Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 28, 2020	Jun. 30, 2019	Jun. 28, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 45,497	$ 49,233	$ 129,205	$ 147,141
Weighted-average common shares outstanding – basic (in shares)	53,985	54,819	54,366	55,101
Effect of dilutive stock options and unvested restricted stock (in shares)	707	949	795	933
Weighted-average common stock outstanding – diluted (in shares)	54,692	55,768	55,161	56,034
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.84	$ 0.90	$ 2.38	$ 2.67
Diluted (in dollars per share)	$ 0.83	$ 0.88	$ 2.34	$ 2.63