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	151,439	151,418				151,418	21
Other comprehensive income (loss)	38,749	38,749			38,749
Distributions paid in 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)			534
Restricted & performance shares released	(12,781)	(12,781)	$ 5	(12,786)
Stock options exercised (in shares)			192
Stock options exercised	1,462	1,462	$ 2	1,460
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 Mar. 31, 2024			267,486
Ending balance at Mar. 31, 2024	1,584,850	1,584,794	$ 2,674	18,963	(156,546)	1,719,703	56
Beginning balance (in shares) at Dec. 31, 2023			267,329
Beginning balance at Dec. 31, 2023	1,537,687	1,537,606	$ 2,672	9,971	(132,202)	1,657,165	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 in 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)			10
Restricted & performance shares released	(111)	(111)		(111)
Stock options exercised (in shares)			147
Stock options exercised	1,127	1,127	$ 2	1,125
Shares issued for Employee Stock Purchase Plan	2	2		2
Ending balance (in shares) at Mar. 31, 2024			267,486
Ending balance at Mar. 31, 2024	1,584,850	1,584,794	$ 2,674	18,963	(156,546)	1,719,703	56
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	6,190	6,135				6,135	55
Other comprehensive income (loss)	(74,305)	(74,305)			(74,305)
Distributions paid in noncontrolling interests	(23)						(23)
Cash dividends per common share	(30,900)	(30,900)				(30,900)
Stock-based compensation	17,027	17,027		17,027
Restricted & performance shares released (in shares)			467
Restricted & performance shares released	(13,848)	(13,848)	$ 5	(13,853)
Stock options exercised (in shares)			27
Stock options exercised	171	171		171
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)			(5,166)
Stock repurchases	(176,251)	(176,251)	$ (51)	(54,548)		(121,652)
Ending balance (in shares) at Mar. 30, 2025			263,503
Ending balance at Mar. 30, 2025	1,573,781	1,573,658	$ 2,635	0	(153,180)	1,724,203	123
Beginning balance (in shares) at Dec. 29, 2024			268,028
Beginning balance at Dec. 29, 2024	1,692,019	1,691,897	$ 2,680	21,153	(187,754)	1,855,818	122
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	5,412	5,388				5,388	24
Other comprehensive income (loss)	34,574	34,574			34,574
Distributions paid in noncontrolling interests	(23)						(23)
Cash dividends per common share	(15,351)	(15,351)				(15,351)
Stock-based compensation	8,885	8,885		8,885
Restricted & performance shares released (in shares)			35
Restricted & performance shares released	(541)	(541)		(541)
Stock options exercised (in shares)			6
Stock options exercised	57	57		57
Stock repurchases (in shares)			(4,566)
Stock repurchases	(151,251)	(151,251)	$ (45)	(29,554)		(121,652)
Ending balance (in shares) at Mar. 30, 2025			263,503
Ending balance at Mar. 30, 2025	$ 1,573,781	$ 1,573,658	$ 2,635	$ 0	$ (153,180)	$ 1,724,203	$ 123