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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, 2023
Jul. 02, 2023
Apr. 02, 2023
Jan. 01, 2023
Oct. 02, 2022
Jul. 03, 2022
Apr. 03, 2022
Jan. 02, 2022
Oct. 01, 2023
Oct. 02, 2022
Oct. 03, 2021
Earnings Per Share [Abstract]                      
Net income attributable to Tetra Tech $ 53,649 $ 60,235 $ 42,830 $ 116,706 $ 82,947 $ 58,650 $ 53,040 $ 68,489 $ 273,420 $ 263,125 $ 232,810
Net income attributable to Tetra Tech $ 53,649 $ 60,235 $ 42,830 $ 116,706 $ 82,947 $ 58,650 $ 53,040 $ 68,489 $ 273,420 $ 263,125 $ 232,810
Weighted-average common shares outstanding – basic (in shares) 53,247 53,231 53,227 53,069 53,148 53,507 53,834 53,937 53,203 53,620 54,078
Effect of diluted stock options and unvested restricted stock (in shares)                 434 543 597
Weighted-average common stock outstanding – diluted (in shares) 53,702 53,653 53,627 53,529 53,667 54,006 54,346 54,577 53,637 54,163 54,675
Earnings per share attributable to Tetra Tech:                      
Basic (in dollars per share) $ 1.01 $ 1.13 $ 0.80 $ 2.20 $ 1.56 $ 1.10 $ 0.99 $ 1.27 $ 5.14 $ 4.91 $ 4.31
Diluted (in dollars per share) $ 1.00 $ 1.12 $ 0.80 $ 2.18 $ 1.55 $ 1.09 $ 0.98 $ 1.25 $ 5.10 $ 4.86 $ 4.26