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 at Sep. 28, 2014	$ 1,013,056	$ 626	$ 402,516	$ (42,538)	$ 651,475	$ 1,012,079	$ 977
Beginning balance (in shares) at Sep. 28, 2014		62,591
Comprehensive income, net of tax:
Net income	39,228				39,074	39,074	154
Foreign currency translation adjustments	(98,287)			(98,144)		(98,144)	(143)
Gain (loss) on cash flow hedge valuations	(2,489)			(2,489)		(2,489)
Comprehensive income (loss) including noncontrolling interests	(61,548)					(61,559)	11
Distributions paid to noncontrolling interests	(515)						(515)
Cash dividends of $0.30, $0.34, and $0.38 per common share for years ended September 27, 2015, October 2, 2016, and October 1, 2017, respectively.	(18,240)				(18,240)	(18,240)
Stock-based compensation	10,926		10,926			10,926
Stock options exercised	8,990	$ 5	8,985			8,990
Stock options exercised (in shares)		510
Shares issued for Employee Stock Purchase Plan	5,203	$ 3	5,200			5,203
Shares issued for Employee Stock Purchase Plan (in shares)		243
Stock repurchases	(100,500)	$ (40)	(100,460)			(100,500)
Stock repurchases (in shares)		(3,963)
Tax benefit for stock options	(574)		(574)			(574)
Ending balance at Sep. 27, 2015	856,798	$ 594	326,593	(143,171)	672,309	856,325	473
Ending balance (in shares) at Sep. 27, 2015		59,381
Comprehensive income, net of tax:
Net income	83,853				83,783	83,783	70
Foreign currency translation adjustments	14,392			14,389		14,389	3
Gain (loss) on cash flow hedge valuations	774			774		774
Comprehensive income (loss) including noncontrolling interests	99,019					98,946	73
Distributions paid to noncontrolling interests	(402)						(402)
Cash dividends of $0.30, $0.34, and $0.38 per common share for years ended September 27, 2015, October 2, 2016, and October 1, 2017, respectively.	(19,735)				(19,735)	(19,735)
Stock-based compensation	12,964		12,964			12,964
Stock options exercised	15,823	$ 9	15,814			15,823
Stock options exercised (in shares)		920
Shares issued for Employee Stock Purchase Plan	4,707	$ 2	4,705			4,707
Shares issued for Employee Stock Purchase Plan (in shares)		209
Stock repurchases	(99,500)	$ (35)	(99,465)			(99,500)
Stock repurchases (in shares)		(3,468)
Tax benefit for stock options	(271)		(271)			(271)
Ending balance at Oct. 02, 2016	869,403	$ 570	260,340	(128,008)	736,357	869,259	144
Ending balance (in shares) at Oct. 02, 2016		57,042
Comprehensive income, net of tax:
Net income	117,917				117,874	117,874	43
Foreign currency translation adjustments	27,902			27,894		27,894	8
Gain (loss) on cash flow hedge valuations	1,614			1,614		1,614
Comprehensive income (loss) including noncontrolling interests	147,433					147,382	51
Distributions paid to noncontrolling interests	(24)						(24)
Cash dividends of $0.30, $0.34, and $0.38 per common share for years ended September 27, 2015, October 2, 2016, and October 1, 2017, respectively.	(21,672)				(21,672)	(21,672)
Stock-based compensation	13,450		13,450			13,450
Stock options exercised	15,094	$ 10	15,084			15,094
Stock options exercised (in shares)		907
Shares issued for Employee Stock Purchase Plan	4,940	$ 2	4,938			4,940
Shares issued for Employee Stock Purchase Plan (in shares)		190
Stock repurchases	(100,000)	$ (23)	(99,977)			(100,000)
Stock repurchases (in shares)		(2,266)
Ending balance at Oct. 01, 2017	$ 928,624	$ 559	$ 193,835	$ (98,500)	$ 832,559	$ 928,453	$ 171
Ending balance (in shares) at Oct. 01, 2017		55,873