Consolidated Statements of Equity - USD ($) shares in Thousands, $ in Thousands	Total	Total Tetra Tech Equity	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive (Loss) Income	Retained Earnings	Non-Controlling Interests
Beginning balance (in shares) at Oct. 02, 2022			264,903
Beginning balance at Oct. 02, 2022	$ 1,183,137	$ 1,183,087	$ 2,650	$ 0	$ (208,144)	$ 1,388,581	$ 50
Comprehensive income, net of tax:
Net income	273,452	273,420				273,420	32
Foreign currency translation adjustments	12,622	12,623			12,623		(1)
Pension	2,638	2,638			2,638
Gain on cash flow hedge valuations	(2,412)	(2,412)			(2,412)
Comprehensive income, net of tax	286,300	286,269					31
Distributions paid to noncontrolling interests	(8)						(8)
Cash dividends	(52,113)	(52,113)				(52,113)
Stock-based compensation	28,607	28,607		28,607
Restricted & performance shares released (in shares)			746
Restricted & performance shares released	(16,833)	(16,833)	$ 7	(16,840)
Stock options exercised (in shares)			97
Stock options exercised	626	626		626
Shares issued for Employee Stock Purchase Plan (in shares)			492
Shares issued for Employee Stock Purchase Plan	12,628	12,628	$ 5	12,623
Reclassification of APIC	0			26,734		(26,734)
Capped call transactions	(38,838)	(38,838)		(51,750)		12,912
Ending balance (in shares) at Oct. 01, 2023			266,238
Ending balance at Oct. 01, 2023	1,403,506	1,403,433	$ 2,662	0	(195,295)	1,596,066	73
Comprehensive income, net of tax:
Net income	333,443	333,382				333,382	61
Foreign currency translation adjustments	115,120	115,120			115,120
Pension	1,300	1,300			1,300
Gain on cash flow hedge valuations	0
Comprehensive income, net of tax	449,863	449,802					61
Distributions paid to noncontrolling interests	(43)						(43)
Cash dividends	(58,828)	(58,828)				(58,828)
Stock-based compensation	31,155	31,155		31,155
Restricted & performance shares released (in shares)			547
Restricted & performance shares released	(12,982)	(12,982)	$ 5	(12,987)
Stock options exercised (in shares)			410
Stock options exercised	3,067	3,067	$ 4	3,063
Shares issued for Employee Stock Purchase Plan (in shares)			522
Shares issued for Employee Stock Purchase Plan	14,675	14,675	$ 6	14,669
Ending balance (in shares) at Sep. 29, 2024			267,717
Ending balance at Sep. 29, 2024	1,830,413	1,830,322	$ 2,677	35,900	(78,875)	1,870,620	91
Comprehensive income, net of tax:
Net income	247,949	247,724				247,724	225
Foreign currency translation adjustments	(17,165)	(17,165)			(17,165)
Pension	263	263			263
Gain on cash flow hedge valuations	0
Comprehensive income, net of tax	231,047	230,822					225
Distributions paid to noncontrolling interests	(120)						(120)
Acquisition	183						183
Cash dividends	(65,039)	(65,039)				(65,039)
Stock-based compensation	33,946	33,946		33,946
Restricted & performance shares released (in shares)			481
Restricted & performance shares released	$ (14,047)	(14,047)	$ 5	(14,052)
Stock options exercised (in shares)	66		66
Stock options exercised	$ 469	469	$ 1	468
Shares issued for Employee Stock Purchase Plan (in shares)			459
Shares issued for Employee Stock Purchase Plan	15,306	15,306	$ 4	15,302
Stock repurchase (in shares)			(7,305)
Stock repurchase	(251,994)	(251,994)	$ (73)	(71,564)		(180,357)
Ending balance (in shares) at Sep. 28, 2025			261,418
Ending balance at Sep. 28, 2025	$ 1,780,164	$ 1,779,785	$ 2,614	$ 0	$ (95,777)	$ 1,872,948	$ 379