Consolidated Statements of Equity (USD $) In Thousands, unless otherwise specified	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Retained Earnings	Total Tetra Tech Equity	Non-Controlling Interests	Comprehensive Income	Total
BALANCE at Oct. 02, 2011	$ 625	$ 399,420	$ 4,754	$ 449,926	$ 854,725	$ 525		$ 855,250
BALANCE (in shares) at Oct. 02, 2011	62,495
Comprehensive income, net of tax:
Net income (loss)				104,380	104,380	352	104,732	104,732
Foreign currency translation adjustments			26,457		26,457	29	26,486	26,486
Gain (loss) on cash flow hedge valuations			(194)		(194)		(194)	(194)
Comprehensive income (loss) including noncontrolling interests					130,643	381	131,024	131,024
Distributions paid to noncontrolling interests						(9)		(9)
Stock-based compensation		10,839			10,839			10,839
Stock options exercised	10	17,525			17,535			17,535
Stock options exercised (in shares)	1,053
Shares issued for Employee Stock Purchase Plan	3	5,297			5,300			5,300
Shares issued for Employee Stock Purchase Plan (in shares)	289
Tax expense (benefit) for stock options		(72)			(72)			(72)
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) including noncontrolling interests					(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) including noncontrolling interests					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