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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. 02, 2022
Jul. 03, 2022
Apr. 03, 2022
Jan. 02, 2022
Oct. 03, 2021
Jun. 27, 2021
Mar. 28, 2021
Dec. 27, 2020
Oct. 02, 2022
Oct. 03, 2021
Sep. 27, 2020
Earnings Per Share [Abstract]                      
Net income attributable to Tetra Tech $ 82,947 $ 58,650 $ 53,040 $ 68,489 $ 82,954 $ 51,903 $ 45,517 $ 52,436 $ 263,125 $ 232,810 $ 173,859
Net income attributable to Tetra Tech $ 82,947 $ 58,650 $ 53,040 $ 68,489 $ 82,954 $ 51,903 $ 45,517 $ 52,436 $ 263,125 $ 232,810 $ 173,859
Weighted-average common shares outstanding – basic (in shares) 53,148 53,507 53,834 53,937 54,019 54,117 54,187 53,927 53,620 54,078 54,235
Effect of diluted stock options and unvested restricted stock (in shares)                 543 597 787
Weighted-average common stock outstanding – diluted (in shares) 53,667 54,006 54,346 54,577 54,597 54,666 54,736 54,637 54,163 54,675 55,022
Earnings per share attributable to Tetra Tech:                      
Basic (in dollars per share) $ 1.56 $ 1.10 $ 0.99 $ 1.27 $ 1.54 $ 0.96 $ 0.84 $ 0.97 $ 4.91 $ 4.31 $ 3.21
Diluted (in dollars per share) $ 1.55 $ 1.09 $ 0.98 $ 1.25 $ 1.52 $ 0.95 $ 0.83 $ 0.96 $ 4.86 $ 4.26 $ 3.16