Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jul. 01, 2018	Jun. 30, 2019	Jul. 01, 2018
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 49,233	$ 33,322	$ 147,141	$ 108,082
Weighted-average common shares outstanding – basic (in shares)	54,819	55,537	55,101	55,780
Effect of dilutive stock options and unvested restricted stock (in shares)	949	853	933	901
Weighted-average common stock outstanding – diluted (in shares)	55,768	56,390	56,034	56,681
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.90	$ 0.60	$ 2.67	$ 1.94
Diluted (in dollars per share)	$ 0.88	$ 0.59	$ 2.63	$ 1.91