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. 29, 2019		54,565
Beginning balance at Sep. 29, 2019	$ 989,464	$ 546	$ 78,132	$ (160,584)	$ 1,071,192	$ 989,286	$ 178
Comprehensive income, net of tax:
Net income	173,890				173,859	173,859	31
Foreign currency translation adjustments	3,435			3,436		3,436	(1)
Gain (loss) on cash flow hedge valuations, net of tax	(4,638)			(4,638)		(4,638)
Comprehensive income, net of tax	172,687					172,657	30
Distributions paid to noncontrolling interests	(154)						(154)
Cash dividends	(34,743)				(34,743)	(34,743)
Stock-based compensation	19,424		19,424			19,424
Restricted & performance shares released (in shares)		212
Restricted & performance shares released	(11,166)	$ 2	(11,168)			(11,166)
Stock options exercised (in shares)		361
Stock options exercised	10,334	$ 4	10,330			10,334
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)		(1,509)
Stock repurchases	(117,188)	$ (15)	(105,432)		(11,741)	(117,188)
Ending balance (in shares) at Sep. 27, 2020		53,797
Ending 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
Gain (loss) on cash flow hedge valuations, net of tax	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)
Gain (loss) on cash flow hedge valuations, net of tax	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	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