Earnings Per Share (''EPS'') - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	9 Months Ended
Jun. 26, 2016	Jun. 28, 2015	Jun. 26, 2016	Jun. 28, 2015
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 25,694	$ 26,206	$ 52,678	$ 70,798
Weighted-average common shares outstanding - basic	57,796	60,207	58,483	61,293
Effect of dilutive stock options and unvested restricted stock	820	585	745	594
Weighted-average common stock outstanding - diluted	58,616	60,792	59,228	61,887
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.44	$ 0.44	$ 0.90	$ 1.16
Diluted (in dollars per share)	$ 0.44	$ 0.43	$ 0.89	$ 1.14

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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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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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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