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. 03, 2021			53,981
Beginning balance at Oct. 03, 2021	$ 1,234,291	$ 1,234,238	$ 540	$ 0	$ (125,028)	$ 1,358,726	$ 53
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	68,499	68,489				68,489	10
Other comprehensive income	1,980	1,980			1,980
Cash dividends per common share	(10,793)	(10,793)				(10,793)
Stock-based compensation	5,828	5,828		5,828
Restricted & performance shares released (in shares)			182
Restricted & performance shares released	(24,949)	(24,949)	$ 2	(18,916)		(6,035)
Stock options exercised (in shares)			20
Stock options exercised	960	960	$ 0	960
Shares issued for Employee Stock Purchase Plan (in shares)			106
Shares issued for Employee Stock Purchase Plan	12,129	12,129	$ 1	12,128
Stock repurchases (in shares)			(290)
Stock repurchases	(50,000)	(50,000)	$ (3)	0		(49,997)
Ending balance (in shares) at Jan. 02, 2022			53,999
Ending balance at Jan. 02, 2022	1,237,945	1,237,882	$ 540	0	(123,048)	1,360,390	63
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	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