Earnings per Share - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 29, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 30, 2018	Sep. 30, 2018	Jul. 01, 2018	Apr. 01, 2018	Dec. 31, 2017	Sep. 29, 2019	Sep. 30, 2018	Oct. 01, 2017
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 11,527	$ 49,233	$ 55,911	$ 41,997	$ 28,802	$ 33,322	$ 28,725	$ 46,034	$ 158,668	$ 136,883	$ 117,874
Weighted-average common shares outstanding – basic (in shares)	54,617	54,819	55,143	55,390	55,341	55,537	55,841	55,855	54,986	55,670	56,911
Effect of diluted stock options and unvested restricted stock (in shares)									950	928	1,002
Weighted-average common stock outstanding – diluted (in shares)	55,618	55,768	55,985	56,366	56,349	56,390	56,673	56,875	55,936	56,598	57,913
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.21	$ 0.90	$ 1.01	$ 0.76	$ 0.52	$ 0.60	$ 0.51	$ 0.82	$ 2.89	$ 2.46	$ 2.07
Diluted (in dollars per share)	$ 0.21	$ 0.88	$ 1.00	$ 0.75	$ 0.51	$ 0.59	$ 0.51	$ 0.81	$ 2.84	$ 2.42	$ 2.04