- Definition

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DeferredCompensationPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration