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 Income (Loss)	Retained Earnings	Non-Controlling Interests
Beginning balance (in shares) at Oct. 01, 2023			266,238
Beginning balance at Oct. 01, 2023	$ 1,403,506	$ 1,403,433	$ 2,662	$ 0	$ (195,295)	$ 1,596,066	$ 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 in 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)			542
Restricted & performance shares released	(12,906)	(12,906)	$ 5	(12,911)
Stock options exercised (in shares)			360
Stock options exercised	2,690	2,690	$ 4	2,686
Shares issued for Employee Stock Purchase Plan (in shares)			522
Shares issued for Employee Stock Purchase Plan	14,677	14,677	$ 5	14,672
Ending balance (in shares) at Jun. 30, 2024			267,662
Ending balance at Jun. 30, 2024	1,666,203	1,666,138	$ 2,676	28,160	(154,689)	1,789,991	65
Beginning balance (in shares) at Mar. 31, 2024			267,486
Beginning balance at Mar. 31, 2024	1,584,850	1,584,794	$ 2,674	18,963	(156,546)	1,719,703	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 in 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)			8
Restricted & performance shares released	(125)	(125)		(125)
Stock options exercised (in shares)			168
Stock options exercised	1,228	1,228		1,226
Ending balance (in shares) at Jun. 30, 2024			267,662
Ending balance at Jun. 30, 2024	1,666,203	1,666,138	$ 2,676	28,160	(154,689)	1,789,991	65
Beginning balance (in shares) at Sep. 29, 2024			267,717
Beginning balance at Sep. 29, 2024	1,830,413	1,830,322	$ 2,677	35,900	(78,875)	1,870,620	91
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	120,073	119,979				119,979	94
Other comprehensive income	13,758	13,758			13,758
Distributions paid in noncontrolling interests	(29)						(29)
Cash dividends per common share	(47,992)	(47,992)				(47,992)
Stock-based compensation	25,789	25,789		25,789
Restricted & performance shares released (in shares)			475
Restricted & performance shares released	(13,942)	(13,942)	$ 6	(13,948)
Stock options exercised (in shares)			35
Stock options exercised	216	216		216
Shares issued for Employee Stock Purchase Plan (in shares)			458
Shares issued for Employee Stock Purchase Plan	15,307	15,307	$ 4	15,303
Stock repurchases (in shares)			(5,933)
Stock repurchases	(201,497)	(201,497)	$ (59)	(63,260)		(138,178)
Ending balance (in shares) at Jun. 29, 2025			262,752
Ending balance at Jun. 29, 2025	1,742,096	1,741,940	$ 2,628	0	(65,117)	1,804,429	156
Beginning balance (in shares) at Mar. 30, 2025			263,503
Beginning balance at Mar. 30, 2025	1,573,781	1,573,658	$ 2,635	0	(153,180)	1,724,203	123
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	113,883	113,844				113,844	39
Other comprehensive income	88,063	88,063			88,063
Distributions paid in noncontrolling interests	(6)						(6)
Cash dividends per common share	(17,092)	(17,092)				(17,092)
Stock-based compensation	8,762	8,762		8,762
Restricted & performance shares released (in shares)			8
Restricted & performance shares released	(94)	(94)	$ 1	(95)
Stock options exercised (in shares)			8
Stock options exercised	45	45		45
Stock repurchases (in shares)			(767)
Stock repurchases	(25,246)	(25,246)	$ (8)	(8,712)		(16,526)
Ending balance (in shares) at Jun. 29, 2025			262,752
Ending balance at Jun. 29, 2025	$ 1,742,096	$ 1,741,940	$ 2,628	$ 0	$ (65,117)	$ 1,804,429	$ 156