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Earnings Per Share - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Oct. 01, 2017	Jul. 02, 2017	Apr. 02, 2017	Jan. 01, 2017	Oct. 02, 2016	Jun. 26, 2016	Mar. 27, 2016	Dec. 27, 2015	Oct. 01, 2017	Oct. 02, 2016	Sep. 27, 2015
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 34,467	$ 29,983	$ 26,862	$ 26,562	$ 31,106	$ 25,694	$ 3,744	$ 23,239	$ 117,874	$ 83,783	$ 39,074
Weighted-average common shares outstanding – basic	56,338	57,184	57,270	57,099	57,309	57,796	58,451	59,058	56,911	58,186	60,913
Effect of diluted stock options and unvested restricted stock									1,002	780	619
Weighted-average common stock outstanding – diluted	57,326	58,161	58,270	58,145	58,192	58,616	59,131	59,793	57,913	58,966	61,532
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.61	$ 0.52	$ 0.47	$ 0.47	$ 0.54	$ 0.44	$ 0.06	$ 0.39	$ 2.07	$ 1.44	$ 0.64
Diluted (in dollars per share)	$ 0.60	$ 0.52	$ 0.46	$ 0.46	$ 0.53	$ 0.44	$ 0.06	$ 0.39	$ 2.04	$ 1.42	$ 0.64

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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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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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