Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jul. 01, 2012	Apr. 01, 2012	Jan. 01, 2012	Oct. 02, 2011	Jul. 03, 2011	Apr. 03, 2011	Jan. 02, 2011	Sep. 30, 2012	Oct. 02, 2011	Oct. 03, 2010
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 30,432	$ 29,054	$ 22,284	$ 22,610	$ 26,399	$ 23,839	$ 17,500	$ 22,301	$ 104,380	$ 90,039	$ 76,819
Weighted-average common shares outstanding - basic (in shares)	63,623,000	63,387,000	63,072,000	62,433,000	62,310,000	62,203,000	62,121,000	61,665,000	63,217,000	62,053,000	61,430,000
Effect of diluted stock options and unvested restricted stock (in shares)									717,000	722,000	657,000
Weighted-average common stock outstanding - diluted (in shares)	64,396,000	64,179,000	63,817,000	63,068,000	62,864,000	62,934,000	62,945,000	62,443,000	63,934,000	62,775,000	62,087,000
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.48	$ 0.46	$ 0.35	$ 0.36	$ 0.42	$ 0.38	$ 0.28	$ 0.36	$ 1.65	$ 1.45	$ 1.25
Diluted (in dollars per share)	$ 0.47	$ 0.45	$ 0.35	$ 0.36	$ 0.42	$ 0.38	$ 0.28	$ 0.36	$ 1.63	$ 1.43	$ 1.24
Options excluded from the calculation of dilutive potential common shares (in shares)									1,900,000	2,600,000	3,100,000