Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 27, 2015	Jun. 28, 2015	Mar. 29, 2015	Dec. 28, 2014	Sep. 28, 2014	Jun. 29, 2014	Mar. 30, 2014	Dec. 29, 2013	Sep. 27, 2015	Sep. 28, 2014	Sep. 29, 2013
Number of weighted-average shares used to compute basic and diluted EPS:
Net income (loss) attributable to Tetra Tech	$ (31,724)	$ 26,206	$ 19,017	$ 25,575	$ 22,585	$ 26,657	$ 31,709	$ 27,315	$ 39,074	$ 108,266	$ (2,141)
Weighted-average common shares outstanding - basic	59,963	60,207	61,153	62,452	63,602	64,566	64,835	64,227	60,913	64,379	64,544
Effect of diluted stock options and unvested restricted stock									619	767
Weighted-average common stock outstanding - diluted	59,963	60,792	61,723	63,112	64,235	65,302	65,710	65,048	61,532	65,146	64,544
Net income (loss) attributable to Tetra Tech per share:
Basic	$ (0.53)	$ 0.44	$ 0.31	$ 0.41	$ 0.36	$ 0.41	$ 0.49	$ 0.43	$ 0.64	$ 1.68	$ (0.03)
Diluted	$ (0.53)	$ 0.43	$ 0.31	$ 0.41	$ 0.35	$ 0.41	$ 0.48	$ 0.42	$ 0.64	$ 1.66	$ (0.03)
Stock options
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares									1,000	0
Potential common shares
Antidilutive securities
Securities excluded from the calculation of dilutive potential common shares											500