Consolidated Statements of Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Retained Earnings	Total Tetra Tech Equity	Non-Controlling Interests	Comprehensive Income
BALANCE at Oct. 03, 2010	$ 748,133	$ 618	$ 368,865	$ 18,763	$ 359,887	$ 748,133
BALANCE (in shares) at Oct. 03, 2010		61,755
Comprehensive income, net of tax:
Net income (loss)	92,982				90,039	90,039	2,943	92,982
Foreign currency translation adjustments	(13,955)			(14,447)		(14,447)	492	(13,955)
Gain (loss) on cash flow hedge valuations	438			438		438		438
Comprehensive income (loss) including noncontrolling interests	79,465					76,030	3,435	79,465
Adjustments for consolidation of variable interest entities	670						670
Noncontrolling interest from business acquisitions	438						438
Acquisition of noncontrolling interests	4,567		6,883			6,883	(2,316)
Distributions paid to noncontrolling interests	(1,702)						(1,702)
Stock-based compensation	10,582		10,582			10,582
Stock options exercised	8,004	4	8,000			8,004
Stock options exercised (in shares)		443
Shares issued for Employee Stock Purchase Plan	5,249	3	5,246			5,249
Shares issued for Employee Stock Purchase Plan (in shares)		297
Tax expense (benefit) for stock options	(156)		(156)			(156)
BALANCE at Oct. 02, 2011	855,250	625	399,420	4,754	449,926	854,725	525
BALANCE (in shares) at Oct. 02, 2011		62,495
Comprehensive income, net of tax:
Net income (loss)	104,732				104,380	104,380	352	104,732
Foreign currency translation adjustments	26,486			26,457		26,457	29	26,486
Gain (loss) on cash flow hedge valuations	(194)			(194)		(194)		(194)
Comprehensive income (loss) including noncontrolling interests	131,024					130,643	381	131,024
Distributions paid to noncontrolling interests	(9)						(9)
Stock-based compensation	10,839		10,839			10,839
Stock options exercised	17,535	10	17,525			17,535
Stock options exercised (in shares)		1,053
Shares issued for Employee Stock Purchase Plan	5,300	3	5,297			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	1,019,867	638	433,009	31,017	554,306	1,018,970	897
BALANCE (in shares) at Sep. 30, 2012		63,837
Comprehensive income, net of tax:
Net income (loss)	(1,506)				(2,141)	(2,141)	635	(1,506)
Foreign currency translation adjustments	(28,817)			(28,770)		(28,770)	(47)	(28,817)
Gain (loss) on cash flow hedge valuations	(389)			(389)		(389)		(389)
Comprehensive income (loss) including noncontrolling interests	(30,712)					(31,300)	588	(30,712)
Distributions paid to noncontrolling interests	(445)						(445)
Stock-based compensation	8,775		8,775			8,775
Stock options exercised	14,881	9	14,872			14,881
Stock options exercised (in shares)		899
Shares issued for Employee Stock Purchase Plan	5,551	3	5,548			5,551
Shares issued for Employee Stock Purchase Plan (in shares)		253
Stock repurchases	(20,000)	(9)	(19,991)			(20,000)
Stock repurchases (in shares)		(855)
Tax expense (benefit) for stock options	886		886			886
BALANCE at Sep. 29, 2013	$ 998,803	$ 641	$ 443,099	$ 1,858	$ 552,165	$ 997,763	$ 1,040
BALANCE (in shares) at Sep. 29, 2013		64,134