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Retirement Plans (Details) - USD ($) $ in Millions | 12 Months Ended
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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 |

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

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- References No definition available. + Details
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Balance Type: | na
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- 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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
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- 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
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