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	219,794	219,771				219,771	23
Other comprehensive income	70,218	70,218			70,218
Cash dividends per common share	(38,268)	(38,268)				(38,268)
Stock-based compensation	21,640	21,640		21,640
Restricted & performance shares released (in shares)			149
Restricted & performance shares released	(16,785)	(16,785)	$ 1	(16,786)
Stock options exercised (in shares)			15
Stock options exercised	425	425		425
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 Jul. 02, 2023			53,243
Ending balance at Jul. 02, 2023	1,452,789	1,452,716	$ 532	17,906	(137,926)	1,572,204	73
Beginning balance (in shares) at Apr. 02, 2023			53,228
Beginning balance at Apr. 02, 2023	1,355,668	1,355,599	$ 532	10,639	(181,381)	1,525,809	69
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	60,238	60,235				60,235	3
Other comprehensive income	43,456	43,455			43,455		1
Cash dividends per common share	(13,840)	(13,840)				(13,840)
Stock-based compensation	7,038	7,038		7,038
Restricted & performance shares released (in shares)			3
Restricted & performance shares released	(105)	(105)		(105)
Stock options exercised (in shares)			12
Stock options exercised	334	334		334
Ending balance (in shares) at Jul. 02, 2023			53,243
Ending balance at Jul. 02, 2023	1,452,789	1,452,716	$ 532	17,906	(137,926)	1,572,204	73
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	237,263	237,228				237,228	35
Other comprehensive income	40,606	40,606			40,606
Distributions paid to noncontrolling interests	(43)						(43)
Cash dividends per common share	(43,303)	(43,303)				(43,303)
Stock-based compensation	23,713	23,713		23,713
Restricted & performance shares released (in shares)			108
Restricted & performance shares released	(12,906)	(12,906)	$ 1	(12,907)
Stock options exercised (in shares)			72
Stock options exercised	2,690	2,690	$ 1	2,689
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 Jun. 30, 2024			53,532
Ending balance at Jun. 30, 2024	1,666,203	1,666,138	$ 535	28,171	(154,689)	1,792,121	65
Beginning balance (in shares) at Mar. 31, 2024			53,497
Beginning balance at Mar. 31, 2024	1,584,850	1,584,794	$ 535	18,972	(156,546)	1,721,833	56
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	85,824	85,810				85,810	14
Other comprehensive income	1,857	1,857			1,857
Distributions paid to noncontrolling interests	(5)						(5)
Cash dividends per common share	(15,522)	(15,522)				(15,522)
Stock-based compensation	8,096	8,096		8,096
Restricted & performance shares released (in shares)			1
Restricted & performance shares released	(125)	(125)		(125)
Stock options exercised (in shares)			34
Stock options exercised	1,228	1,228		1,228
Ending balance (in shares) at Jun. 30, 2024			53,532
Ending balance at Jun. 30, 2024	$ 1,666,203	$ 1,666,138	$ 535	$ 28,171	$ (154,689)	$ 1,792,121	$ 65