Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Retained Earnings	Total Tetra Tech Equity	Non-Controlling Interests
Beginning balance (in shares) at Sep. 27, 2020		53,797
Beginning balance at Sep. 27, 2020	$ 1,037,373	$ 538	$ 0	$ (161,786)	$ 1,198,567	$ 1,037,319	$ 54
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	97,976				97,953	97,953	23
Other comprehensive income	46,734			46,730		46,730	4
Cash dividends per common share	(18,410)				(18,410)	(18,410)
Stock-based compensation	10,566		10,566			10,566
Restricted & performance shares released (in shares)		211
Restricted & performance shares released	(17,488)	$ 2	(17,490)			(17,488)
Stock options exercised (in shares)		275
Stock options exercised	9,771	$ 3	9,768			9,771
Shares issued for Employee Stock Purchase Plan (in shares)		124
Shares issued for Employee Stock Purchase Plan	10,706	$ 1	10,705			10,706
Stock repurchases (in shares)		(249)
Stock repurchases	(30,000)	$ (2)	(13,549)		(16,449)	(30,000)
Ending balance (in shares) at Mar. 28, 2021		54,158
Ending balance at Mar. 28, 2021	$ 1,147,228	$ 542	0	(115,056)	1,261,661	1,147,147	81
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend paid per share (in dollars per share)	$ 0.34
Beginning balance (in shares) at Dec. 27, 2020		54,193
Beginning balance at Dec. 27, 2020	$ 1,105,254	$ 542	0	(127,919)	1,232,563	1,105,186	68
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	45,528				45,517	45,517	11
Other comprehensive income	12,865			12,863		12,863	2
Cash dividends per common share	(9,212)				(9,212)	(9,212)
Stock-based compensation	5,668		5,668			5,668
Restricted & performance shares released (in shares)		2
Restricted & performance shares released	(158)		(158)			(158)
Stock options exercised (in shares)		77
Stock options exercised	2,275	$ 1	2,274			2,275
Shares issued for Employee Stock Purchase Plan	8		8			8
Stock repurchases (in shares)		(114)
Stock repurchases	(15,000)	$ (1)	(7,792)		(7,207)	(15,000)
Ending balance (in shares) at Mar. 28, 2021		54,158
Ending balance at Mar. 28, 2021	$ 1,147,228	$ 542	0	(115,056)	1,261,661	1,147,147	81
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend paid per share (in dollars per share)	$ 0.17
Beginning balance (in shares) at Oct. 03, 2021		53,981
Beginning balance at Oct. 03, 2021	$ 1,234,291	$ 540	0	(125,028)	1,358,726	1,234,238	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)						(31)
Cash dividends per common share	(21,562)				(21,562)	(21,562)
Stock-based compensation	12,392		6,357		6,035	12,392
Restricted & performance shares released (in shares)		185
Restricted & performance shares released	(25,055)	$ 2	(19,023)		(6,034)	(25,055)
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	$ 1	12,128			12,129
Stock repurchases (in shares)		(618)
Stock repurchases	(100,000)	$ (6)	(667)		(99,327)	(100,000)
Ending balance (in shares) at Apr. 03, 2022		53,683
Ending balance at Apr. 03, 2022	$ 1,244,642	$ 537	0	(115,303)	1,359,367	1,244,601	41
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend paid per share (in dollars per share)	$ 0.40
Beginning balance (in shares) at Jan. 02, 2022		53,999
Beginning balance at Jan. 02, 2022	$ 1,237,945	$ 540	0	(123,048)	1,360,390	1,237,882	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
Distributions paid to noncontrolling interests	(31)						(31)
Cash dividends per common share	(10,769)				(10,769)	(10,769)
Stock-based compensation	6,564		529		6,035	6,564
Restricted & performance shares released (in shares)		3
Restricted & performance shares released	(106)		(107)		1	(106)
Stock options exercised (in shares)		9
Stock options exercised	244		244			244
Shares issued for Employee Stock Purchase Plan	1		1			1
Stock repurchases (in shares)		(328)
Stock repurchases	(50,000)	$ (3)	(667)		(49,330)	(50,000)
Ending balance (in shares) at Apr. 03, 2022		53,683
Ending balance at Apr. 03, 2022	$ 1,244,642	$ 537	$ 0	$ (115,303)	$ 1,359,367	$ 1,244,601	$ 41
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend paid per share (in dollars per share)	$ 0.20