Quarterly Financial Information-Unaudited (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Oct. 01, 2017	Jul. 02, 2017	Apr. 02, 2017	Jan. 01, 2017	Oct. 02, 2016	Jun. 26, 2016	Mar. 27, 2016	Dec. 27, 2015	Oct. 01, 2017	Oct. 02, 2016	Sep. 27, 2015
Business acquisition
Acquisition and integration expenses									$ 0	$ 19,548	$ 0
Revenue	$ 735,188	$ 685,539	$ 663,781	$ 668,851	$ 728,508	$ 666,869	$ 627,384	$ 560,708	2,753,360	2,583,469	2,299,321
Operating income	54,647	45,884	42,956	39,855	47,190	39,085	16,650	32,930	183,342	135,855	87,684
Net income attributable to Tetra Tech	$ 34,467	$ 29,983	$ 26,862	$ 26,562	$ 31,106	$ 25,694	$ 3,744	$ 23,239	$ 117,874	$ 83,783	$ 39,074
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.61	$ 0.52	$ 0.47	$ 0.47	$ 0.54	$ 0.44	$ 0.06	$ 0.39	$ 2.07	$ 1.44	$ 0.64
Diluted (in dollars per share)	$ 0.60	$ 0.52	$ 0.46	$ 0.46	$ 0.53	$ 0.44	$ 0.06	$ 0.39	$ 2.04	$ 1.42	$ 0.64
Weighted-average common shares outstanding:
Basic (in shares)	56,338	57,184	57,270	57,099	57,309	57,796	58,451	59,058	56,911	58,186	60,913
Diluted (in shares)	57,326	58,161	58,270	58,145	58,192	58,616	59,131	59,793	57,913	58,966	61,532
Coffey
Business acquisition
Acquisition and integration expenses					$ 2,600	$ 1,000	$ 15,900			$ 19,500
Coffey | Interest expense
Business acquisition
Pre-payment charges paid							$ 1,900			$ 1,900