Earnings Per Share ("EPS") (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Jul. 01, 2012	Jul. 03, 2011	Jul. 01, 2012	Jul. 03, 2011
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 29,054	$ 23,839	$ 73,948	$ 63,639
Weighted-average common shares outstanding - basic	63,387,000	62,203,000	63,054,000	61,967,000
Effect of diluted stock options and unvested restricted stock (in shares)	792,000	731,000	698,000	778,000
Weighted-average common stock outstanding - diluted (in shares)	64,179,000	62,934,000	63,752,000	62,745,000
Earnings per share attributable to Tetra Tech
Basic (in dollars per share)	$ 0.46	$ 0.38	$ 1.17	$ 1.03
Diluted (in dollars per share)	$ 0.45	$ 0.38	$ 1.16	$ 1.01
Options excluded from the calculation of dilutive potential common shares	2,100,000	2,900,000	2,900,000	2,700,000