Quarterly Financial Information-Unaudited (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
Acquisition and integration expenses									$ 19,548
Pre-payment charges paid			$ 1,900						1,900
Impairment charge					$ 60,800					$ 60,800
Impairment charge after tax					57,300					57,300
Revenue	$ 728,508	$ 666,869	627,384	$ 560,708	578,394	$ 575,108	$ 564,763	$ 581,056	2,583,469	2,299,321	$ 2,483,814
Operating income (loss)	47,190	39,085	16,650	32,930	(20,047)	40,721	30,398	36,612	135,855	87,684	153,833
Net income (loss) 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
Earnings (loss) per share attributable to Tetra Tech (1):
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
Weighted-average common shares outstanding:
Basic (in shares)	57,309	57,796	58,451	59,058	59,963	60,207	61,153	62,452	58,186	60,913	64,379
Diluted (in shares)	58,192	58,616	59,131	59,793	59,963	60,792	61,723	63,112	58,966	61,532	65,146
Coffey
Acquisition and integration expenses	$ 2,600	$ 1,000	$ 15,900						$ 19,500
Coffey | Interest expense.
Pre-payment charges paid			$ 1,900