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 Sep. 27, 2009	$ 646,478	$ 613	$ 350,571	$ 12,226	$ 283,068	$ 646,478
BALANCE (in shares) at Sep. 27, 2009		61,257
Comprehensive income:
Net income	76,819				76,819	76,819		76,819
Foreign currency translation adjustment	6,874			6,874		6,874		6,874
Foreign currency hedge	(337)			(337)		(337)		(337)
Comprehensive income including noncontrolling interests	83,356					83,356		83,356
Stock-based compensation	10,178		10,178			10,178
Stock options exercised	2,513	3	2,510			2,513
Stock options exercised (in shares)		291
Shares issued for Employee Stock Purchase Plan	4,742	2	4,740			4,742
Shares issued for Employee Stock Purchase Plan (in shares)		207
Tax benefit for stock options	866		866			866
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 income	92,982				90,039	90,039	2,943	92,982
Foreign currency translation adjustment	(13,955)			(14,447)		(14,447)	492	(13,955)
Foreign currency hedge	438			438		438		438
Comprehensive income 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 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 income	104,732				104,380	104,380	352	104,732
Foreign currency translation adjustment	26,486			26,457		26,457	29	26,486
Foreign currency hedge	(194)			(194)		(194)		(194)
Comprehensive income 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 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