Consolidated Statements of Equity - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Retained Earnings	Total Tetra Tech Equity	Non-Controlling Interests	Comprehensive Income	Total
BALANCE at Sep. 30, 2012	$ 638	$ 433,009	$ 31,017	$ 554,306	$ 1,018,970	$ 897		$ 1,019,867
BALANCE (in shares) at Sep. 30, 2012	63,837
Comprehensive income, net of tax:
Net income (loss)				(2,141)	(2,141)	635	$ (1,506)	(1,506)
Foreign currency translation adjustments			(28,770)		(28,770)	(47)	(28,817)	(28,817)
Gain (loss) on cash flow hedge valuations			(389)		(389)		(389)	(389)
Comprehensive income (loss), net of tax					(31,300)	588	(30,712)	(30,712)
Distributions paid to noncontrolling interests						(445)		(445)
Stock-based compensation		8,775			8,775			8,775
Stock options exercised	$ 9	14,872			14,881			14,881
Stock options exercised (in shares)	899
Shares issued for Employee Stock Purchase Plan	$ 3	5,548			5,551			5,551
Shares issued for Employee Stock Purchase Plan (in shares)	253
Stock repurchases	$ (9)	(19,991)			(20,000)			(20,000)
Stock repurchases (in shares)	(855)
Tax expense (benefit) for stock options		886			886			886
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 (loss)				108,266	108,266	409	108,675	108,675
Foreign currency translation adjustments			(45,425)		(45,425)	(55)	(45,480)	(45,480)
Gain (loss) on cash flow hedge valuations			1,029		1,029		1,029	1,029
Comprehensive income (loss), net of tax					63,870	354	64,224	64,224
Distributions paid to noncontrolling interests						(417)		(417)
Dividends				(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 (loss)				39,074	39,074	154	39,228	39,228
Foreign currency translation adjustments			(98,144)		(98,144)	(143)	(98,287)	(98,287)
Gain (loss) on cash flow hedge valuations			(2,489)		(2,489)		(2,489)	(2,489)
Comprehensive income (loss), net of tax					(61,559)	11	$ (61,548)	(61,548)
Distributions paid to noncontrolling interests						(515)		(515)
Dividends				(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