Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 30, 2012	Jul. 01, 2012	Apr. 01, 2012	Jan. 01, 2012	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Number of weighted-average shares used to compute basic and diluted EPS:
Net income (loss) attributable to Tetra Tech	$ 25,200	$ (78,385)	$ 24,820	$ 26,224	$ 30,432	$ 29,054	$ 22,284	$ 22,610	$ (2,141)	$ 104,380	$ 90,039
Weighted-average common shares outstanding - basic	64,272,000	64,832,000	64,551,000	63,864,000	63,623,000	63,387,000	63,072,000	62,433,000	64,544,000	63,217,000	62,053,000
Effect of diluted stock options and unvested restricted stock										717,000	722,000
Weighted-average common stock outstanding - diluted	64,853,000	64,832,000	65,472,000	64,608,000	64,396,000	64,179,000	63,817,000	63,068,000	64,544,000	63,934,000	62,775,000
Net income (loss) attributable to Tetra Tech per share:
Basic (in dollars per share)	$ 0.39	$ (1.21)	$ 0.38	$ 0.41	$ 0.48	$ 0.46	$ 0.35	$ 0.36	$ (0.03)	$ 1.65	$ 1.45
Diluted (in dollars per share)	$ 0.39	$ (1.21)	$ 0.38	$ 0.41	$ 0.47	$ 0.45	$ 0.35	$ 0.36	$ (0.03)	$ 1.63	$ 1.43
Options
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares										1,900,000	2,600,000
Potential common shares
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares									500,000