Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 29, 2020	Mar. 31, 2019	Mar. 29, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 36,397	$ 55,911	$ 83,707	$ 97,907
Weighted-average common shares outstanding – basic (in shares)	54,699	55,143	54,541	55,237
Effect of dilutive stock options and unvested restricted stock (in shares)	764	842	839	924
Weighted-average common stock outstanding – diluted (in shares)	55,463	55,985	55,380	56,161
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.67	$ 1.01	$ 1.53	$ 1.77
Diluted (in dollars per share)	$ 0.66	$ 1.00	$ 1.51	$ 1.74