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	52,448				52,436	52,436	12
Other comprehensive income (loss)	33,869			33,867		33,867	2
Cash dividends per common share	(9,198)				(9,198)	(9,198)
Stock-based compensation	4,898		4,898			4,898
Restricted & performance shares released (in shares)		209
Restricted & performance shares released	(17,330)	$ 2	(17,332)			(17,330)
Stock options exercised (in shares)		198
Stock options exercised	7,495	$ 2	7,493			7,495
Shares issued for Employee Stock Purchase Plan (in shares)		124
Shares issued for Employee Stock Purchase Plan	10,699	$ 1	10,698			10,699
Stock repurchases (in shares)		(135)
Stock repurchases	(15,000)	$ (1)	(5,757)		(9,242)	(15,000)
Ending balance (in shares) at Dec. 27, 2020		54,193
Ending balance at Dec. 27, 2020	1,105,254	$ 542	0	(127,919)	1,232,563	1,105,186	68
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	68,499				68,489	68,489	10
Other comprehensive income (loss)	1,980			1,980		1,980
Cash dividends per common share	(10,793)				(10,793)	(10,793)
Stock-based compensation	5,828		5,828			5,828
Restricted & performance shares released (in shares)		182
Restricted & performance shares released	(24,949)	$ 2	(18,916)		(6,035)	(24,949)
Stock options exercised (in shares)		20
Stock options exercised	960		960			960
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)		(290)
Stock repurchases	(50,000)	$ (3)			(49,997)	(50,000)
Ending balance (in shares) at Jan. 02, 2022		53,999
Ending balance at Jan. 02, 2022	$ 1,237,945	$ 540	$ 0	$ (123,048)	$ 1,360,390	$ 1,237,882	$ 63