Earnings Per Share ("EPS") (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	6 Months Ended
Apr. 01, 2012	Apr. 03, 2011	Apr. 01, 2012	Apr. 03, 2011
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 22,284	$ 17,500	$ 44,894	$ 39,801
Weighted-average common shares outstanding - basic	63,072,000	62,121,000	62,846,000	61,836,000
Effect of diluted stock options and unvested restricted stock (in shares)	745,000	824,000	651,000	802,000
Weighted-average common stock outstanding - diluted (in shares)	63,817,000	62,945,000	63,497,000	62,638,000
Earnings per share attributable to Tetra Tech
Basic (in dollars per share)	$ 0.35	$ 0.28	$ 0.71	$ 0.64
Diluted (in dollars per share)	$ 0.35	$ 0.28	$ 0.71	$ 0.64
Options excluded from the calculation of dilutive potential common shares	2,400,000	2,300,000	3,000,000	2,800,000

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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- Definition

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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- Definition

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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- Definition

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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- Definition

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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