Earnings Per Share (''EPS'') (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 28, 2015	Jun. 29, 2014	Jun. 28, 2015	Jun. 29, 2014
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 26,206	$ 26,657	$ 70,798	$ 85,680
Weighted-average common shares outstanding - basic	60,207	64,566	61,293	64,683
Effect of diluted stock options and unvested restricted stock	585	736	594	810
Weighted-average common stock outstanding - diluted	60,792	65,302	61,887	65,493
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.44	$ 0.41	$ 1.16	$ 1.32
Diluted (in dollars per share)	$ 0.43	$ 0.41	$ 1.14	$ 1.31
Stock options
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares	1,100	700	1,300	0