Consolidated Statements of Equity - USD ($) shares in Thousands, $ in Thousands	Total	Adjustment	Total Tetra Tech Equity	Total Tetra Tech Equity Adjustment	Common Stock	Common Stock Adjustment	Additional Paid-in Capital	Accumulated Other Comprehensive (Loss) Income	Retained Earnings	Retained Earnings Adjustment	Non-Controlling Interests
Beginning balance (in shares) at Oct. 03, 2021					53,981	215,926
Beginning balance at Oct. 03, 2021	$ 1,234,291	$ 0	$ 1,234,238	$ 0	$ 540	$ 2,160	$ 0	$ (125,028)	$ 1,358,726	$ (2,160)	$ 53
Comprehensive income, net of tax:
Net income	263,164		263,125						263,125		39
Foreign currency translation adjustments	(94,933)		(94,922)					(94,922)			(11)
Pension	0
Gain on cash flow hedge valuations	11,806		11,806					11,806
Comprehensive income, net of tax	180,037		180,009								28
Distributions paid to noncontrolling interests	(31)										(31)
Cash dividends	(46,099)		(46,099)						(46,099)
Stock-based compensation	26,227		26,227				26,227
Restricted & performance shares released (in shares)					944
Restricted & performance shares released	(25,223)		(25,223)		$ 10		(25,233)
Stock options exercised (in shares)					229
Stock options exercised	1,806		1,806				1,806
Shares issued for Employee Stock Purchase Plan (in shares)					531
Shares issued for Employee Stock Purchase Plan	12,129		12,129		$ 5		12,124
Stock repurchases (in shares)					(6,708)
Stock repurchases	(200,000)		(200,000)		$ (65)		(14,884)		(185,051)
Reclassification of APIC	0		0				(40)		40
Ending balance (in shares) at Oct. 02, 2022					264,903
Ending balance at Oct. 02, 2022	1,183,137		1,183,087		$ 2,650		0	(208,144)	1,388,581		50
Comprehensive income, net of tax:
Net income	273,452		273,420						273,420		32
Foreign currency translation adjustments	12,622		12,623					12,623			(1)
Pension	2,638		2,638					2,638
Gain on cash flow hedge valuations	(2,412)		(2,412)					(2,412)
Comprehensive income, net of tax	286,300		286,269								31
Distributions paid to noncontrolling interests	(8)										(8)
Cash dividends	(52,113)		(52,113)						(52,113)
Stock-based compensation	28,607		28,607				28,607
Restricted & performance shares released (in shares)					746
Restricted & performance shares released	(16,833)		(16,833)		$ 7		(16,840)
Stock options exercised (in shares)					97
Stock options exercised	626		626				626
Shares issued for Employee Stock Purchase Plan (in shares)					492
Shares issued for Employee Stock Purchase Plan	12,628		12,628		$ 5		12,623
Reclassification of APIC	0		0				26,734		(26,734)
Capped call transactions	(38,838)		(38,838)				(51,750)		12,912
Ending balance (in shares) at Oct. 01, 2023					266,238
Ending balance at Oct. 01, 2023	1,403,506		1,403,433		$ 2,662		0	(195,295)	1,596,066		73
Comprehensive income, net of tax:
Net income	333,443		333,382						333,382		61
Foreign currency translation adjustments	115,120		115,120					115,120
Pension	1,300		1,300					1,300
Gain on cash flow hedge valuations	0
Comprehensive income, net of tax	449,863		449,802								61
Distributions paid to noncontrolling interests	(43)										(43)
Cash dividends	(58,828)		(58,828)						(58,828)
Stock-based compensation	31,155		31,155				31,155
Restricted & performance shares released (in shares)					547
Restricted & performance shares released	$ (12,982)		(12,982)		$ 5		(12,987)
Stock options exercised (in shares)	410				410
Stock options exercised	$ 3,067		3,067		$ 4		3,063
Shares issued for Employee Stock Purchase Plan (in shares)					522
Shares issued for Employee Stock Purchase Plan	14,675		14,675		$ 6		14,669
Ending balance (in shares) at Sep. 29, 2024					267,717
Ending balance at Sep. 29, 2024	$ 1,830,413		$ 1,830,322		$ 2,677		$ 35,900	$ (78,875)	$ 1,870,620		$ 91