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	159,556	159,536				159,536	20
Other comprehensive income (loss)	26,762	26,763			26,763		(1)
Cash dividends per common share	(24,428)	(24,428)				(24,428)
Stock-based compensation	14,602	14,602		14,602
Restricted & performance shares released (in shares)			146
Restricted & performance shares released	(16,680)	(16,680)	$ 1	(16,681)
Stock options exercised (in shares)			3
Stock options exercised	91	91		91
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 Apr. 02, 2023			53,228
Ending balance at Apr. 02, 2023	1,355,668	1,355,599	$ 532	10,639	(181,381)	1,525,809	69
Beginning balance (in shares) at Jan. 01, 2023			53,226
Beginning balance at Jan. 01, 2023	1,323,967	1,323,908	$ 532	3,281	(175,126)	1,495,221	59
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	42,841	42,830				42,830	11
Other comprehensive income (loss)	(6,256)	(6,255)			(6,255)		(1)
Cash dividends per common share	(12,242)	(12,242)				(12,242)
Stock-based compensation	7,418	7,418		7,418
Restricted & performance shares released (in shares)			1
Restricted & performance shares released	(94)	(94)		(94)
Stock options exercised (in shares)			1
Stock options exercised	34	34		34
Ending balance (in shares) at Apr. 02, 2023			53,228
Ending balance at Apr. 02, 2023	1,355,668	1,355,599	$ 532	10,639	(181,381)	1,525,809	69
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	151,439	151,418				151,418	21
Other comprehensive income (loss)	38,749	38,749			38,749
Distributions paid to noncontrolling interests	(38)						(38)
Cash dividends per common share	(27,781)	(27,781)				(27,781)
Stock-based compensation	15,617	15,617		15,617
Restricted & performance shares released (in shares)			107
Restricted & performance shares released	(12,781)	(12,781)	$ 1	(12,782)
Stock options exercised (in shares)			38
Stock options exercised	1,462	1,462	$ 1	1,461
Shares issued for Employee Stock Purchase Plan (in shares)			104
Shares issued for Employee Stock Purchase Plan	14,677	14,677	$ 1	14,676
Ending balance (in shares) at Mar. 31, 2024			53,497
Ending balance at Mar. 31, 2024	1,584,850	1,584,794	$ 535	18,972	(156,546)	1,721,833	56
Beginning balance (in shares) at Dec. 31, 2023			53,466
Beginning balance at Dec. 31, 2023	1,537,687	1,537,606	$ 534	9,979	(132,202)	1,659,295	81
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	76,459	76,446				76,446	13
Other comprehensive income (loss)	(24,344)	(24,344)			(24,344)
Distributions paid to noncontrolling interests	(38)						(38)
Cash dividends per common share	(13,908)	(13,908)				(13,908)
Stock-based compensation	7,976	7,976		7,976
Restricted & performance shares released (in shares)			2
Restricted & performance shares released	(111)	(111)		(111)
Stock options exercised (in shares)			29
Stock options exercised	1,127	1,127	$ 1	1,126
Shares issued for Employee Stock Purchase Plan	2	2		2
Ending balance (in shares) at Mar. 31, 2024			53,497
Ending balance at Mar. 31, 2024	$ 1,584,850	$ 1,584,794	$ 535	$ 18,972	$ (156,546)	$ 1,721,833	$ 56