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	121,548	121,529				121,529	19
Other comprehensive income	9,725	9,725			9,725
Distributions paid to noncontrolling interests	$ (31)	0					(31)
Dividend paid per share (in dollars per share)	$ 0.40
Cash dividends per common share	$ (21,562)	(21,562)				(21,562)
Stock-based compensation	12,392	12,392		6,357		6,035
Restricted & performance shares released (in shares)			185
Restricted & performance shares released	(25,055)	(25,055)	$ 2	(19,023)		(6,034)
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)			(618)
Stock repurchases	(100,000)	(100,000)	$ (6)	(667)		(99,327)
Ending balance (in shares) at Apr. 03, 2022			53,683
Ending balance at Apr. 03, 2022	1,244,642	1,244,601	$ 537	0	(115,303)	1,359,367	41
Beginning balance (in shares) at Jan. 02, 2022			53,999
Beginning balance at Jan. 02, 2022	1,237,945	1,237,882	$ 540	0	(123,048)	1,360,390	63
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	53,049	53,040				53,040	9
Other comprehensive income	7,745	7,745			7,745		0
Distributions paid to noncontrolling interests	$ (31)	0					(31)
Dividend paid per share (in dollars per share)	$ 0.20
Cash dividends per common share	$ (10,769)	(10,769)				(10,769)
Stock-based compensation	6,564	6,564		529		6,035
Restricted & performance shares released (in shares)			3
Restricted & performance shares released	(106)	(106)	$ 0	(107)		1
Stock options exercised (in shares)			9
Stock options exercised	244	244	$ 0	244
Shares issued for Employee Stock Purchase Plan (in shares)			0
Shares issued for Employee Stock Purchase Plan	1	1	$ 0	1
Stock repurchases (in shares)			(328)
Stock repurchases	(50,000)	(50,000)	$ (3)	(667)		(49,330)
Ending balance (in shares) at Apr. 03, 2022			53,683
Ending balance at Apr. 03, 2022	1,244,642	1,244,601	$ 537	0	(115,303)	1,359,367	41
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	159,556	159,536				159,536	20
Other comprehensive income	$ 26,762	26,763			26,763		(1)
Dividend paid per share (in dollars per share)	$ 0.46
Cash dividends per common share	$ (24,428)	(24,428)				(24,428)
Stock-based compensation	14,602	14,602		14,602
Restricted & performance shares released (in shares)			146
Restricted & performance shares released	(16,680)	(16,680)	$ 1	(16,681)
Stock options exercised (in shares)			3
Stock options exercised	91	91	$ 0	91
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 Apr. 02, 2023			53,228
Ending balance at Apr. 02, 2023	1,355,668	1,355,599	$ 532	10,639	(181,381)	1,525,809	69
Beginning balance (in shares) at Jan. 01, 2023			53,226
Beginning balance at Jan. 01, 2023	1,323,967	1,323,908	$ 532	3,281	(175,126)	1,495,221	59
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	42,841	42,830				42,830	11
Other comprehensive income	$ (6,256)	(6,255)			(6,255)		(1)
Dividend paid per share (in dollars per share)	$ 0.23
Cash dividends per common share	$ (12,242)	(12,242)				(12,242)
Stock-based compensation	7,418	7,418		7,418
Restricted & performance shares released (in shares)			1
Restricted & performance shares released	(94)	(94)	$ 0	(94)
Stock options exercised (in shares)			1
Stock options exercised	34	34	$ 0	34
Ending balance (in shares) at Apr. 02, 2023			53,228
Ending balance at Apr. 02, 2023	$ 1,355,668	$ 1,355,599	$ 532	$ 10,639	$ (181,381)	$ 1,525,809	$ 69