Earnings Per Share - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Oct. 02, 2016	Jun. 26, 2016	Mar. 27, 2016	Dec. 27, 2015	Sep. 27, 2015	Jun. 28, 2015	Mar. 29, 2015	Dec. 28, 2014	Oct. 02, 2016	Sep. 27, 2015	Sep. 28, 2014
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 31,106	$ 25,694	$ 3,744	$ 23,239	$ (31,724)	$ 26,206	$ 19,017	$ 25,575	$ 83,783	$ 39,074	$ 108,266
Weighted-average common shares outstanding - basic	57,309	57,796	58,451	59,058	59,963	60,207	61,153	62,452	58,186	60,913	64,379
Effect of dilutive stock options and unvested restricted stock									780	619	767
Weighted-average common stock outstanding - diluted	58,192	58,616	59,131	59,793	59,963	60,792	61,723	63,112	58,966	61,532	65,146
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.54	$ 0.44	$ 0.06	$ 0.39	$ (0.53)	$ 0.44	$ 0.31	$ 0.41	$ 1.44	$ 0.64	$ 1.68
Diluted (in dollars per share)	$ 0.53	$ 0.44	$ 0.06	$ 0.39	$ (0.53)	$ 0.43	$ 0.31	$ 0.41	$ 1.42	$ 0.64	$ 1.66