Revenue and Contract Balances - Summary of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 28, 2025	Dec. 29, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 1,210,663	$ 1,420,561
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	572,442	519,822
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	543,098	598,948
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	95,123	301,791
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	272,598	501,848
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	171,473	202,987
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	225,352	233,591
International
Disaggregation of Revenue [Line Items]
Revenue	$ 541,240	$ 482,135

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.

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+ Details

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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+ Details

TETRA TECH INC 10-Q