Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	6 Months Ended
Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 28,725	$ 26,862	$ 74,759	$ 53,424
Weighted-average common shares outstanding – basic (in shares)	55,841	57,270	55,900	57,171
Effect of dilutive stock options and unvested restricted stock (in shares)	832	1,000	925	1,023
Weighted-average common stock outstanding – diluted (in shares)	56,673	58,270	56,825	58,194
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.51	$ 0.47	$ 1.34	$ 0.93
Diluted (in dollars per share)	$ 0.51	$ 0.46	$ 1.32	$ 0.92

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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