Consolidated Statements of Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (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
Comprehensive income, net of tax:
Net income	232,831				232,810	232,810	21
Foreign currency translation adjustments	30,644			30,641		30,641	3
Net pension adjustments	0
Gain on cash flow hedge valuations	6,117			6,117		6,117
Comprehensive income, net of tax	269,592					269,568	24
Distributions paid to noncontrolling interests	(25)						(25)
Cash dividends	(40,041)				(40,041)	(40,041)
Stock-based compensation	23,067		23,067			23,067
Restricted & performance shares released (in shares)		215
Restricted & performance shares released	(17,630)	$ 3	(17,633)			(17,630)
Stock options exercised (in shares)		324
Stock options exercised	11,250	$ 3	11,247			11,250
Shares issued for Employee Stock Purchase Plan (in shares)		124
Shares issued for Employee Stock Purchase Plan	10,705	$ 1	10,704			10,705
Stock repurchases (in shares)		(479)
Stock repurchases	(60,000)	$ (5)	(27,385)		(32,610)	(60,000)
Ending balance (in shares) at Oct. 03, 2021		53,981
Ending balance at Oct. 03, 2021	1,234,291	$ 540	0	(125,028)	1,358,726	1,234,238	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)
Net pension adjustments	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)		190
Restricted & performance shares released	(25,223)	$ 2	(25,225)			(25,223)
Stock options exercised (in shares)		46
Stock options exercised	1,806	$ 0	1,806			1,806
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)		(1,342)
Stock repurchases	(200,000)	$ (13)	(14,936)		(185,051)	(200,000)
Ending balance (in shares) at Oct. 02, 2022		52,981
Ending balance at Oct. 02, 2022	1,183,137	$ 530	0	(208,144)	1,390,701	1,183,087	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)
Net pension adjustments	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)		149
Restricted & performance shares released	$ (16,833)	$ 1	(16,834)			(16,833)
Stock options exercised (in shares)	20	19
Stock options exercised	$ 626	$ 0	626			626
Shares issued for Employee Stock Purchase Plan (in shares)		99
Shares issued for Employee Stock Purchase Plan	12,628	$ 1	12,627			12,628
Reclassification of APIC	0		26,724		(26,724)	0
Capped call transactions	(38,838)		(51,750)		12,912	(38,838)
Ending balance (in shares) at Oct. 01, 2023		53,248
Ending balance at Oct. 01, 2023	$ 1,403,506	$ 532	$ 0	$ (195,295)	$ 1,598,196	$ 1,403,433	$ 73