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	180,205	180,179				180,179	26
Other comprehensive income (loss)	(32,779)	(32,778)			(32,778)		(1)
Distributions paid to noncontrolling interests	(31)	0					(31)
Cash dividends per common share	(33,873)	(33,873)				(33,873)
Stock-based compensation	19,104	19,104		19,104		0
Restricted & performance shares released (in shares)			189
Restricted & performance shares released	(25,193)	(25,193)	$ 2	(25,195)		0
Stock options exercised (in shares)			29
Stock options exercised	1,205	1,205	$ 0	1,205
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)			(986)
Stock repurchases	(150,000)	(150,000)	$ (10)	(7,242)		(142,748)
Ending balance (in shares) at Jul. 03, 2022			53,319
Ending balance at Jul. 03, 2022	1,205,058	1,205,011	$ 533	0	(157,806)	1,362,284	47
Beginning balance (in shares) at Apr. 03, 2022			53,683
Beginning balance at Apr. 03, 2022	1,244,642	1,244,601	$ 537	0	(115,303)	1,359,367	41
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	58,657	58,650				58,650	7
Other comprehensive income (loss)	(42,504)	(42,503)			(42,503)		(1)
Cash dividends per common share	(12,311)	(12,311)				(12,311)
Stock-based compensation	6,712	6,712		12,747		(6,035)
Restricted & performance shares released (in shares)			4
Restricted & performance shares released	(138)	(138)	$ 0	(6,172)		6,034
Stock repurchases (in shares)			(368)
Stock repurchases	(50,000)	(50,000)	$ (4)	(6,575)		(43,421)
Ending balance (in shares) at Jul. 03, 2022			53,319
Ending balance at Jul. 03, 2022	1,205,058	1,205,011	$ 533	0	(157,806)	1,362,284	47
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 (loss)	70,218	70,218			70,218		0
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	$ 0	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 (loss)	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)	$ 0	(105)
Stock options exercised (in shares)			12
Stock options exercised	334	334	$ 0	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