Consolidated Statements of Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Total Tetra Tech Equity	Non-Controlling Interests	Total
BALANCE at Sep. 29, 2013	$ 641	$ 443,099	$ 1,858	$ 552,165	$ 997,763	$ 1,040	$ 998,803
BALANCE (in shares) at Sep. 29, 2013	64,134
Comprehensive income, net of tax:
Net income				108,266	108,266	409	108,675
Foreign currency translation adjustments			(45,425)		(45,425)	(55)	(45,480)
Gain (loss) on cash flow hedge valuations			1,029		1,029		1,029
Comprehensive income (loss), net of tax					63,870	354	64,224
Distributions paid to noncontrolling interests						(417)	(417)
Cash dividends of $0.14, $0.30 and $0.34 per common share for years ended September 28, 2014, September 27, 2015 and October 2, 2016 respectively.				(8,956)	(8,956)		(8,956)
Stock-based compensation		10,374			10,374		10,374
Stock options exercised	$ 13	22,956			22,969		22,969
Stock options exercised (in shares)	1,263
Shares issued for Employee Stock Purchase Plan	$ 2	5,597			5,599		5,599
Shares issued for Employee Stock Purchase Plan (in shares)	246
Stock repurchases	$ (30)	(79,970)			(80,000)		(80,000)
Stock repurchases (in shares)	(3,052)
Tax expense (benefit) for stock options		460			460		460
BALANCE at Sep. 28, 2014	$ 626	402,516	(42,538)	651,475	1,012,079	977	1,013,056
BALANCE (in shares) at Sep. 28, 2014	62,591
Comprehensive income, net of tax:
Net income				39,074	39,074	154	39,228
Foreign currency translation adjustments			(98,144)		(98,144)	(143)	(98,287)
Gain (loss) on cash flow hedge valuations			(2,489)		(2,489)		(2,489)
Comprehensive income (loss), net of tax					(61,559)	11	(61,548)
Distributions paid to noncontrolling interests						(515)	(515)
Cash dividends of $0.14, $0.30 and $0.34 per common share for years ended September 28, 2014, September 27, 2015 and October 2, 2016 respectively.				(18,240)	(18,240)		(18,240)
Stock-based compensation		10,926			10,926		10,926
Stock options exercised	$ 5	8,985			8,990		8,990
Stock options exercised (in shares)	510
Shares issued for Employee Stock Purchase Plan	$ 3	5,200			5,203		5,203
Shares issued for Employee Stock Purchase Plan (in shares)	243
Stock repurchases	$ (40)	(100,460)			(100,500)		(100,500)
Stock repurchases (in shares)	(3,963)
Tax expense (benefit) for stock options		(574)			(574)		(574)
BALANCE at Sep. 27, 2015	$ 594	326,593	(143,171)	672,309	856,325	473	856,798
BALANCE (in shares) at Sep. 27, 2015	59,381
Comprehensive income, net of tax:
Net income				83,783	83,783	70	83,853
Foreign currency translation adjustments			14,389		14,389	3	14,392
Gain (loss) on cash flow hedge valuations			774		774		774
Comprehensive income (loss), net of tax					98,946	73	99,019
Distributions paid to noncontrolling interests						(402)	(402)
Cash dividends of $0.14, $0.30 and $0.34 per common share for years ended September 28, 2014, September 27, 2015 and October 2, 2016 respectively.				(19,735)	(19,735)		(19,735)
Stock-based compensation		12,964			12,964		12,964
Stock options exercised	$ 9	15,814			15,823		15,823
Stock options exercised (in shares)	920
Shares issued for Employee Stock Purchase Plan	$ 2	4,705			4,707		4,707
Shares issued for Employee Stock Purchase Plan (in shares)	209
Stock repurchases	$ (35)	(99,465)			(99,500)		(99,500)
Stock repurchases (in shares)	(3,468)
Tax expense (benefit) for stock options		(271)			(271)		(271)
BALANCE at Oct. 02, 2016	$ 570	$ 260,340	$ (128,008)	$ 736,357	$ 869,259	$ 144	$ 869,403
BALANCE (in shares) at Oct. 02, 2016	57,042