Consolidated Statements of Equity - USD ($) shares in Thousands, $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Total Tetra Tech Equity	Total Tetra Tech Equity Cumulative Effect, Period of Adoption, Adjustment	Non-Controlling Interests
Beginning balance (in shares) at Sep. 30, 2018			55,349
Beginning balance at Sep. 30, 2018	$ 967,100	$ (2,767)	$ 553	$ 148,803	$ (127,350)	$ 944,965	$ (2,767)	$ 966,971	$ (2,767)	$ 129
Comprehensive income, net of tax:
Net income	158,761					158,668		158,668		93
Foreign currency translation adjustments	(20,866)				(21,109)			(21,109)		243
Gain on cash flow hedge valuations	(12,125)				(12,125)			(12,125)
Comprehensive income, net of tax	125,770							125,434		336
Distributions paid to noncontrolling interests	(287)									(287)
Cash dividends	(29,674)					(29,674)		(29,674)
Stock-based compensation	17,618			17,618				17,618
Restricted & performance shares released (in shares)			183
Restricted & performance shares released	(6,893)		$ 2	(6,895)				(6,893)
Stock options exercised (in shares)			448
Stock options exercised	11,751		$ 5	11,746				11,751
Shares issued for Employee Stock Purchase Plan (in shares)			148
Shares issued for Employee Stock Purchase Plan	6,846		$ 2	6,844				6,846
Stock repurchases (in shares)			(1,563)
Stock repurchases	(100,000)		$ (16)	(99,984)				(100,000)
Ending balance (in shares) at Sep. 29, 2019			54,565
Ending 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	(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	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		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