Quarterly Financial Information – Unaudited - Summary of Quarterly Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 29, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Oct. 01, 2023	Jul. 02, 2023	Apr. 02, 2023	Jan. 01, 2023	Sep. 29, 2024	Oct. 01, 2023	Oct. 02, 2022
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 1,374,473	$ 1,344,323	$ 1,251,616	$ 1,228,267	$ 1,260,611	$ 1,208,947	$ 1,158,226	$ 894,766	$ 5,198,679	$ 4,522,550	$ 3,504,048
Income from operations	143,343	128,630	117,683	111,081	107,377	97,675	61,011	92,050	500,737	358,113	340,446
Net income attributable to Tetra Tech	96,154	85,810	76,446	74,972	53,649	60,235	42,830	116,706	333,382	273,420	263,125
Net income attributable to Tetra Tech	$ 96,154	$ 85,810	$ 76,446	$ 74,972	$ 53,649	$ 60,235	$ 42,830	$ 116,706	$ 333,382	$ 273,420	$ 263,125
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.36	$ 0.32	$ 0.29	$ 0.28	$ 0.20	$ 0.23	$ 0.16	$ 0.44	$ 1.25	$ 1.03	$ 0.98
Diluted (in dollars per share)	$ 0.35	$ 0.32	$ 0.28	$ 0.28	$ 0.20	$ 0.22	$ 0.16	$ 0.44	$ 1.23	$ 1.02	$ 0.97
Weighted-average common shares outstanding:
Basic (in shares)	267,687	267,575	267,420	266,585	266,235	266,155	266,135	265,345	267,364	266,015	268,100
Diluted (in shares)	271,656	270,260	269,375	268,690	268,510	268,265	268,135	267,645	270,042	268,185	270,815