Earnings per Share - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 27, 2020	Jun. 28, 2020	Mar. 29, 2020	Dec. 29, 2019	Sep. 29, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 30, 2018	Sep. 27, 2020	Sep. 29, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 44,654	$ 45,497	$ 36,397	$ 47,310	$ 11,527	$ 49,233	$ 55,911	$ 41,997	$ 173,859	$ 158,668	$ 136,883
Weighted-average common shares outstanding – basic (in shares)	53,841	53,985	54,699	54,560	54,617	54,819	55,143	55,390	54,235	54,986	55,670
Effect of diluted stock options and unvested restricted stock (in shares)									787	950	928
Weighted-average common stock outstanding – diluted (in shares)	54,603	54,692	55,463	55,438	55,618	55,768	55,985	56,366	55,022	55,936	56,598
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.83	$ 0.84	$ 0.67	$ 0.87	$ 0.21	$ 0.90	$ 1.01	$ 0.76	$ 3.21	$ 2.89	$ 2.46
Diluted (in dollars per share)	$ 0.82	$ 0.83	$ 0.66	$ 0.85	$ 0.21	$ 0.88	$ 1.00	$ 0.75	$ 3.16	$ 2.84	$ 2.42