Earnings Per Share (''EPS'') (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	6 Months Ended
Mar. 29, 2015	Mar. 30, 2014	Mar. 29, 2015	Mar. 30, 2014
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 19,017	$ 31,709	$ 44,592	$ 59,023
Weighted-average common shares outstanding - basic	61,153,000	64,835,000	61,816,000	64,670,000
Effect of diluted stock options and unvested restricted stock	570,000	875,000	615,000	847,000
Weighted-average common stock outstanding - diluted	61,723,000	65,710,000	62,431,000	65,517,000
Earnings per share attributable to Tetra Tech:
Basic	$ 0.31us-gaap_EarningsPerShareBasic	$ 0.49us-gaap_EarningsPerShareBasic	$ 0.72us-gaap_EarningsPerShareBasic	$ 0.91us-gaap_EarningsPerShareBasic
Diluted	$ 0.31us-gaap_EarningsPerShareDiluted	$ 0.48us-gaap_EarningsPerShareDiluted	$ 0.71us-gaap_EarningsPerShareDiluted	$ 0.90us-gaap_EarningsPerShareDiluted
Stock options
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares	1,300,000	300,000	1,300,000	0

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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