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	129,234				129,205	129,205	29
Other comprehensive income (loss)	(17,154)			(17,148)		(17,148)	(6)
Distributions paid to noncontrolling interests	(69)						(69)
Cash dividends per common share	(25,590)				(25,590)	(25,590)
Stock-based compensation	13,494		13,494			13,494
Restricted & performance shares released (in shares)		210
Restricted & performance shares released	(11,143)	$ 2	(11,145)			(11,143)
Stock options exercised (in shares)		283
Stock options exercised	8,263	$ 3	8,260			8,263
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)		(1,338)
Stock repurchases	(102,188)	$ (13)	(97,455)		(4,720)	(102,188)
Ending balance (in shares) at Jun. 28, 2020		53,888
Ending balance at Jun. 28, 2020	993,026	$ 539	0	(177,732)	1,170,087	992,894	132
Beginning balance (in shares) at Mar. 29, 2020		54,142
Beginning balance at Mar. 29, 2020	950,238	$ 541	10,473	(199,391)	1,138,485	950,108	130
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	45,503				45,497	45,497	6
Other comprehensive income (loss)	21,655			21,659		21,659	(4)
Cash dividends per common share	(9,175)				(9,175)	(9,175)
Stock-based compensation	4,057		4,057			4,057
Restricted & performance shares released (in shares)		(5)
Restricted & performance shares released	(45)		(45)			(45)
Stock options exercised (in shares)		14
Stock options exercised	336		336			336
Stock repurchases (in shares)		(263)
Stock repurchases	(19,543)	$ (2)	(14,821)		(4,720)	(19,543)
Ending balance (in shares) at Jun. 28, 2020		53,888
Ending balance at Jun. 28, 2020	993,026	$ 539	0	(177,732)	1,170,087	992,894	132
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	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)	$ 2	(17,591)			(17,589)
Stock options exercised (in shares)		304
Stock options exercised	10,703	$ 4	10,699			10,703
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)		(367)
Stock repurchases	(45,000)	$ (4)	(20,074)		(24,922)	(45,000)
Ending balance (in shares) at Jun. 27, 2021		54,071
Ending balance at Jun. 27, 2021	1,192,495	$ 541	0	(102,401)	1,294,260	1,192,400	95
Beginning balance (in shares) at Mar. 28, 2021		54,158
Beginning 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]
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	$ 1	931			932
Stock repurchases (in shares)		(118)
Stock repurchases	(15,000)	$ (2)	(6,525)		(8,473)	(15,000)
Ending balance (in shares) at Jun. 27, 2021		54,071
Ending balance at Jun. 27, 2021	$ 1,192,495	$ 541	$ 0	$ (102,401)	$ 1,294,260	$ 1,192,400	$ 95