Consolidated Statements of Equity (USD $) In Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Total Tetra Tech Equity	Non-Controlling Interests	Comprehensive Income
Balance at Sep. 28, 2008	$ 511,514	$ 599	$ 314,860	$ 15	$ 196,040	$ 511,514
Balance (in shares) at Sep. 28, 2008		59,875
Comprehensive income:
Net income	87,028				87,028	87,028		87,028
Foreign currency translation adjustment	12,211			12,211		12,211		12,211
Comprehensive income	99,239					99,239		99,239
Stock-based compensation	9,392		9,392			9,392
Stock options exercised	19,836	12	19,824			19,836
Stock options exercised (in shares)		1,219
Shares issued for Employee Stock Purchase Plan	3,340	2	3,338			3,340
Shares issued for Employee Stock Purchase Plan (in shares)		163
Tax benefit for stock options	3,157		3,157			3,157
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	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	90,039
Foreign currency translation adjustment	(13,955)			(14,447)		(14,447)	492	(14,447)
Foreign currency hedge	438			438		438		438
Comprehensive income	79,465					76,030	3,435	76,030
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