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. 29, 2019		54,565
Beginning balance at Sep. 29, 2019	$ 989,464	$ 546	$ 78,132	$ (160,584)	$ 1,071,192	$ 989,286	$ 178
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	83,730				83,707	83,707	23
Other comprehensive income (loss)	(38,809)			(38,807)		(38,807)	(2)
Distributions paid to noncontrolling interests	(69)						(69)
Cash dividends per common share	(16,414)				(16,414)	(16,414)
Stock-based compensation	9,437		9,437			9,437
Restricted & performance shares released (in shares)		215
Restricted & performance shares released	(11,098)	$ 2	(11,100)			(11,098)
Stock options exercised (in shares)		269
Stock options exercised	7,927	$ 3	7,924			7,927
Shares issued for Employee Stock Purchase Plan (in shares)		168
Shares issued for Employee Stock Purchase Plan	8,715	$ 1	8,714			8,715
Stock repurchases (in shares)		(1,075)
Stock repurchases	(82,645)	$ (11)	(82,634)			(82,645)
Ending balance (in shares) at Mar. 29, 2020		54,142
Ending balance at Mar. 29, 2020	950,238	$ 541	10,473	(199,391)	1,138,485	950,108	130
Beginning balance (in shares) at Dec. 29, 2019		54,728
Beginning balance at Dec. 29, 2019	1,026,765	$ 547	60,747	(145,015)	1,110,312	1,026,591	174
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	36,413				36,397	36,397	16
Other comprehensive income (loss)	(54,380)			(54,376)		(54,376)	(4)
Distributions paid to noncontrolling interests	(56)						(56)
Cash dividends per common share	(8,224)				(8,224)	(8,224)
Stock-based compensation	4,955		4,955			4,955
Restricted & performance shares released (in shares)		30
Restricted & performance shares released	(280)	$ 1	(281)			(280)
Stock options exercised (in shares)		215
Stock options exercised	6,513	$ 2	6,511			6,513
Stock repurchases (in shares)		(831)
Stock repurchases	(61,468)	$ (9)	(61,459)			(61,468)
Ending balance (in shares) at Mar. 29, 2020		54,142
Ending balance at Mar. 29, 2020	950,238	$ 541	10,473	(199,391)	1,138,485	950,108	130
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 (loss)	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
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 (loss)	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