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 Sep. 27, 2020			53,797
Beginning balance at Sep. 27, 2020	$ 1,037,373	$ 1,037,319	$ 538	$ 0	$ (161,786)	$ 1,198,567	$ 54
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	149,900	149,856				149,856	44
Other comprehensive income (loss)	59,391	59,385			59,385		6
Distributions paid to noncontrolling interests	(9)						(9)
Cash dividends per common share	(29,241)	(29,241)				(29,241)
Stock-based compensation	16,261	16,261		16,261
Restricted & performance shares released (in shares)			213
Restricted & performance shares released	(17,589)	(17,589)	$ 2	(17,591)
Stock options exercised (in shares)			304
Stock options exercised	10,703	10,703	$ 4	10,699
Shares issued for Employee Stock Purchase Plan (in shares)			124
Shares issued for Employee Stock Purchase Plan	10,706	10,706	$ 1	10,705
Stock repurchases (in shares)			(367)
Stock repurchases	(45,000)	(45,000)	$ (4)	(20,074)		(24,922)
Ending balance (in shares) at Jun. 27, 2021			54,071
Ending balance at Jun. 27, 2021	1,192,495	1,192,400	$ 541	0	(102,401)	1,294,260	95
Beginning balance (in shares) at Mar. 28, 2021			54,158
Beginning balance at Mar. 28, 2021	1,147,228	1,147,147	$ 542	0	(115,056)	1,261,661	81
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	51,924	51,903				51,903	21
Other comprehensive income (loss)	12,657	12,655			12,655		2
Distributions paid to noncontrolling interests	(9)						(9)
Cash dividends per common share	(10,831)	(10,831)				(10,831)
Stock-based compensation	5,695	5,695		5,695
Restricted & performance shares released (in shares)			2
Restricted & performance shares released	(101)	(101)		(101)
Stock options exercised (in shares)			29
Stock options exercised	932	932	$ 1	931
Stock repurchases (in shares)			(118)
Stock repurchases	(15,000)	(15,000)	$ (2)	(6,525)		(8,473)
Ending balance (in shares) at Jun. 27, 2021			54,071
Ending balance at Jun. 27, 2021	1,192,495	1,192,400	$ 541	0	(102,401)	1,294,260	95
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)						(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		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)		(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