Earnings per Share - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jul. 01, 2018	Apr. 01, 2018	Dec. 31, 2017	Oct. 01, 2017	Jul. 02, 2017	Apr. 02, 2017	Jan. 01, 2017	Sep. 30, 2018	Oct. 01, 2017	Oct. 02, 2016
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 28,802	$ 33,322	$ 28,725	$ 46,034	$ 34,467	$ 29,983	$ 26,862	$ 26,562	$ 136,883	$ 117,874	$ 83,783
Weighted-average common shares outstanding – basic (in shares)	55,341	55,537	55,841	55,855	56,338	57,184	57,270	57,099	55,670	56,911	58,186
Effect of diluted stock options and unvested restricted stock (in shares)									928	1,002	780
Weighted-average common stock outstanding – diluted (in shares)	56,349	56,390	56,673	56,875	57,326	58,161	58,270	58,145	56,598	57,913	58,966
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.52	$ 0.60	$ 0.51	$ 0.82	$ 0.61	$ 0.52	$ 0.47	$ 0.47	$ 2.46	$ 2.07	$ 1.44
Diluted (in dollars per share)	$ 0.51	$ 0.59	$ 0.51	$ 0.81	$ 0.60	$ 0.52	$ 0.46	$ 0.46	$ 2.42	$ 2.04	$ 1.42