Earnings Per Share (''EPS'') (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 27, 2016	Mar. 29, 2015	Mar. 27, 2016	Mar. 29, 2015
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 3,744	$ 19,017	$ 26,983	$ 44,592
Weighted-average common shares outstanding - basic	58,451	61,153	58,819	61,816
Effect of dilutive stock options and unvested restricted stock	680	570	708	615
Weighted-average common stock outstanding - diluted	59,131	61,723	59,527	62,431
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.06	$ 0.31	$ 0.46	$ 0.72
Diluted (in dollars per share)	$ 0.06	$ 0.31	$ 0.45	$ 0.71
Stock options
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares	800	1,300	500	1,300