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 Oct. 01, 2017			55,873
Beginning balance at Oct. 01, 2017	$ 928,624		$ 559	$ 193,835	$ (98,500)	$ 832,559		$ 928,453		$ 171
Comprehensive income, net of tax:
Net income	136,957					136,883		136,883		74
Foreign currency translation adjustments	(29,720)				(29,656)			(29,656)		(64)
Gain on cash flow hedge valuations	806				806			806
Comprehensive income, net of tax	108,043							108,033		10
Distributions paid to noncontrolling interests	(52)									(52)
Cash dividends	(24,477)					(24,477)		(24,477)
Stock-based compensation	19,582			19,582				19,582
Restricted & performance shares released (in shares)			277
Restricted & performance shares released	(8,871)		$ 3	(8,874)				(8,871)
Stock options exercised (in shares)			549
Stock options exercised	13,511		$ 5	13,506				13,511
Shares issued for Employee Stock Purchase Plan (in shares)			142
Shares issued for Employee Stock Purchase Plan	5,740		$ 1	5,739				5,740
Stock repurchases (in shares)			(1,492)
Stock repurchases	(75,000)		$ (15)	(74,985)				(75,000)
Ending balance (in shares) at Sep. 30, 2018			55,349
Ending 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)
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)	355		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