Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	9 Months Ended
Jun. 29, 2025	Jun. 30, 2024
Statement of Cash Flows [Abstract]
Income tax refunds	$ 13.6	$ 4.0

Amount of income tax refund received from tax jurisdiction.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2A
-Publisher FASB
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Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 740
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-URI https://asc.fasb.org/1943274/2147482685/740-10-50-22

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-Topic 230
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TETRA TECH INC 10-Q