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 Oct. 01, 2017		55,873
Beginning balance at Oct. 01, 2017	$ 928,624	$ 559	$ 193,834	$ (98,500)	$ 832,560	$ 928,453	$ 171
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	108,144				108,082	108,082	62
Other comprehensive (loss) income	(33,248)			(33,238)		(33,238)	(10)
Distributions paid to noncontrolling interests	(21)					0	(21)
Cash dividends per common share	(17,836)				(17,836)	(17,836)
Stock-based compensation	15,519		15,519			15,519
Restricted & performance shares released (in shares)		272
Restricted & performance shares released	(8,825)	$ 3	(8,828)			(8,825)
Stock options exercised (in shares)		537
Stock options exercised	13,196	$ 5	13,191			13,196
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 Jul. 01, 2018		55,332
Ending balance at Jul. 01, 2018	936,293	$ 553	144,470	(131,738)	922,806	936,091	202
Beginning balance (in shares) at Oct. 01, 2017		55,873
Beginning balance at Oct. 01, 2017	928,624	$ 559	193,834	(98,500)	832,560	928,453	171
Ending balance (in shares) at Sep. 30, 2018		55,349
Ending balance at Sep. 30, 2018	967,100	$ 553	148,803	(127,350)	944,965	966,971	129
Beginning balance (in shares) at Apr. 01, 2018		55,749
Beginning balance at Apr. 01, 2018	940,229	$ 557	161,387	(118,044)	896,148	940,048	181
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	33,345				33,322	33,322	23
Other comprehensive (loss) income	(13,696)			(13,694)		(13,694)	(2)
Cash dividends per common share	(6,664)				(6,664)	(6,664)
Stock-based compensation	6,814		6,814			6,814
Restricted & performance shares released (in shares)		(1)
Restricted & performance shares released	(5)	$ 1	(6)			(5)
Stock options exercised (in shares)		56
Stock options exercised	1,258	$ 0	1,258			1,258
Shares issued for Employee Stock Purchase Plan (in shares)		0
Shares issued for Employee Stock Purchase Plan	14	$ 0	14			14
Stock repurchases (in shares)		(472)
Stock repurchases	(25,002)	$ (5)	(24,997)			(25,002)
Ending balance (in shares) at Jul. 01, 2018		55,332
Ending balance at Jul. 01, 2018	936,293	$ 553	144,470	(131,738)	922,806	936,091	202
Beginning balance (in shares) at Sep. 30, 2018		55,349
Beginning balance at Sep. 30, 2018	967,100	$ 553	148,803	(127,350)	944,965	966,971	129
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	147,227				147,141	147,141	86
Other comprehensive (loss) income	(19,486)			(19,728)		(19,728)	242
Distributions paid to noncontrolling interests	(276)					0	(276)
Cash dividends per common share	(21,489)				(21,489)	(21,489)
Stock-based compensation	12,717		12,717			12,717
Restricted & performance shares released (in shares)		180
Restricted & performance shares released	(6,840)	$ 2	(6,842)			(6,840)
Stock options exercised (in shares)		293
Stock options exercised	7,759	$ 3	7,756			7,759
Shares issued for Employee Stock Purchase Plan (in shares)		148
Shares issued for Employee Stock Purchase Plan	6,847	$ 2	6,845			6,847
Stock repurchases (in shares)		(1,256)
Stock repurchases	(75,000)	$ (13)	(74,987)			(75,000)
Ending balance (in shares) at Jun. 30, 2019		54,714
Ending balance at Jun. 30, 2019	1,015,793	$ 547	94,292	(147,078)	1,067,851	1,015,612	181
Beginning balance (in shares) at Mar. 31, 2019		54,947
Beginning balance at Mar. 31, 2019	990,606	$ 549	111,277	(148,217)	1,026,837	990,446	160
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	49,251				49,233	49,233	18
Other comprehensive (loss) income	1,142			1,139		1,139	3
Cash dividends per common share	(8,219)				(8,219)	(8,219)
Stock-based compensation	4,122		4,122			4,122
Restricted & performance shares released (in shares)		1
Restricted & performance shares released	(33)	$ 1	(34)			(33)
Stock options exercised (in shares)		142
Stock options exercised	3,924	$ 1	3,923			3,924
Stock repurchases (in shares)		(376)
Stock repurchases	(25,000)	$ (4)	(24,996)			(25,000)
Ending balance (in shares) at Jun. 30, 2019		54,714
Ending balance at Jun. 30, 2019	$ 1,015,793	$ 547	$ 94,292	$ (147,078)	$ 1,067,851	$ 1,015,612	$ 181