Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Total Tetra Tech Equity	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Retained Earnings	Non-Controlling Interests
Beginning balance (in shares) at Oct. 02, 2022			52,981
Beginning balance at Oct. 02, 2022	$ 1,183,137	$ 1,183,087	$ 530	$ 0	$ (208,144)	$ 1,390,701	$ 50
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	116,715	116,706				116,706	9
Other comprehensive income (loss)	33,018	33,018			33,018
Cash dividends per common share	(12,186)	(12,186)				(12,186)
Stock-based compensation	7,184	7,184		7,184
Restricted & performance shares released (in shares)			145
Restricted & performance shares released	(16,586)	(16,586)	$ 1	(16,587)
Stock options exercised (in shares)			2
Stock options exercised	57	57	$ 0	57
Shares issued for Employee Stock Purchase Plan (in shares)			98
Shares issued for Employee Stock Purchase Plan	12,628	12,628	$ 1	12,627
Ending balance (in shares) at Jan. 01, 2023			53,226
Ending balance at Jan. 01, 2023	1,323,967	1,323,908	$ 532	3,281	(175,126)	1,495,221	59
Beginning balance (in shares) at Oct. 01, 2023			53,248
Beginning balance at Oct. 01, 2023	1,403,506	1,403,433	$ 532	0	(195,295)	1,598,196	73
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	74,980	74,972				74,972	8
Other comprehensive income (loss)	63,093	63,093			63,093
Cash dividends per common share	(13,873)	(13,873)				(13,873)
Stock-based compensation	7,641	7,641		7,641
Restricted & performance shares released (in shares)			105
Restricted & performance shares released	(12,670)	(12,670)	$ 1	(12,671)
Stock options exercised (in shares)			9
Stock options exercised	335	335	$ 0	335
Shares issued for Employee Stock Purchase Plan (in shares)			104
Shares issued for Employee Stock Purchase Plan	14,675	14,675	$ 1	14,674
Ending balance (in shares) at Dec. 31, 2023			53,466
Ending balance at Dec. 31, 2023	$ 1,537,687	$ 1,537,606	$ 534	$ 9,979	$ (132,202)	$ 1,659,295	$ 81